The Project Gutenberg eBook, The Royal Observatory Greenwich, by E. Walter
(Edwared Walter) Maunder
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Title: The Royal Observatory Greenwich
A Glance at Its History and Work
Author: E. Walter (Edwared Walter) Maunder
Release Date: November 12, 2013 [eBook #44167]
Language: English
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[Illustration: FLAMSTEED, THE FIRST ASTRONOMER ROYAL.
(_From the portrait in the 'Historia Coelestis.'_)]
THE ROYAL OBSERVATORY GREENWICH
A Glance at Its History and Work
by
E. WALTER MAUNDER, F.R.A.S.
With Many Portraits and Illustrations from
Old Prints and Original Photographs
London
The Religious Tract Society
56 Paternoster Row, and 65 St. Paul's Churchyard
1900
London:
Printed by William Clowes and Sons, Limited,
Stamford Street and Charing Cross.
PREFACE
I was present on one occasion at a popular lecture delivered in
Greenwich, when the lecturer referred to the way in which so many
English people travel to the ends of the earth in order to see
interesting or wonderful places, and yet entirely neglect places
of at least equal importance in their own land. 'Ten minutes' walk
from this hall,' he said, 'is Greenwich Observatory, the most famous
observatory in the world. Most of you see it every day of your
lives, and yet I dare say that not one in a hundred of you has ever
been inside.'
Whether the lecturer was justified in the general scope of his
stricture or not, the particular instance he selected was certainly
unfortunate. It was not the fault of the majority of his audience
that they had not entered Greenwich Observatory, since the
regulations by which it is governed forbade them doing so. These
rules are none too stringent, for the efficiency of the institution
would certainly suffer if it were made a 'show' place, like a
picture gallery or museum. The work carried on therein is too
continuous and important to allow of interruption by daily streams
of sightseers.
To those who may at some time or other visit the Observatory it
may be of interest to have at hand a short account of its history,
principal instruments, and work. To the far greater number who
will never be able to enter it, but who yet feel an interest in
it, I would trust that this little book may prove some sort of a
substitute for a personal visit.
I would wish to take this opportunity of thanking the Astronomer
Royal for his kind permission to reproduce some of the astronomical
photographs taken at the Observatory and to photograph the domes
and instruments. I would also express my thanks to Miss Airy, for
permission to reproduce the photograph of Sir G. B. Airy; to Mr. J.
Nevil Maskelyne, F.R.A.S., for the portrait of Dr. Maskelyne; to Mr.
Bowyer, for procuring the portraits of Bliss and Pond; to Messrs.
Edney and Lacey, for many photographs of the Royal Observatory;
and to the Editor of _Engineering_, for permission to copy two
engravings of the Astrographic telescope.
E. W. M.
ROYAL OBSERVATORY, GREENWICH,
_August, 1900_.
[Illustration: THE NEW BUILDING.
(_From a photograph by Mr. Lacey._)]
CONTENTS
CHAPTER PAGE
I. INTRODUCTION 13
II. FLAMSTEED 25
III. HALLEY AND HIS SUCCESSORS 60
IV. AIRY 102
V. THE OBSERVATORY BUILDINGS 124
VI. THE TIME DEPARTMENT 146
VII. THE TRANSIT AND CIRCLE DEPARTMENTS 181
VIII. THE ALTAZIMUTH DEPARTMENT 205
IX. THE MAGNETIC AND METEOROLOGICAL DEPARTMENTS 228
X. THE HELIOGRAPHIC DEPARTMENT 251
XI. THE SPECTROSCOPIC DEPARTMENT 266
XII. THE ASTROGRAPHIC DEPARTMENT 284
XIII. THE DOUBLE-STAR DEPARTMENT 303
INDEX 317
LIST OF ILLUSTRATIONS
PAGE
FLAMSTEED, THE FIRST ASTRONOMER ROYAL _Frontispiece_
THE NEW BUILDING 7
GENERAL VIEW OF THE OBSERVATORY BUILDINGS FROM THE
NEW DOME 12
FLAMSTEED'S SEXTANT 36
THE ROYAL OBSERVATORY IN FLAMSTEED'S TIME 44
THE 'CAMERA STELLATA' IN FLAMSTEED'S TIME 52
EDMUND HALLEY 61
HALLEY'S QUADRANT 69
JAMES BRADLEY 72
GRAHAM'S ZENITH SECTOR 77
NATHANIEL BLISS 83
NEVIL MASKELYNE 87
HADLEY'S QUADRANT 91
JOHN POND 96
GEORGE BIDDELL AIRY, ASTRONOMER ROYAL 103
THE ASTRONOMER ROYAL'S ROOM 110
THE SOUTH-EAST TOWER 115
W. H. M. CHRISTIE, ASTRONOMER ROYAL 121
THE ASTRONOMER ROYAL'S HOUSE 127
THE COURTYARD 130
PLAN OF OBSERVATORY AT PRESENT TIME 134
THE GREAT CLOCK AND PORTER'S LODGE 147
THE CHRONOGRAPH 158
THE TIME-DESK 164
HARRISON'S CHRONOMETER 165
THE CHRONOMETER ROOM 167
THE CHRONOMETER OVEN 171
THE TRANSIT PAVILION 174
'LOST IN THE BIRKENHEAD' 179
THE TRANSIT CIRCLE 189
THE MURAL CIRCLE 195
AIRY'S ALTAZIMUTH 208
NEW ALTAZIMUTH BUILDING 211
THE NEW ALTAZIMUTH 213
THE NEW OBSERVATORY AS SEEN FROM FLAMSTEED'S OBSERVATORY 219
THE SELF-REGISTERING THERMOMETERS 235
THE ANEMOMETER ROOM, NORTH-WEST TURRET 240
THE ANEMOMETER TRACE 243
MAGNETIC PAVILION--EXTERIOR 246
MAGNETIC PAVILION--INTERIOR 248
THE DALLMEYER PHOTO-HELIOGRAPH 254
PHOTOGRAPH OF A GROUP OF SUN-SPOTS 259
THE GREAT NEBULA IN ORION 269
THE HALF-PRISM SPECTROSCOPE ON THE SOUTH-EAST EQUATORIAL 273
THE WORKSHOP 276
THE 30-INCH REFLECTOR WITH THE NEW SPECTROSCOPE
ATTACHED 278
'CHART PLATE' OF THE PLEIADES 286
THE CONTROL PENDULUM AND THE BASE OF THE THOMPSON
TELESCOPE 289
THE ASTROGRAPHIC TELESCOPE 291
THE DRIVING CLOCK OF THE ASTROGRAPHIC TELESCOPE 294
THE THOMPSON TELESCOPE IN THE NEW DOME 297
THE NEBULÆ OF THE PLEIADES 300
DOUBLE-STAR OBSERVATION WITH THE SOUTH-EAST EQUATORIAL 308
THE SOUTH-EAST DOME WITH THE SHUTTER OPEN 314
[Illustration: GENERAL VIEW OF THE OBSERVATORY BUILDINGS FROM THE
NEW DOME.
(_From a photograph by Mr. Lacey._)]
THE ROYAL OBSERVATORY
GREENWICH
CHAPTER I
INTRODUCTION
I had parted from a friend one day just as he met an acquaintance
of his to whom I was unknown. 'Who is that?' said the newcomer,
referring to me. My friend replied that I was an astronomer from
Greenwich Observatory.
'Indeed; and what does he do there?'
This question completely exhausted my friend's information, for as
his tastes did not lead him in the direction of astronomy, he had
at no time ever concerned himself to inquire as to the nature of my
official duties. 'Oh--er--why--he _observes_, don't you know?' and
the answer, vague as it was, completely slaked the inquirer's thirst
for knowledge.
It is not every one who has such exceedingly nebulous ideas of an
astronomer's duties. More frequently we find that the inquirer has
already formed a vivid and highly-coloured picture of the astronomer
at his 'soul-entrancing work.' Resting on a comfortable couch,
fixed at a luxurious angle, the eye-piece of some great and perfect
instrument brought most conveniently to his eye, there passes
before him, in grand procession, a sight such as the winter nights,
when clear and frosty, give to the ordinary gazer, but increased
ten thousand times in beauty, brilliance, and wonder by the power
of his telescope. For him Jupiter reveals his wind-drifted clouds
and sunset colours; for him Saturn spreads his rings; for him
the snows of Mars fall and melt, and a thousand lunar plains are
ramparted with titanic crags; his are the star-clusters, where suns
in their first warm youth swarm thicker than hiving bees; his the
faint veils of nebulous smoke, the first hint of shape in worlds
about to be, or, perchance, the last relics of worlds for ever
dead. And beside the enjoyment of all this entrancing spectacle of
celestial beauty, the fortunate astronomer sits at his telescope and
_discovers_--always he _discovers_.
This, or something like it, is a very popular conception of an
astronomer's experiences and duty; and consequently many, when they
are told that 'discoveries' are not made at Greenwich, are inclined
to consider that the Observatory has failed in its purpose. An
astronomer without 'discoveries' to his record is like an angler
who casts all day and comes home without fish--obviously an idle or
incompetent person.
Again, it is considered that astronomy is a most transcendental
science. It deals with infinite distances, with numbers beyond
all power of human intellect to appreciate, and therefore it is
supposed, on the one hand, that it is a most elevating study,
keeping the mind continually on the stretch of ecstasy, and, on the
other hand, that it is utterly removed from all connection with
practical, everyday, ordinary life.
These ideas as to the Royal Observatory, or ideas like them, are
very widely current, and they are, in every respect, exactly and
wholly wrong. First of all, Greenwich Observatory was originally
founded, and has been maintained to the present day, for a strictly
practical purpose. Next, instead of leading a life of dreamy ecstasy
or transcendental speculation, the astronomer has, perhaps, more
than any man, to give the keenest attention to minute practical
details. His life, on the one side, approximates to that of the
engineer; on the other, to that of the accountant. Thirdly, the
professional astronomer has hardly anything to do with the show
places of the sky. It is quite possible that there are many people
whose sole opportunity of looking through a telescope is the penny
peep through the instrument of some itinerant showman, who may have
seen more of these than an active astronomer in a lifetime; while as
to 'discoveries,' these lie no more within the scope of our national
observatory than do geographical discoveries within that of the
captain and officers of an ocean liner.
If it is not to afford the astronomer beautiful spectacles, nor to
enable him to make thrilling discoveries, what is the purpose of
Greenwich Observatory?
First and foremost, it is to assist navigation. The ease and
certainty with which to-day thousands of miles of ocean are
navigated have ceased to excite any wonder. We do not even think
about it. We go down to the docks and see, it may be, one steamer
bound for Halifax, another for New York, a third for Charleston,
a fourth for the West Indies, a fifth for Rio de Janeiro; and we
unhesitatingly go on board the one bound for our chosen destination,
without the faintest misgiving as to its direction. We have no more
doubt about the matter than we have in choosing our train at a
railway station. Yet, whilst the train is obliged to follow a narrow
track already laid for it, from which it cannot swerve an inch,
the steamer goes forth to traverse for many days an ocean without
a single fixed mark or indication of direction; and it is exposed,
moreover, to the full force of winds and currents, which may turn it
from its desired path.
But for this facility of navigation, Great Britain could never have
obtained her present commercial position and world-wide empire.
'For the Lord our God most High,
He hath made the deep as dry;
He has smote for us a pathway,
To the ends of all the earth.'
Part of this facility is, of course, due to the invention of the
steam engine, but much less than is generally supposed. Even yet
the clippers, with their roods of white canvas, are not entirely
superseded; and if we could conceive of all steamships being
suddenly annihilated, ere long the sailing vessels would again, as
of yore, prove the
'Swift shuttles of an empire's loom,
That weave us main to main.'
But with the art of navigation thrust back into its condition of a
hundred and fifty years ago, it is doubtful whether a sufficient
tide of commerce could be carried on to keep our home population
supplied, or to maintain a sufficiently close political connection
between these islands and our colonies.
Navigation was in a most primitive condition even as late as
the middle of last century. Then the method of finding a ship's
longitude at sea was the insufficient one of dead reckoning. In
other words, the direction and speed of the ship were estimated as
closely as possible, and so the position was carried on from day to
day. The uncertainty of the method was very great, and many terrible
stories might be told of the disastrous consequences which might,
and often did, follow in the train of this method by guess-work.
It will be sufficient, however, to cite the instance of Commodore
Anson. He wanted to make the island of Juan Fernandez, where he
hoped to obtain fresh water and provisions, and to recruit his
crew, many of whom were suffering from that scourge of old-time
navigators--scurvy. He got into its latitude easily enough, and
ran eastward, believing himself to be west of the island. He was,
however, really east of it, and therefore made the mainland of
America. He had therefore to turn round and sail westwards, losing
many days, during which the scurvy increased upon his crew, many of
whom died from the terrible disease before he reached the desired
island.
The necessity for finding out a ship's place when at sea had not
been very keenly felt until the end of the fifteenth century. It
was always possible for the sailor to ascertain his latitude pretty
closely, either by observing the height of the pole-star at night
or the height of the sun at noonday; and so long as voyages were
chiefly confined to the Mediterranean Sea, and the navigators were
content for the most part to coast from point to point, rarely
losing sight of land, the urgency of solving the second problem--the
longitude of the ship--was not so keenly felt. But immediately the
discoveries of the great Portuguese and Spanish navigators brought a
wider, bolder navigation into vogue, it became a matter of the first
necessity.
To take, for example, the immortal voyage of Christopher Columbus.
His purpose in setting out into the west was to discover a new
way to India. The Venetians and Genoese practically possessed the
overland route across the Isthmus of Suez and down the Red Sea.
Vasco da Gama had opened out the route eastward round the Cape.
Firmly convinced that the world was a globe, Columbus saw that a
third route was possible, namely, one nearly due west; and when,
therefore, he reached the Bahamas, after traversing some 66° of
longitude, he believed that he was in the islands of the China Sea,
some 230° from Spain. Those who followed him still laboured under
the same impression, and when they reached the mainland of America,
believed that they were close to the shores of India, which was
still distant from them by half the circumference of the globe.
Little by little the intrepid sailors of the sixteenth century
forced their way to a true knowledge of the size of the globe, and
of the relative position of the great continents. But this knowledge
was only attained after many disasters and terrible miseries; and
though a new kind of navigation was established--the navigation of
the open ocean, far away from any possible landmark, a navigation as
different as could be conceived from the old method of coasting--yet
it remained terribly risky and uncertain throughout the sixteenth
century. Therefore many mathematicians endeavoured to solve the
problem of determining the position of a ship when at sea. Their
suggestions, however, remained entirely fruitless at the time,
though in several instances they struck upon principles which are
being employed at the present day.
The first country to profit by the discovery of America was Spain,
and hence Spain was the first to feel keenly the pinch of the
problem. In 1598, therefore, Philip III. offered a prize of 100,000
crowns to any one who would devise a method by which a captain of
a vessel could determine his position when out of sight of land.
Holland, which had recently started on its national existence, and
which was challenging the colonial empire of Spain, followed very
shortly after with the offer of a reward of 30,000 florins. Not very
long after the offer of these rewards, a master mind did work out a
simple method for determining the longitude, a method theoretically
complete, though practically it proved inapplicable. This was
Galileo, who, with his newly invented telescope, had discovered that
Jupiter was attended by four satellites.
At first sight such a discovery, however interesting, would seem to
have not the slightest bearing upon the sailor's craft, or upon the
commercial progress of one nation or another. But Galileo quickly
saw in it the promise of great practical usefulness. The question
of the determination of the place of a ship when in the open ocean
really resolved itself into this: How could the navigator ascertain
at any time what was the true time, say at the port from which he
sailed? As already pointed out, it was possible, by observing the
height of the sun at noon, or of the pole-star at night, to infer
the latitude of the ship. The longitude was the point of difficulty.
Now, the longitude may be expressed as the difference between the
local time of the place of observation and the local time at the
place chosen as the standard meridian. The sailor could, indeed,
obtain his own local time by observations of the height of the sun.
The sun reached its greatest height at local noon, and a number of
observations before and after noon would enable him to determine
this with sufficient nicety.
But how was he to determine when he, perhaps, was half-way across
the Atlantic, what was the local time at Genoa, Cadiz, Lisbon,
Bristol, or Amsterdam, or whatever was the port from which he
sailed? Galileo thought out a way by which the satellites of Jupiter
could give him this information.
For as they circle round their primary, they pass in turn into its
shadow, and are eclipsed by it. It needed, then, only that the
satellites should be so carefully watched, that their motions,
and, consequently, the times of their eclipses could be foretold.
It would follow, then, that if the mariner had in his almanac the
local time of the standard city at which a given satellite would
enter into eclipse, and he were able to note from the deck of his
vessel the disappearance of the tiny point, he would ascertain the
difference between the local times of the two places, or, in other
words, the difference of their longitudes.
The plan was simplicity itself, but there were difficulties
in carrying it out, the greatest being the impossibility of
satisfactorily making telescopic observations from the moving deck
of a ship at sea. Nor were the observations sufficiently sharp to be
of much help. The entry of a satellite into the shadow of Jupiter is
in most cases a somewhat slow process, and the moment of complete
disappearance would vary according to the size of the telescope, the
keenness of the observer's sight, and the transparency of the air.
As the power and commerce of Spain declined, two other nations
entered into the contest for the sovereignty of the seas, and with
that sovereignty predominance in the New World of America--France
and England. The problem of the longitude at sea, or, as already
pointed out, what amounts to the same thing, the problem how to
determine when at sea the local time at some standard place, became,
in consequence, of greater necessity to them.
The standard time would be easily known, if a thoroughly good
chronometer which did not change its rate, and which was set to the
standard time before starting, was carried on board the ship. This
plan had been proposed by Gemma Frisius as early as 1526, but at the
time was a mere suggestion, as there were no chronometers or watches
sufficiently good for the purpose. There was, however, another
method of ascertaining the standard time. The moon moves pretty
quickly amongst the stars, and at the present time, when its motions
are well known, it is possible to draw up a table of its distances
from a number of given stars at definite times for long periods in
advance. This is actually done to-day in the _Nautical Almanac_,
the moon's distance from certain stars being given for every three
hours of Greenwich time. It is possible, then, by measuring these
distances, and making, as in the case of the latitude, certain
corrections, to find out the time at Greenwich. In short, the whole
sky may be considered as a vast clock set to Greenwich time, the
stars being the numbers on the dial face, and the moon the hand (for
this clock has only one hand) moving amongst them.
The local apparent time--that is, the time at the place at which the
ship itself was--is a simpler matter. It is noon at any place when
the sun is due south--or, as we may put it a little differently,
when it culminates--that is, when it reaches its highest point.
To find the longitude at sea, therefore, it was necessary to be
able to predict precisely the apparent position of the moon in
the sky for any time throughout the entire year, and it was also
necessary that the places of the stars themselves should be very
accurately known. It was therefore to gather the materials for a
better knowledge of the motions of the moon and the position of the
stars that Greenwich Observatory was founded, whilst the _Nautical
Almanac_ was instituted to convey this information to mariners in a
convenient form.
This proposal was actually made in the reign of Charles II. by
a Frenchman, Le Sieur de Saint-Pierre, who, having secured an
introduction to the Duchess of Portsmouth, endeavoured to obtain a
reward for his scheme. It would appear that he had simply borrowed
the idea from a book which an eminent French mathematician brought
out forty years before, without having himself any real knowledge
of the subject. But when the matter was brought before the king's
notice, he desired some of the leading scientific men of the day to
report upon its practicability, and the Rev. John Flamsteed was the
man selected for the task. He reported that the scheme in itself
was a good one, but impracticable in the then state of science. The
king, who, in spite of the evil reputation which he has earned for
himself, took a real interest in science, was startled when this was
reported to him, and commanded the man who had drawn his attention
to these deficiencies 'to apply himself,' as the king's astronomer,
'with the most exact care and diligence to the Rectifying the Tables
of the Motions of the Heavens and the Places of the Fixed Stars,
in order to find out the so much desired Longitude at Sea, for the
perfecting the Art of Navigation.'
This man, the Rev. John Flamsteed, was accordingly appointed first
Astronomer Royal at the meagre salary of £100 a year, with full
permission to provide himself with the instruments he might require,
at his own expense. He followed out the task assigned to him with
extreme devotion, amidst many difficulties and annoyances, until his
death in 1719. He has been succeeded by seven Astronomers Royal,
each of whom has made it his first object to carry out the original
scheme of the institution; and the chief purpose of Greenwich
Observatory to-day, as when it was founded in 1675, is to observe
the motions of the sun, moon, and planets, and to issue accurate
star catalogues.
It will be seen, therefore, that the establishment of Greenwich
Observatory arose from the actual necessity of the nation. It was
an essential step in its progress towards its present position as
the first commercial nation. No thoughts of abstract science were
in the minds of its founders; there was no desire to watch the
cloud-changes on Jupiter, or to find out what Sirius was made of.
The Observatory was founded for the benefit of the Royal Navy and of
the general commerce of the realm; and, in essence, that which was
the sole object of its foundation at the beginning has continued to
be its first object down to the present time.
It was impossible that the work of the Observatory should be always
confined within the above limits, and it will be my purpose, in the
pages which follow, to describe when and how the chief expansions of
its programme have taken place. But assistance to navigation is now,
and has always been, the dominant note in its management.
CHAPTER II
FLAMSTEED
For the first century of its existence, the lives of its Astronomers
Royal formed practically the history of the Royal Observatory.
During this period, the Observatory was itself so small that the
Astronomer Royal, with a single assistant, sufficed for the entire
work. Everything, therefore, depended upon the ability, energy,
and character of the actual director. There was no large organized
staff, established routine, or official tradition, to keep the
institution moving on certain lines, irrespective of the personal
qualities of the chief. It was specially fortunate, therefore, that
the first four Astronomers Royal, Flamsteed, Halley, Bradley, and
Maskelyne (for Bliss, the immediate successor of Bradley, reigned
for so short a time that he may be practically left out of the
count), were all men of the most conspicuous ability.
It will be convenient to divide the history of the first seven
Astronomers Royal into three sections. In the first, we have the
founder, John Flamsteed, a pathetic and interesting figure, whom
we seem to know with especial clearness, from the fulness of the
memorials which he has left to us. He was succeeded by the man
who was, indeed, best fitted to succeed him, but whom he most
hated. The second to the sixth Astronomers Royal formed what we
might almost speak of as a dynasty, each in turn nominating his
successor, who had entered into more or less close connection with
the Observatory during the lifetime of the previous director; and
the lives of these five may well form the second section. The line
was interrupted after the resignation of the sixth Astronomer Royal,
and the third section will be devoted to the seventh director,
Airy, under whom the Observatory entered upon its modern period of
expansion.
'God suffers not man to be idle, although he swim in the midst
of delights; for when He had placed His own image (Adam) in a
paradise so replenished (of His goodness) with varieties of all
things, conducing as well to his pleasure as sustenance, that
the earth produced of itself things convenient for both,--He yet
(to keep him out of idleness) commands him to till, prune, and
dress his pleasant, verdant habitation; and to add (if it might
be) some lustre, grace, or conveniency to that place, which, as
well as he, derived its original from his Creator.'
In these words JOHN FLAMSTEED begins the first of several
autobiographies which he has handed down to us; this particular one
being written before he attained his majority, 'to keep myself from
idleness and to recreate myself.'
'I was born,' he goes on, 'at Denby, in Derbyshire, in the year
1646, on the 19th day of August, at 7 hours 16 minutes after
noon. My father, named Stephen, was the third son of Mr. William
Flamsteed, of Little Hallam; my mother, Mary, was the daughter
of Mr. John Spateman, of Derby, ironmonger. From these two I
derived my beginning, whose parents were of known integrity,
honesty, and fortune, as they [were] of equal extraction and
ingenuity; betwixt whom I [was] tenderly educated (by reason of
my natural weakness, which required more than ordinary care)
till I was aged three years and a fortnight; when my mother
departed, leaving my father a daughter, then not a month old,
with me, then weak, to his fatherly care and provision.'
The weakly, motherless boy became at an early age a voracious
reader. At first, he says--
'I began to affect the volubility and ranting stories of
romances; and at twelve years of age I first left off the
wild ones, and betook myself to read the better sort of
them, which, though they were not probable, yet carried no
seeming impossibility in the fiction. Afterwards, as my reason
increased, I gathered other real histories; and by the time I
was fifteen years old I had read, of the ancients, Plutarch's
_Lives_, Appian's and Tacitus's _Roman Histories_, Holingshed's
_History of the Kings of England_, Davies's _Life of Queen
Elizabeth_, Saunderson's of _King Charles the First_, Heyling's
_Geography_, and many others of the moderns; besides a company
of romances and other stories, of which I scarce remember a
tenth at present.'
Flamsteed received his education at the free school at Derby, where
he continued until the Whitsuntide of 1662, when he was nearly
sixteen years of age. Two years earlier than this, however, a great
misfortune fell upon him.
'At fourteen years of age,' he writes, 'when I was nearly
arrived to be the head of the free-school, [I was] visited with
a fit of sickness, that was followed with a consumption and
other distempers, which yet did not so much hinder me in my
learning, but that I still kept my station till the form broke
up, and some of my fellows went to the Universities; for which,
though I was designed, my father thought it not advisable to
send me, by reason of my distemper.'
This was a keen disappointment to him, but seems to have really
been the means of determining his career. The sickly, suffering boy
could not be idle, though 'a day's short reading caused so violent
a headache;' and a month or two after he had left school, he had
a book lent to him--Sacrobosco's _De Sphæra_, in Latin--which was
the beginning of his mathematical studies. A partial eclipse of the
sun in September of the same year seems to have first drawn his
attention to astronomical observation, and during the winter his
father, who had himself a strong passion for arithmetic, instructed
him in that science.
It was astonishing how quickly his appetite for his new subjects
grew. The _Art of Dialling_, the calculation of tables of the sun's
altitudes for all hours of the day, and for different latitudes,
and the construction of a quadrant--'of which I was not meanly
joyful'--were the occupations of this winter of illness.
In 1664 he made the acquaintanceship of two friends, Mr. George
Linacre and Mr. William Litchford; the former of whom taught him to
recognize many of the fixed stars, whilst the latter was the means
of his introduction to a knowledge of the motions of the planets.
'I had now completed eighteen years, when the winter came
on, and thrust me again into the chimney; whence the heat
and dryness of the preceding summer had happily once before
withdrawn me.'
The following year, 1665, was memorable to him 'for the appearance
of the comet,' and for a journey which he made to Ireland to be
'stroked' for his rheumatic disorder by Valentine Greatrackes, a
kind of mesmerist, who had the repute of effecting wonderful cures.
The journey, of which he gives a full and vivid account, occupied
a month; but though he was a little better, the following winter
brought him no permanent benefit.
But, ill or well, he pressed on his astronomical studies. A large
partial eclipse of the sun was due the following June; he computed
the particulars of it for Derby, and observed the eclipse itself to
the best of his ability. He argued out for himself 'the equation
of time'; the difference, that is, between time as given by the
actual sun, or 'apparent time,' and that given by a perfect clock,
or 'mean time.' He drew up a catalogue of seventy stars, computing
their right ascensions, declinations, longitudes, and latitudes
for the year 1701; he attempted to determine the inclination of
the ecliptic, the mean length of the tropical year, and the actual
distance of the earth from the sun. And these were the recreations
of a sickly, suffering young man, not yet twenty-one years of age,
and who had only begun the study of arithmetic, such as fractions
and the rule of three, four years previously!
His next attempt was almanac-making, in the which he improved
considerably upon those current at the time. His almanac for 1670
was rejected, however, and returned to him, and, not to lose
his whole labour, he sent his calculations of an eclipse of the
sun, and of five occultations of stars by the moon, which he had
undertaken for the almanac, to the Royal Society. He sent the paper
anonymously, or, rather, signed it with an anagram, 'In mathesi a
sole fundes,' for 'Johannes Flamsteedius.' His covering letter ends
thus:--
'Excuse, I pray you, this juvenile heat for the concerns of
science and want of better language, from one who, from the
sixteenth year of his age to this instant, hath only served one
bare apprenticeship in these arts, under the discouragement of
friends, the want of health, and all other instructors except
his better genius.'
This letter was dated November 4, 1669, and on January 14, Mr.
Oldenburg, the secretary of the Society, replied to him in a letter
which the young man cannot but have felt encouraging and flattering
to the highest degree.
'Though you did what you could to hide your name from us,'
he writes, 'yet your ingenious and useful labours for the
advancement of Astronomy addressed to the noble President of the
Royal Society, and some others of that illustrious body, did
soon discover you to us, upon our solicitous inquiries after
their worthy author.'
And after congratulating him upon his skill, and encouraging him
to furnish further similar papers, he signs himself, 'Your very
affectionate friend and real servant'--no unmeaning phrase, for the
friendship then commenced ceased only with Oldenburg's life.
The following June, his father, pleased with the notice that some
of the leading scientific men of the day were taking of his son,
sent him up to London, that he might be personally acquainted with
them; and he then was introduced to Sir Jonas Moore, the Surveyor
of the Ordnance, who made him a present of Townley's micrometer,
and promised to furnish him with object-glasses for telescopes at
moderate rates.
On his return journey he called at Cambridge, where he visited Dr.
Barrow and Newton, and entered his name in Jesus College.
It was not until the following year, 1671, that he was enabled to
complete his own observatory, as he had had to wait long for the
lenses which Sir Jonas Moore and Collins had promised to procure
for him. He still laboured under several difficulties, in that he
had no good means for measuring time, pendulum clocks not then
being common. He, therefore, with a practical good sense which was
characteristic, refrained from attempting anything which lay out of
his power to do well, and he devoted himself to such observations as
did not require any very accurate knowledge of the time. At the same
time, he was careful to ascertain the time of his observations as
closely as possible, by taking the altitudes of the stars.
The next four years seem to have passed exceedingly pleasantly to
him. The notes of ill-health are few. He was making rapid progress
in his acquaintanceship with the work of other astronomers,
particularly with those of the three marvellously gifted young
men--Horrox, Crabtree, and Gascoigne--who had passed away shortly
before his own birth. He was making new friends in scientific
circles, and, in particular, Sir Jonas Moore was evidently esteeming
him more and more highly. In 1674 he became more intimate with
Newton, the occasion which led to this acquaintanceship being the
amusing one, that his assistance was asked by Newton, who had
found himself unable to adjust a microscope, having forgotten its
object-glass--not the only instance of the great mathematician's
absent-mindedness.
The same year he took his degree of A.M. at Cambridge, designing
to enter the Church; but Sir Jonas Moore was extremely anxious to
give him official charge of an observatory, and was urging the Royal
Society to build an astronomical observatory at Chelsea College,
which then belonged to that body. He therefore came up to London,
and resided some months with Sir Jonas Moore at the Tower. But
shortly after his coming up to London, 'an accident happened,' to
use his own expression, that hastened, if it did not occasion, the
building of Greenwich Observatory.
'A Frenchman that called himself Le Sieur de St. Pierre, having
some small skill in astronomy, and made an interest with a
French lady, then in favour at Court, proposed no less than
the discovery of the Longitude, and had procured a kind of
Commission from the King to the Lord Brouncker, Dr. Ward (Bishop
of Salisbury), Sir Christopher Wren, Sir Charles Scarborough,
Sir Jonas Moore, Colonel Titus, Dr. Pell, Sir Robert Murray,
Mr. Hook, and some other ingenious gentlemen about the town and
Court, to receive his proposals, with power to elect, and to
receive into their number, any other skilful persons; and having
heard them, to give the King an account of them, with their
opinion whether or no they were practicable, and would show
what he pretended. Sir Jonas Moore carried me with him to one
of their meetings, where I was chosen into their number; and,
after, the Frenchman's proposals were read, which were:
'(1) To have the year and day of the observations.
'(2) The height of two stars, and on which side of the meridian
they appeared.
'(3) The height of the moon's two limbs.
'(4) The height of the pole--all to degrees and minutes.
'It was easy to perceive, from these demands, that the
sieur understood not that the best lunar tables differed
from the heavens; and that, therefore, his demands were not
sufficient for determining the longitude of the place where
such observations were, or should be, made, from that to which
the lunar tables were fitted, which I represented immediately
to the company. But they, considering the interests of his
patroness at Court, desired to have him furnished according to
his demands. I undertook it; and having gained the moon's true
place by observations made at Derby, February 23, 1672, and
November 12, 1673, gave him observations such as he demanded.
The half-skilled man did not think they could have been given
him, and cunningly answered "_They were feigned_." I delivered
them to Dr. Pell, February 19, 1674-5, who, returning me his
answer some time after, I wrote a letter in English to the
commissioners, and another in Latin to the sieur, to assure him
they were not feigned, and to show them that, if they had been,
yet if we had astronomical tables that would give us the two
places of the fixed stars and the moon's true places, both in
longitude and latitude, nearer than to half a minute, we might
hope to find the longitude of places by lunar observations, but
not by such as he demanded. But that we were so far from having
the places of the fixed stars true, that the Tychonic Catalogues
often erred ten minutes or more; that they were uncertain to
three or four minutes, by reason that Tycho assumed a faulty
obliquity of the ecliptic, and had employed only plain sights
in his observations: and that the best lunar tables differ
one-quarter, if not one-third, of a degree from the heavens;
and lastly, that he might have learnt better methods than he
proposed, from his countryman Morin, whom he had best consult
before he made any more demands of this nature.'
This was in effect to tell St. Pierre that his proposal was neither
original nor practicable. If St. Pierre had but consulted Morin's
writings (Morin himself had died more than eighteen years before),
he would have known that practically the same proposal had been
laid before Cardinal Richelieu in 1634, and had been rejected, as
quite impracticable in the then state of astronomical knowledge.
Possibly Flamsteed meant further to intimate that St. Pierre had
simply stolen his method from Morin, hoping to trade it off upon
the government of another country; in which case he would no doubt
regard Flamsteed's letter as a warning that he had been found out.
Flamsteed continues:--
'I heard no more of the Frenchman after this; but was told
that, my letters being shown King Charles, he startled at
the assertion of the fixed stars' places being false in the
catalogue; said, with some vehemence, "He must have them anew
observed, examined, and corrected, for the use of his seamen;"
and further (when it was urged to him how necessary it was to
have a good stock of observations taken for correcting the
motions of the moon and planets), with the same earnestness,
"he must have it done." And when he was asked Who could, or who
should do it? "The person (says he) that informs you of them."
Whereupon I was appointed to it, with the incompetent allowance
aforementioned; but with assurances, at the same time, of such
further additions as thereafter should be found requisite for
carrying on the work.'
[Illustration: FLAMSTEED'S SEXTANT.
(_From an engraving in the 'Historia Coelestis.'_)]
Thus, in his twenty-ninth year, John Flamsteed became the first
Astronomer Royal. In many ways he was an ideal man for the post.
In the twelve years which had passed since he left school he had
accomplished an amazing amount of work. Despite his constant
ill-health and severe sufferings, and the circumstance--which may
be inferred from many expressions in his autobiographies--that he
assisted his father in his business, he had made himself master,
perhaps more thoroughly than any of his contemporaries, of the
entire work of a practical astronomer as it was then understood.
He was an indefatigable computer; the calculation of tables of the
motions of the moon and planets, which should as faithfully as
possible represent their observed positions, had had an especial
attraction for him, and, as has been already mentioned, some years
before his appointment he had drawn up a catalogue of stars,
based upon the observations of Tycho Brahe. More than that, he
had not been a merely theoretical worker, he had been a practical
observer of very considerable skill, and, in the dearth of suitable
instruments, had already made one or two for himself, and had
contemplated the making of others. In his first letter to Sir Jonas
Moore he asks for instruction as to the making of object-glasses
for telescopes, for he was quite prepared to set about the task of
making his own. In addition to his tireless industry, which neither
illness nor suffering could abate, he was a man of singularly exact
and business-like habits. The precision with which he preserves and
records the dates of all letters received or sent is an illustration
of this. On the other hand, he had the defects of his circumstances
and character. His numerous autobiographical sketches betray him,
not indeed as a conceited man, in the ordinary sense of the word,
but as an exceedingly self-conscious one. Devout and high-principled
he most assuredly was, but, on the other hand, he shows in almost
every line he wrote that he was one who could not brook anything
like criticism or opposition.
Such a man, however efficient, was little likely to be happy as the
first incumbent of a new and important government post; but there
was another circumstance which was destined to cause him greater
unhappiness still.
If we believe, as surely we must, that not only the moral and the
physical progress of mankind is watched over and controlled by
God's good Providence, but its intellectual progress as well, then
there can be no doubt that John Flamsteed was raised up at this
particular time, not merely to found Greenwich Observatory, and to
assist the solution of the problem of the longitude at sea, but
also, and chiefly, to become the auxiliary to a far greater mind,
the journeyman to a true master-builder. But for the founding of
Greenwich Observatory, and for John Flamsteed's observations made
therein, the working out of Newton's grand theory of gravitation
must have been hindered, and its acceptance by the men of science
of his time immensely delayed. We cannot regard as accidental the
combination, so fortunate for us, of Newton, the great world-genius,
to work out the problem, of Flamsteed, the painstaking observer, to
supply him with the materials for his work, and of the newly-founded
institution, Greenwich Observatory, where Flamsteed was able to
gather those materials together. This is the true debt that we owe
to Flamsteed, that, little as he understood the position in which
he had been placed from the standpoint from which we see it to-day,
yet, to the extent of his ability, and as far as he conceived it
in accordance with his duty, he gave Newton such assistance as he
could.
This is how we see the matter to-day. It wore a very different
aspect in Flamsteed's eyes; and the two following documents, the
one, the warrant founding the Observatory and making him Astronomer
Royal; the other, the warrant granting him a salary, will go far to
explain his position in the matter. He had a high-sounding, official
position, which could not fail to impress him with a sense of
importance; whilst his salary was so insufficient that he naturally
regarded himself as absolute owner of his own work.
_'Warrant for the Payment of Mr. Flamsteed's Salary._
'Charles Rex.
'Whereas, we have appointed our trusty and well-beloved John
Flamsteed, Master of Arts, our astronomical observator,
forthwith to apply himself with the most exact care and
diligence to the rectifying the tables of the motions of the
heavens, and the places of the fixed stars, so as to find out
the so-much-desired longitude of places for the perfecting the
art of navigation, Our will and pleasure is, and we do hereby
require and authorize you, for the support and maintenance of
the said John Flamsteed, of whose abilities in astronomy we have
very good testimony, and are well satisfied, that from time
to time you pay, or cause to be paid, unto him, the said John
Flamsteed, or his assigns, the yearly salary or allowance of
one hundred pounds per annum; the same to be charged and borne
upon the quarter-books of the Office of the Ordnance, and paid
to him quarterly, by even and equal portions, by the Treasurer
of our said office, the first quarter to begin and be accompted
from the feast of St. Michael the Archangel last past, and so to
continue during our pleasure. And for so doing, this shall be as
well unto you, as to the Auditors of the Exchequer, for allowing
the same, and all other our officers and ministers whom it may
concern, a full and sufficient warrant.
'Given at our Court at Whitehall, the 4th day of March, 1674-5.
'By his Majesty's Command,
'J. WILLIAMSON.
'To our right-trusty and well-beloved Counsellor, Sir
Thomas Chichely, Knt., Master of our Ordnance, and to the
Lieutenant-General of our Ordnance, and to the rest of the
Officers of our Ordnance, now and for the time being, and to all
and every of them.'
_'Warrant for Building the Observatory._
'Charles Rex.
'Whereas, in order to the finding out of the longitude of places
for perfecting navigation and astronomy, we have resolved to
build a small observatory within our park at Greenwich, upon
the highest ground, at or near the place where the Castle
stood, with lodging-rooms for our astronomical observator and
assistant, Our will and pleasure is, that according to such plot
and design as shall be given you by our trusty and well-beloved
Sir Christopher Wren, Knight, our surveyor-general of the place
and scite of the said observatory, you cause the same to be
fenced in, built and finished with all convenient speed, by such
artificers and workmen as you shall appoint thereto, and that
you give order unto our Treasurer of the Ordnance for the paying
of such materials and workmen as shall be used and employed
therein, out of such monies as shall come to your hands for old
and decayed powder, which hath or shall be sold by our order of
the 1st of January last, provided that the whole sum, so to be
expended or paid, shall not exceed five hundred pounds; and our
pleasure is, that all our officers and servants belonging to our
said park be assisting to those that you shall appoint, for the
doing thereof, and for so doing, this shall be to you, and to
all others whom it may concern, a sufficient warrant.
'Given at our Court at Whitehall, the 22nd day of June, 1675, in
the 27th year of our reign.
'By his Majesty's Command,
'J. WILLIAMSON.
'To our right-trusty and well-beloved Counsellor, Sir Thomas
Chichely, Knt., Master-General of our Ordnance.'
The first question that arose, when it had been determined to found
the new Observatory, was where it was to be placed. Hyde Park was
suggested, and Sir Jonas Moore recommended Chelsea College, where
he had already thought of establishing Flamsteed in a private
observatory. Fortunately, both these localities were set aside in
favour of one recommended by Sir Christopher Wren. There was a small
building on the top of the hill in the Royal Park of Greenwich
belonging to the Crown, and which was now of little or no use.
Visible from the city, and easily accessible by that which was then
the best and most convenient roadway, the river Thames, it was yet
so completely out of town as to be entirely safe from the smoke
of London. In Greenwich Park, too, but on the more easterly hill,
Charles I. had contemplated setting up an observatory, but the
pressure of events had prevented him carrying out his intention.
A further practical advantage was that materials could be easily
transported thither. The management of public affairs under Charles
II. left much to be desired in the matter of efficiency and economy,
and it was not very easy to procure what was wanted for the erection
of a purely scientific building. However, the matter was arranged.
A gate-house demolished in the Tower supplied wood; iron, and
lead, and bricks were supplied from Tilbury Fort, and these could
be easily brought by water to the selected site. The sum of £500,
actually £520, was further allotted from the results of a sale
of spoilt gunpowder; and with these limited resources Greenwich
Observatory was built.
The foundation-stone was laid August 10, 1675, and Flamsteed
amused himself by drawing the horoscope of the Observatory, a
fact which--in spite of his having written across the face of the
horoscope _Risum teneatis amici?_ (Can you keep from laughter, my
friends?), and his having two or three years before written very
severely against the imposture of astrology--has led some modern
astrologers to claim him as a believer in their cult. He actually
entered into residence July 10, 1676.
[Illustration: THE ROYAL OBSERVATORY IN FLAMSTEED'S DAY.
(_From an engraving in the 'Historia Coelestis.'_)]
His position was not a bright one. The Government had, indeed,
provided him with a building for his observatory, and a small house
for his own residence, but he had no instrument and no assistant.
The first difficulty was partly overcome for the moment by gifts
or loans from Sir Jonas Moore, and by one or two small loans from
the Royal Society. The death of this great friend and patron,
four years after the founding of the Observatory, and only three
years after his entering into residence, deprived him of several
of these; it was with difficulty that he maintained against Sir
Jonas' heirs his claim to the instruments which Sir Jonas had given
him. There was nothing for him to do but to make his instruments
himself, and in 1683 he built a mural quadrant of fifty inches
radius. His circumstances improved the following year, when Lord
North gave him the living of Burstow, near Horley, Surrey, Flamsteed
having received ordination almost at the time of his appointment
to the Astronomer Royalship. We have little or no account of the
way in which he fulfilled his duties as a clergyman. Evidently he
considered that his position as Astronomer Royal had the first
claim upon him. At the same time, comparatively early in life he
had expressed his desire to fill the clerical office, and he was a
man too conscientious to neglect any duty that lay upon him. That
in spite of his feeble health he often journeyed to and fro between
Burstow and Greenwich we know; and we may take it as certain that at
a time when the standard of clerical efficiency was extremely low,
he was not one of those who
'For their bellies' sake,
Creep and intrude and climb into the fold.'
His chief source of income, however, seems to have been the private
pupils whom he took in mathematics and astronomy. These numbered in
the years 1676 to 1709 no fewer than 140; and as many of them were
of the very first and wealthiest families in the kingdom, the gain
to Flamsteed in money and influence must have been considerable. But
it was most distasteful work. It was in no sense that which he felt
to be his duty, and which he had at heart. It was undertaken from
sheer, hard necessity, and he grudged bitterly the time and strength
which it diverted from his proper calling.
How faithfully he followed that, one single circumstance will show.
In the thirteen years ending 1689, he made 20,000 observations, and
had revised single-handed the whole of the theories and tables of
the heavenly bodies then in use.
In 1688 the death of his father brought him a considerable accession
of means, and, far more important, the assistance of Abraham
Sharp,[1] the first and most distinguished of the long list of
Greenwich assistants, men who, though far less well known than the
Astronomers Royal, have contributed scarcely less in their own field
to the high reputation of the Observatory.
[1] Abraham Sharp had been with Flamsteed earlier than this--in 1684
and 1685.
Sharp was not only a most careful and indefatigable calculator,
he was what was even more essential for Flamsteed--a most skilful
instrument-maker; and he divided for him a new mural arc of 140° and
seven feet radius, with which he commenced operations on December
12, 1689. Above all, Sharp became his faithful and devoted friend
and adherent, and no doubt his sympathy strengthened Flamsteed to
endure the trouble which was at hand.
That trouble began in 1694, when Newton visited the Royal
Observatory. At that time Flamsteed, though he had done so much,
had published nothing, and Newton, who had made his discovery of
the laws of gravitation some few years before, was then employed
in deducing from them a complete theory of the moon's motion. This
work was one of absolutely first importance. In the first place
and chiefly, upon the success with which it could be carried out,
depended undoubtedly the acceptance of the greatest discovery
which has yet been made in physical science. Secondarily--and this
should, and no doubt did, appeal to Flamsteed--the perfecting of
our knowledge of the movements of the moon was a primary part of
the very work which he was commissioned to do as Astronomer Royal.
Newton was, therefore, anxious beyond everything to receive the
best possible observations of the moon's places, and he came to
Flamsteed, as to the man from whom he had a right to expect to
receive a supply of them. At first Flamsteed seems to have given
these as fully as he was able; but it is evident that Newton chafed
at the necessity for these frequent applications to Flamsteed, and
to the constant need of putting pressure upon him. Flamsteed, on the
other hand, as clearly evidently resented this continual demand.
Feeling, as he keenly did, that, though he had been named Astronomer
Royal, he had been left practically entirely without support; his
instruments were entirely his own, either made or purchased by
himself; his nominal salary of £100 was difficult to get, and did
not nearly cover the actual current expenses of his position, he not
unnaturally regarded his observations as his own exclusive property.
He had a most natural dislike for his observations to be published,
except after such reduction as he himself had carried through,
and in the manner which he himself had chosen. The idea which was
ever before him was that of carrying out a single great work that
should not only be a monument to his own industry and skill, but
should also raise the name of England amongst scientific nations. He
complained of it, therefore, both as a personal wrong and an injury
to the country when some observations of Cassini's were combined
with some observations of his own in order to deduce a better orbit
for a comet.
Unknown to himself, therefore, he was called upon to decide a
question that has proved fundamental to the policy of Greenwich
Observatory, and he decided it wrongly--the question of publication.
Newton had urged upon him as early as 1691 that he should not wait
until he had formed an exhaustive catalogue of all the brighter
stars, but that he should publish at once a catalogue of a few,
which might serve as standards; but Flamsteed would not hear of it.
He failed to see that his office had been created for a definite
practical purpose, not for the execution of some great scheme,
however important to science. All his work of thirty years had done
nothing to forward navigation so long as he published nothing. But
if, year by year, he had published the places of the moon and of a
few standard stars, he would have advanced the art immensely and
yet have not hindered himself from eventually bringing out a great
catalogue. No doubt the little incident of Newton's difficulty
with the microscope, of which he had forgotten the object-glass,
had given Flamsteed a low opinion of Newton's qualifications as a
practical astronomer. If so, he was wrong, for Newton's insight
into practical matters was greater than Flamsteed's own, and his
practical skill was no less, though his absent-mindedness might
occasionally lead him into an absurd mistake.
The following extract from Flamsteed's own 'brief History of the
Observatory' gives an account of his view of Newton's action towards
him in desiring the publication of his star catalogue, and at the
same time it illustrates Flamsteed's touchy and suspicious nature.
'Whilst Mr. Flamsteed was busied in the laborious work of
the catalogue of the fixed stars, and forced often to watch
and labour by night, to fetch the materials for it from the
heavens, that were to be employed by day, he often, on Sir Isaac
Newton's instances, furnished him with observations of the
moon's places, in order to carry on his correction of the lunar
theory. A civil correspondence was carried on between them; only
Mr. Flamsteed could not but take notice that as Sir Isaac was
advanced in place, so he raised himself in his conversation and
became more magisterial. At last, finding that Mr. Flamsteed
had advanced far in his designed catalogue by the help of his
country calculators, that he had made new lunar tables, and
was daily advancing on the other planets, Sir Isaac Newton
came to see him (Tuesday, April 11, 1704); and desiring, after
dinner, to be shown in what forwardness his work was, had so
much of the catalogue of the fixed stars laid before him as was
then finished; together with the maps of the constellations,
both those drawn by T. Weston and P. Van Somer, as also his
collation of the observed places of Saturn and Jupiter, with
the Rudolphine numbers. Having viewed them well, he told Mr.
Flamsteed he would (_i.e._ he was desirous to) recommend them
to the Prince _privately_. Mr. Flamsteed (who had long been
sensible of his partiality, and heard how his two flatterers
cried Sir Isaac's performances up, was sensible of the snare in
the word _privately_) answered that would not do; and (upon Sir
Isaac's demanding "why not?") that then the Prince's attendants
would tell him these were but curiosities of no great use, and
persuade him to save that expense, that there might be the
more for them to beg of him: and that the recommendation must
be made _publicly_, to prevent any such suggestions. Sir Isaac
apprehended right, that he was understood, and his designs
defeated: and so took his leave not well satisfied with the
refusal.
'It was November following ere Mr. Flamsteed heard from him any
more: when, considering with himself that what he had done was
not well understood, he set himself to examine how many folio
pages his work when printed would fill; and found upon an easy
computation that they would at least take up 1400. Being amazed
at this, he set himself to consider them more seriously; drew
up an estimate of them; and, to obviate the misrepresentations
of Dr. S[loane] and some others, who had given out that what he
had was inconsiderable, he delivered a copy of the estimate to
Mr. Hodgson, then lately chosen a member of the Royal Society,
with directions to deliver it to a friend, who he knew would do
him justice; and, on this fair account, obviate those unjust
reports which had been studiously spread to his prejudice. It
happened soon after, Mr. Hodgson being at a meeting, spied this
person there, at the other side of the room; and therefore gave
the paper to one that stood in some company betwixt them, to be
handed to him. But the gentleman, mistaking his request, handed
to the Secretary [Dr. Sloane], who, being a Physician, and not
acquainted with astronomical terms, did not read it readily.
Whereupon another in the company took it out of his hands;
and, having read it distinctly, desired that the works therein
mentioned might be recommended to the Prince; the charge of
printing them being too great either for the author or the Royal
Society. Sir Isaac closed in with this.'
[Illustration: THE 'CAMERA STELLATA' IN FLAMSTEED'S TIME.
(_From an engraving in the 'Historia Coelestis.'_)]
The work was in consequence recommended to Prince George of Denmark,
the Queen's Consort; but it was not till November 10, 1705, that
the contract for the printing was signed. Two years later, the
observations which he had made with his sextant in his first
thirteen years of office were printed. Then came the difficulty of
the catalogue. It was not complete to Flamsteed's satisfaction, and
he was most unwilling to let it pass out of his hands. However,
two manuscripts, comprising some three-quarters of the whole, were
deposited with referees, the first of these being sealed up. The
seal was broken with Flamsteed's concurrence; but the fact that it
had been so broken was made by him the subject of bitter complaint
later. At this critical juncture Prince George died, and a stop
was put to the progress of the printing. Two years more elapsed
without any advance being made, and then, in order to check any
further obstruction, a committee of the Royal Society was appointed
as a Board of Visitors to visit and inspect the Observatory, and so
maintain a control over the Astronomer Royal. This was naturally
felt by so sensitive a man as Flamsteed as a most intolerable wrong,
and when he found that the printing of his catalogue had been placed
in the hands of Halley as editor, a man for whom he had conceived
the most violent distrust, he absolutely refused to furnish the
Visitors with any further material. This led to, perhaps, the most
painful scene in the lives either of Newton or Flamsteed. Flamsteed
was summoned to meet the Council of the Royal Society at their rooms
in Crane Court. A quorum was not present, and so the interview was
not official, and no record of it is preserved in the archives.
Flamsteed has himself described it with great particularity in more
than one document, and it is only too easy to understand the scene
that took place. Newton was a man who had an absolutely morbid dread
of anything like controversy, and over and over again would have
preferred to have buried his choicest researches, rather than to
have encountered the smallest conflict of the kind. He was perhaps,
therefore, the worst man to deal with a high-principled, sensitive,
and obstinate man who was in the wrong, and yet who had been so
hardly dealt with that it was most natural for him to think himself
wholly in the right. Flamsteed adhered absolutely to his position,
from which it is clear it would have been extremely difficult for
the greatest tact and consideration to have dislodged him. Newton,
on his part, simply exerted his authority, and, that failing, was
reduced to the miserable extremity of calling names. The scene is
described by Flamsteed himself, in a letter to Abraham Sharp, as
follows:--
'I have had another contest with the President[2] of the
Royal Society, who had formed a plot to make my instruments
theirs; and sent for me to a Committee, where only himself and
two physicians (Dr. Sloane, and another as little skilful as
himself) were present. The President ran himself into a great
heat, and very indecent passion. I had resolved aforehand
his kn--sh talk should not move me; showed him that all the
instruments in the Observatory were my own; the mural arch and
voluble quadrant having been made at my own charge, the rest
purchased with my own money, except the sextant and two clocks,
which were given me by Sir Jonas Moore, with Mr. Towneley's
micrometer, his gift, some years before I came to Greenwich.
This nettled him; for he has got a letter from the Secretary of
State for the Royal Society to be Visitors of the Observatory,
and he said, "_as good have no observatory as no instruments_."
I complained then of my catalogue being printed by Raymer,
without my knowledge, and that I was _robbed of the fruit of my
labours_. At this he fired, and called me all the ill names,
puppy, etc., that he could think of. All I returned was, I put
him in mind of his passion, desired him to govern it, and keep
his temper: this made him rage worse, and he told me how much
I had received from the Government in thirty-six years I had
served. I asked what he had done for the £500 per annum that he
had received ever since he had settled in London. This made him
calmer; but finding him going to burst out again, I only told
him my catalogue, half finished, was delivered into his hands,
on his own request, sealed up. He could not deny it, but said
Dr. Arbuthnott had procured the Queen's order for opening it.
This, I am persuaded, was false; or it was got after it had been
opened. I said nothing to him in return; but, with a little
more spirit than I had hitherto showed, told them that God (who
was seldom spoken of with due reverence in that meeting) had
hitherto prospered all my labours, and I doubted not would do so
to a happy conclusion; took my leave and left them. Dr. Sloane
had said nothing all this while; the other Doctor told me I was
proud, and insulted the President, and ran into the same passion
with the President. At my going out, I called to Dr. Sloane,
told him he had behaved himself civilly, and thanked him for it.
I saw Raymer after, drank a dish of coffee with him, and told
him, still calmly, of the villany of his conduct, and called it
_blockish_. Since then they let me be quiet; but how long they
will do so I know not, nor am I solicitous.'
[2] Sir Isaac Newton.
The Visitors continued the printing, Halley being the editor, and
the work appeared in 1712 under the title of _Historia Coelestis_.
This seemed to Flamsteed the greatest wrong of all. The work as it
appeared seemed to him so full of errors, wilfully or accidentally
inserted, as to be the greatest blot upon his fair fame, and he
set himself, though now an old man, to work it out _de novo_ and
at his own expense. To that purpose he devoted the remaining seven
years of his life. Few things can be more pathetic than the letters
which he wrote in that period referring to it. He was subject to
the attacks of one of the cruelest of all diseases--the stone; he
was at all times liable to distracting headaches. He had been, from
his boyhood, a great sufferer from rheumatism, and yet, in spite of
all, he resolutely pushed on his self-appointed task. The following
extract from one of his letters will give a more vivid idea of the
brave old man than much description:--
'I can still, I praise God for it, walk from my door to the
Blackheath gate and back, with a little resting at some benches
I have caused to be set up betwixt them. But I found myself so
tired with getting up the hill when I return from church, that
at last I have bought a sedan, and am carried thither in state
on Sunday mornings and back; I hope I may employ it in the
afternoons, though I have not hitherto, by reason of the weather
is too cold for me.'
After the death of Queen Anne, a change in the ministry enabled
him to secure that three hundred copies of the total impression of
four hundred of the _Historia Coelestis_ were handed over to him.
These, except the first volume, containing his sextant observations
(which had received his own approval), he burned, 'as a sacrifice
to heavenly truth.' His own great work had advanced so far that the
first volume was printed, and much of the second, when he himself
died, on the last day of 1719. He was buried in the chancel of
Burstow Church.
The completion of his work took ten years more; a work of piety and
regard on the part of his assistant, Joseph Crosthwait.
When compared with the catalogues that have gone before, it was
a work of wonderful accuracy. Nevertheless, as Caroline Herschel
showed, nearly a century later, not a few errors had crept into it.
Some of the stars are non-existent, others have been catalogued in
more than one constellation; important stars have been altogether
omitted. Perhaps the most serious fault arises from the neglect of
Flamsteed to accept from Newton a practical hint, namely, to read
the barometer and thermometer at the time of his observations.
Nevertheless, the work accomplished was not only wonderful under the
untoward conditions in which Flamsteed was placed; it was wonderful
in itself, winning from Airy the following high encomium:--
'In regard not only to accuracy of observation, and to detail in
publication of the methods of observing, but also to steadiness
of system followed through many years, and to completeness of
calculation of the useful results deduced from the observations,
this work may shame any other collection of observations in this
or any other country.'
This catalogue was not Flamsteed's only achievement. He had
determined the latitude of the Observatory, the obliquity of the
ecliptic, and the position of the equinoctial points. He thought out
an original method of obtaining the absolute right ascensions of
stars by differential observations of the places of the stars and
the sun near to both equinoxes. He had revised and improved Horrox's
theory of the lunar motions, which was by far the best existing in
Flamsteed's day. He showed the existence of the long inequality of
Jupiter and Saturn; that is to say, the periodic influence which
they exercise upon each other. He determined the time in which the
sun rotates on its axis, and the position of that axis. He observed
an apparent movement of the stars in the course of a year, which he
ascribed, though erroneously, to the stellar parallax, and which was
explained by the third Astronomer Royal, Bradley.
Flamsteed not only met with harsh treatment during his lifetime; he
has not yet received, except from a few, anything like the meed of
appreciation which is his just due; but, at least, his successors in
the office have not forgotten him. They have been proud that their
official residence should be known as Flamsteed House, and his name
is inscribed over the main entrance of the latest and finest of
the Observatory buildings, and his bust looks forth from its front
towards the home where he laboured so devotedly for nearly fifty
years. But he has received little honour, save at Greenwich, and--in
spite of the proverb--in his other home, the village of Burstow, in
Surrey, of which he was for many years the rector. Here a stained
glass window representing, appropriately, the Adoration of the Magi,
has been recently set up to his memory, largely through the interest
taken in his history by an amateur astronomer of the neighbourhood,
Mr. W. Tebb, F.R.A.S.
No instrument of Flamsteed's remains in the Observatory, his wife
removing them after his death. But we may consider his principal
instrument, the mural quadrant made for him by Abraham Sharp, as
represented by the remains of a quadrant by the same artist, which
was presented to the Observatory by the Rev. N. S. Heineken, in
1865, and now hangs over the door of the transit room.
CHAPTER III
HALLEY AND HIS SUCCESSORS
There is no need to give the lives of the succeeding Astronomers
Royal so fully as that of Flamsteed. Not that they were inferior
men to him; on the contrary, there can be little doubt that
we ought to reckon some of them as his superiors, but, in the
case of several, their best work was done apart from Greenwich
Observatory, and before they came to it.
This was particularly the case with EDMUND HALLEY. Born on October
29, 1656, he was ten years the junior of Flamsteed. Like Flamsteed,
he came of a Derbyshire family, though he was born at Haggerston,
in the parish of St. Leonard's, Shoreditch. He was educated at St.
Paul's School, where he made very rapid progress, and already showed
the bent of his mind. He learnt to make dials; he made himself so
thoroughly acquainted with the heavens that it is said, 'If a star
were displaced in the globe he would presently find it out,' and
he observed the changes in the direction of the mariner's compass.
In 1673 he went to Queen's College, Oxford, where he observed a
sunspot in July and August, 1676, and an occultation of Mars. This
was not his first astronomical observation, as, in June, 1675,
he had observed an eclipse of the moon from his father's house in
Winchester Street.
[Illustration: EDMUND HALLEY.
(_From an old print._)]
A much wider scheme of work than such merely casual observations
now entered his mind, possibly suggested to him by Flamsteed's
appointment to the direction of the new Royal Observatory. This was
to make a catalogue of the southern stars. Tycho's places for the
northern stars were defective enough, but there was no catalogue at
all of stars below the horizon of Tycho's observatory. Here, then,
was a field entirely unworked, and young Halley was so eager to
enter upon it that he would not wait at Oxford to obtain his degree,
but was anxious to start at once for the southern hemisphere.
His father, who was wealthy and proud of his gifted son, strongly
supported him in his project. The station he selected was St.
Helena, an unfortunate choice, as the skies there were almost
always more or less clouded, and rain was frequent during his stay.
However, he remained there a year and a half, and succeeded in
making a catalogue of 341 stars. This catalogue was finally reduced
by Sharp, and included in the third volume of Flamsteed's _Historia
Coelestis_.
In 1678 he was elected Fellow of the Royal Society, and the
following year he was chosen to represent that society in a
discussion with Hevelius. The question at issue was as to whether
more accurate observations of the place of a star could be obtained
by the use of sights without optical assistance, or by the use of
a telescope. The next year he visited the Paris Observatory, and,
later in the same tour, the principal cities of the Continent.
Not long after his return from this tour, Halley was led to that
undertaking for which we owe him the greatest debt of gratitude, and
which must be regarded as his greatest achievement.
Some fifty years before, the great Kepler had brought out the third
of his well-known laws of planetary motion. These laws stated
that the planets move round the sun in ellipses, of which the sun
occupies one of the foci; that the straight line joining any planet
with the sun moves over equal areas of space in equal periods of
time; and, lastly, that the squares of the times in which the
several planets complete a revolution round the sun are proportional
to the cubes of their mean distances from it. These three laws were
deduced from actual examination of the movements of the planets.
Kepler did not work out any underlying cause of which these three
laws were the consequence.
But the desire to find such an underlying cause was keen amongst
astronomers, and had given rise to many researches. Amongst those
at work on the subject was Halley himself. He had seen, and been
able to prove, that if the planets moved in circles round the sun,
with the sun in the centre, then the law of the relation between the
times of revolution and the distances of the planets would show that
the attractive force of the sun varied inversely as the square of
the distance. The actual case, however, of motion in an ellipse was
too hard for him, and he could not deal with it. Halley therefore
went up to Cambridge to consult Newton, and, to his wonder and
delight, found that the latter had already completely solved the
problem, and had proved that Kepler's three laws of planetary motion
were summed up in one, namely, that the sun attracted the planets to
it with a force inversely proportional to the square of the distance.
Halley was most enthusiastic over this great discovery, and he at
once strongly urged Newton to publish it. Newton's unwillingness to
do so was great, but at length Halley overcame his reluctance; and
the Royal Society not being able at the time to afford the expense,
Halley took the charges upon himself, although his own resources had
been recently seriously damaged by the death of his father.
The publication of Newton's _Principia_, which, but for him, might
never have seen the light, and most certainly would have been long
delayed, is Halley's highest claim to our gratitude. But, apart
from this, his record of scientific achievement is indeed a noble
one. Always, from boyhood, he had taken a great interest in the
behaviour of the magnetic compass, and he now followed up the study
of its variations with the greatest energy. For this purpose it was
necessary that he should travel, in view of the great importance of
the subject to navigation. King William III. gave him a captain's
commission in the Royal Navy--a curious and interesting illustration
of the close connection between astronomy and the welfare of our
navy--and placed him in command of a 'pink,' that is to say, a small
vessel with pointed stern, named the Paramour, in which he proceeded
to the southern ocean. His first voyage was unfortunate, but the
Paramour was recommissioned in 1699, and he sailed in it as far as
south latitude 52°.
In 1701 and the succeeding year he made further voyages in the
Paramour, surveying the tides and coasts of the British Channel
and of the Adriatic, and helping in the fortification of Trieste.
He became Savilian Professor of Geometry at Oxford in 1703, having
failed twelve years previously to secure the Savilian Professorship
of Astronomy, mainly through the opposition of Flamsteed, who had
already formed a strong prejudice against him, which some writers
have traced to Halley's detection of several errors in one of
Flamsteed's tide-tables, others to Halley's supposed materialistic
views. Probably the difference was innate in the two men. There was
likely to be but little sympathy between the strong, masterful man
of action and society and the secluded, self-conscious, suffering
invalid. At any rate, in the contest between Newton and Flamsteed,
which has been already described, Halley took warmly the side of the
former, and was appointed to edit the publication of Flamsteed's
results, and, on the death of the latter, to succeed him at the
Royal Observatory.
The condition of things at Greenwich when Halley succeeded to
the post of Astronomer Royal in 1720 was most discouraging. The
instruments there had all belonged to Flamsteed, and therefore,
most naturally, had been removed by his widow. The Observatory
had practically to be begun _de novo_, and Halley had now almost
attained the age at which in the present day an Astronomer
Royal would have to retire. More fortunate, however, than his
predecessor, he was able to get a grant for instruments, and he
equipped the Observatory as well as the resources of the time
permitted, and his transit instrument and great eight-foot quadrant
still hang upon the Observatory walls.
As Astronomer Royal his great work was the systematic observation
of the positions of the moon through an entire _saros_. As is well
known, a period of eighteen years and ten or eleven days brings the
sun and moon very nearly into the same positions relatively to the
earth which they occupied at the commencement of the period. This
period was well known to the ancient Chaldeans, who gave it its
name, since they had noticed that eclipses of the sun or eclipses of
the moon recurred at intervals of the above length. It was Halley's
desire to obtain such a set of observations of the moon through an
entire _saros_ period as to be able to deduce therefrom an improved
set of tables of the moon's motion. It was an ambitious scheme for
a man so much over sixty to undertake, nevertheless he carried it
through successfully.
His desire to complete this scheme, and to found upon it improved
lunar tables, hindered him from publishing his observations, for
he feared that others might make use of them before he was in a
position to complete his work himself. This omission to publish
troubled Newton, who, as President of the Royal Society--the
Greenwich Board of Visitors having lapsed at Queen Anne's
death--drew attention at a meeting of the Royal Society, March 2,
1727, to Halley's disobedience of the order issued under Queen
Anne, for the prompt communication of the Observatory results. That
Newton should thus have put public pressure upon Halley, the man to
whom he was so much indebted, and with whom there was so close an
affection, is sufficient proof that his similar attitude towards
Flamsteed was one of principle and not of arbitrariness. Halley, on
his side, stood firm, as Flamsteed had done, urging the danger that,
by publishing before he had completed his task, he might give an
opportunity to others to forestall his results. It is said--probably
without sufficient ground--that this refusal broke Newton's heart
and caused his death. Certainly Halley's writings in that very year
show his reverence and affection for Newton to have been as keen and
lively as ever.
Halley's work at the Observatory went on smoothly, on the lines he
had laid down for himself, for ten years after Newton's death; but
in 1737 he had a stroke of paralysis, and his health, which had
been remarkably robust up to that time, began to give way. He died
January 14, 1742, and was buried in the cemetery of Lee Church.
As an astronomer, his services to the science rank higher than those
of his predecessor; but as Astronomer Royal, as director, that is to
say, of Greenwich Observatory, he by no means accomplished as much
as Flamsteed had done. Professor Grant, in his _History of Physical
Astronomy_, says that he seems to have undervalued those habits of
minute attention which are indispensable to the attainment of a high
degree of excellence in the practice of astronomical observation. He
was far from being sufficiently careful as to the adjustment of his
instruments, the going of his clocks, or the recording of his own
observations. The important feature of his administration was that
under him the Observatory was first supplied with instruments which
belonged to it.
[Illustration: HALLEY'S QUADRANT.
(_From an old print._)]
His astronomical work apart from the Observatory was of the first
importance. He practically inaugurated the study of terrestrial
magnetism, and his map giving the results of his observations during
his voyage in the Paramour introduced a new and most useful style
of recording observations. He joined together by smooth curves
places of equal variation, the result being that the chart shows at
a glance, not merely the general course of the variation over the
earth's surface, but its value at any spot within the limits of the
chart.
Another work which has justly made his name immortal was the
prediction of the return of the comet which is called by his name,
to which reference will be made later. Another great scheme, and
one destined to bear much fruit, was the working out of a plan to
determine the distance of the sun by observations of the transit of
Venus.
Of attractive appearance, pleasing manners, and ready wit, loyal,
generous, and free from self-seeking, he probably was one of the
most personally engaging men who ever held the office.
The salary of the Astronomer Royal remained under Halley at the same
inadequate rate which it had done under Flamsteed--£100, without
provision for an assistant. But in 1729 Queen Caroline, learning
that Halley had actually had a captain's commission in the Royal
Navy, secured for him a post-captain's pay.
[Illustration: JAMES BRADLEY.
(_From the painting by Hudson._)]
Halley's work is represented at the Observatory by two of his
instruments which are still preserved there, and which hang on the
west wall of the present transit room: the Iron Quadrant afterwards
made famous by the observations of Bradley, and 'Halley's Transit,'
the first of the great series of instruments upon which the fame of
Greenwich chiefly rests. This transit instrument seems to have been
set up in a small room at the west end of what is now known as the
North Terrace. His quadrant was mounted on the pier which is now the
base of the pier of the astrographic telescope. This pier was the
first extension which the Observatory received from the original
building.
On the breakdown of his health Halley nominated as his successor,
James Bradley; indeed, it is stated that he offered to resign
in his favour. He had known him then for over twenty years, and
that keen and generous appreciation of merit in others which was
characteristic of Halley had led him very early to recognize
Bradley's singular ability.
* * * * *
JAMES BRADLEY was born in 1692 or 1693, of an old North of England
family. His birthplace was Sherbourne, in Gloucestershire, and he
was educated at North Leach Grammar School and at Baliol College,
Oxford. During the years of his undergraduateship he resided much
with his uncle, the Rev. James Pound, Rector of Wanstead, Essex, an
ardent amateur astronomer, a frequent visitor at the Observatory
in Flamsteed's time, and one of the most accurate observers in the
country. From him, no doubt, he derived his love of the science,
and possibly some of his skill in observation.
Bradley's earliest observations seem to have been devoted to the
phenomena of Jupiter's satellites and to the measures of double
stars. The accuracy with which he followed up the first drew the
attention of Halley, and so began a friendship which lasted through
life. His observations of double stars, particularly of Castor, only
just failed to show him the orbital movement of the pair, because
his attention was drawn to other subjects before it had become
sufficiently obvious.
In 1719 Bradley and his uncle made an attempt to determine the
distance of the sun through observations of Mars when in opposition,
observations which were so accurate that they sufficed to show that
the distance of the sun could not be greater than 125 millions
of miles, nor less than about 94 millions. The lower limit which
they thus found has proved to be almost exactly correct, our best
modern determinations giving it as 93 millions. The instrument with
which the observations were made was a novel one, being 'moved by a
machine that made it to keep pace with the stars;' in other words,
it was the first, or nearly the first, example of what we should now
call a clock-driven equatorial.
That same year he was offered the Vicarage of Bridstow, near Ross,
in Monmouthshire, where, having by that time taken priest's orders,
he was duly installed, July, 1720. To this was added the sinecure
Rectory of Llandewi-Velgry; but he held both livings only a very
short time. In 1721 the death of Dr. John Keill rendered vacant the
Savilian Professorship of Astronomy at Oxford, for which Bradley
became a candidate, and was duly elected, and resigned his livings
in consequence.
It was whilst he was Savilian Professor that Bradley made that
great discovery which will always be associated with his name.
Though professor at Oxford, he had continued to assist his uncle,
Mr. Pound, at his observations at Wanstead, and after the death of
the latter he still lived there as much as possible, and continued
his astronomical work. But in 1725 he was invited by Mr. Samuel
Molyneux, who had set up a twenty-four-foot telescope made by
Graham as a zenith tube at his house on Kew Green, to verify some
observations which he was making. These were of the star Gamma
Draconis, a star which passes through the zenith of London, and
which, therefore, had been much observed both by Flamsteed and
Hooke, inasmuch as by fixing a telescope in an absolutely vertical
position--a position which could be easily verified--it was easy to
ascertain if there was any minute change in the apparent position
of the star. Dr. Hooke had declared that there was such a change,
a change due to the motion of the earth in its orbit, which would
prove that the star was not an infinite distance from the earth, the
seeming change of its place in the sky corresponding to the change
in the place of the earth from which the observer was viewing it.
Bradley found at once that there was such a change--a marked one. It
amounted to as much as 1´´ of arc in three days; but it was not in
the direction in which the parallax of the star would have moved
it, but in the opposite. Whether, therefore, the star was near
enough to show any parallax or not, some other cause was giving rise
to an apparent displacement of the star, which entirely masked and
overcame the effect of parallax.
So far, Bradley had but come to the same point which Flamsteed
had reached. Flamsteed had detected precisely the same apparent
displacement of stars, and, like Hooke, had ascribed it to
parallax. Cassini had shown that this could not be the case, as
the displacement was in the wrong direction; and there the matter
had rested. Bradley now set to follow the question up. Other stars
beside Gamma Draconis were found to show a displacement of the same
general character, but the amount varied with their distance from
the plane of the ecliptic, the earth's orbit. The first explanation
suggested was that the axis of the earth, which moves very nearly
parallel to itself as the earth moves round the sun, underwent a
slight regular 'wobble' in the course of a year. To check this,
a star was observed on the opposite side of the pole from Gamma
Draconis; then Bradley investigated as to whether refraction might
explain the difficulty, but again without success. He now was
most keenly interested in the problem, and he purchased a zenith
telescope of his own, made, like that of Molyneux, by Graham,
and mounted it in his aunt's house at Wanstead, and observed
continuously with it. The solution of the problem came at last to
him as he was boating on the Thames. Watching a vane at the top of
the mast, he saw with surprise that it shifted its direction every
time that the boat was put about. Remarking to the boatmen that it
was very odd that the wind should change just at the same moment
that there was a shift in the boat's course, they replied that there
was no change in the wind at all, and that the apparent change of
the vane was simply due to the change of direction of the motion of
the boat.
[Illustration: GRAHAM'S ZENITH SECTOR.
(_From an old print._)]
This supplied Bradley with a key to the solution of the mystery
that had troubled him so long. It had been discovered long before
this that light does not travel instantaneously from place to
place, but takes an appreciable time to pass from one member of
the solar system to another. This had been discovered by Römer
from observations of the satellites of Jupiter. He had noted that
the eclipses of the satellites always fell late of the computed
time, when Jupiter was at his greatest distance from the earth;
and Bradley's own work in the observation of those satellites had
brought the fact most intimately under his own acquaintance. The
result of the boating incident taught him, then, that he might look
upon light as analogous to the wind blowing on the boat. As the
wind, so long as it was steady, would seem to blow from one fixed
quarter so long as the boat was also in rest, but as it seemed
to shift its direction when the boat was moving and changed its
direction, so he saw that the light coming from a particular star
must seem to slightly change the direction in which it came, or, in
other words, the apparent position of the star, to correspond with
the movement of the earth in its orbit round the sun.
This was the celebrated discovery of the Aberration of Light,
a triumph of exact observation and of clear insight. As to the
exactness of Bradley's observations, it is sufficient to say that
his determination of the value of the 'Constant of Aberration' gave
it as 20·39´´; the value adopted to-day is 20·47´´.
On the death of Halley, in 1742, Bradley was appointed to succeed
him. He found the Observatory in as utterly disheartening a
condition as his predecessors had done. As already mentioned, Halley
had not the same qualifications as an observer that Flamsteed
had. He was, further, an old man when appointed to the post, he
had no assistant provided for him, and the last five years of his
life his health and strength had entirely given way. Under these
circumstances, it was no wonder that Bradley found the instruments
of the Observatory in a deplorable state. Nevertheless, he set to
work most energetically, and in the year of his appointment he
made 1500 observations in the last five months of the year. He was
particularly earnest in examining the condition and the errors of
his instruments; and as their defects became known to him, he was
more and more anxious for a better equipment. He moved the Royal
Society, therefore, to apply on his behalf for the instruments he
required; and a petition from that body, in 1748, obtained what
in those days must be considered the generous grant of £1000,
the proceeds of the sale of old Admiralty stores. The principal
instruments purchased therewith were a mural quadrant and a transit
instrument, both eight feet in focal length, still preserved on the
walls of the transit-room. It is interesting also to note that,
following in the steps of Halley, and forecasting, as it were, the
magnetic observatory which Airy would found, he devoted £20 of the
grant to purchasing magnetic instruments.
Meantime he had continued his observations on aberration, and had
discovered that the aberration theory was not sufficient entirely
to account for the apparent changes in places of stars which he had
discovered. A second cause was at work, a movement of the earth's
axis, a 'wobble' in its inclination, technically known as Nutation,
which is due to the action of the moon, and goes through its course
in a period of nineteen years.
Beside these two great discoveries of aberration and nutation,
Bradley's reputation rests upon his magnificent observations of the
places of more than three thousand stars. This part of his work was
done with such thoroughness, that the star-places deduced from them
form the basis of most of our knowledge as to the actual movements
of individual stars. In particular, he was careful to investigate
and to correct for the errors of his instrument, and to determine
the laws of refraction, introducing corrections for changes in the
readings of thermometer and barometer. His tables of refraction
were used, indeed, for seventy years after his death. Of his other
labours it may be sufficient to refer to his determination of the
longitudes of Lisbon and of New York, and to his effort to ascertain
the parallax of the sun and moon, in combination with La Caille, who
was observing at the Cape of Good Hope.
As Astronomer Royal, Bradley's great achievement was the high
standard to which he raised the practical work of observation. From
his day onwards, also, there was always at least one assistant.
His first assistant was his own nephew, John Bradley, who received
the munificent salary of ten shillings a week. Still, this was
not out of proportion to the then salary of the Astronomer Royal,
which practically amounted only to £90. However, in 1752, Bradley
was awarded a Crown pension of £250 a year. He refused the living
of Greenwich, which was offered him in order to increase his
emoluments, on the ground that he could not suitably fulfil the
double office. Bradley's later assistants were Charles Mason and
Charles Green.
Bradley's last work was the preparation for the observations of the
transit of Venus of 1761, according to the lines laid down by his
predecessor, Halley. His health gave way, and he became subject to
melancholia, so that the actual observations were taken by the Rev.
Nathaniel Bliss, who succeeded him in his office after his death, in
1762. He was buried at Minchinhampton.
So far as we know Bradley's character, he seems to have been a
gentle, modest, unassuming man, entirely free from self-seeking,
and indifferent to personal gain. He was in many ways an ideal
astronomer, exact, methodical, and conscientious to the last degree.
His skill as an observer was his chief characteristic; and though
his abilities were not equal as a mathematician or a mechanician,
yet, on the one hand, he had a very clear insight into the meaning
of his observations, and, on the other, he was skilful enough to
himself adjust, repair, and improve his instruments.
Of Bradley's instruments, there are still preserved his famous
twelve-and-a-half-foot zenith sector, with which he made his two
great discoveries; his brass quadrant, which in 1750 he substituted
for Halley's iron quadrant; his transit instrument, and equatorial
sector. Bradley added to the buildings of the Observatory that
portion which is now represented by the upper and lower computing
rooms, and the chronometer room, which adjoins the latter. This
room--the chronometer room--was his transit room, and the position
of the shutters is still marked by the window in the roof.
* * * * *
The Rev. NATHANIEL BLISS, who succeeded Bradley, only held the
office for a couple of years, and during that time was much at
Oxford. He, therefore, has left no special mark behind him as
Astronomer Royal.
He was born November 28, 1700. His father, like himself, Nathaniel
Bliss, was a gentleman, of Bisley, Gloucestershire.
[Illustration: NATHANIEL BLISS.
(_From an engraving on an old pewter flagon._)]
Bliss graduated at Pembroke College, Oxford, as B.A. in 1720,
and M.A. in 1723. He became the Rector of St. Ebb's, Oxford, in
1736, and on Halley's death succeeded him as Savilian Professor of
Geometry. He supplied Bradley with his observations of Jupiter's
satellites, and from time to time, at his request, rendered him some
assistance at the Royal Observatory. This was particularly the case,
as has been already mentioned, with respect to the transit of
Venus of 1761, the observations of which were carried out by Bliss,
owing to Bradley's ill-health. It was natural, therefore, that on
Bradley's death he should succeed to the vacant post; but he held
it too short a time to do any distinctive work. Such observations
as he made seem to have been entirely in continuation of Bradley's.
He took a great interest, however, in the improvement of clocks, a
department in which so much was being done at this time by Graham,
Ellicott, and others.
* * * * *
NEVIL MASKELYNE, the fifth Astronomer Royal, was, like Bliss, a
close friend of Bradley's. He was the third son of a wealthy country
gentleman, Edmund Maskelyne, of Purton, in Wiltshire. Maskelyne was
born in London, October 6, 1732, and was educated at Westminster
School. Thence he proceeded to Cambridge, where he graduated seventh
Wrangler in 1754. He was ordained to the curacy of Barnet in 1755,
and, twenty years later, was presented by his nephew, Lord Clive, to
the living of Shrawardine, in Shropshire. In 1782 he was presented
by his college to the Rectory of North Runcton, Norfolk.
The event which turned his thoughts in the direction of astronomy
was the solar eclipse of July 25, 1748; and about the time that he
was appointed to the curacy of Barnet he became acquainted with
Bradley, then the Astronomer Royal, to whom he gave great assistance
in the preparation of his table of refractions.
Like Halley before him, he made an astronomical expedition to the
island of St. Helena. This was for the special purpose of observing
the transit of Venus of June 6, 1761, Bradley having induced the
Royal Society to send him out for that purpose. Here he stayed
ten months, and made many observations. But though the transit
of Venus was his special object, it was not the chief result of
the expedition: not because clouds hindered his observations, but
because the voyage gave him the especial bent of his life.
Halley had actually held a captain's commission in the Royal Navy,
and commanded a ship; Maskelyne, more than any of the Astronomers
Royal before or since, made the improvement of the practical
business of navigation his chief aim. None of all the incumbents of
the office kept its original charter--'To find the so much desired
Longitude at Sea, for the perfecting the Art of Navigation,' so
closely before him.
The solution of the problem was at hand at this time--its solution
in two different ways. On the one hand, the offer by the Government
of a reward of £20,000 for a clock or watch which should go so
perfectly at sea, notwithstanding the tossing of the ship and the
wide changes of temperature to which it might be exposed, that the
navigator might at any moment learn the true Greenwich time from
it, had brought out the invention of Harrison's time-keeper; on the
other hand, the great improvement that had now taken place in the
computation of tables of the moon's motion, and the more accurate
star-catalogues now procurable, had made the method of 'lunars,'
suggested a hundred and thirty years before by the Frenchman, Morin,
and others, a practicable one.
[Illustration: NEVIL MASKELYNE.]
In principle, the method of finding the longitude from 'lunars,'
that is to say, from measurements of the distances between the moon
and certain stars, is an exceedingly simple one. In actual practice,
it involves a very toilsome calculation, beside exact and careful
observation. The principle, as already mentioned, is simply this:
The moon travels round the sky, making a complete circuit of the
heavens in between twenty-seven and twenty-eight days. It thus
moves amongst the stars, roughly speaking, its own diameter, in
about an hour. When once its movements were sufficiently well known
to be exactly predicted, almanacs could be drawn up in which the
Greenwich time of its reaching any definite point of the sky could
be predicted long beforehand; or, what comes to the same thing,
its distances from a number of suitable stars could be given for
definite intervals of Greenwich time. It is only necessary, then, to
measure the distances between the moon and some of these stars, and
by comparing them with the distances given in the almanac, the exact
time at Greenwich can be inferred. As has been already pointed out,
the determination of the latitude of the ship and of the local time
at any place where the ship is, is not by any means so difficult
a matter; but the local time being known and the Greenwich time,
the difference between these gives the longitude; and the latitude
having been also ascertained, the exact position of the ship is
known.
There are, of course, difficulties in the way of working out this
method. One is, that whilst it takes the sun but twenty-four hours
to move round the sky from one noon to the next, and consequently
its movements, from which the local time is inferred, are fairly
rapid, the moon takes nearly twenty-eight days to move amongst the
stars from the neighbourhood of one particular star round to that
particular star again. Consequently, it is much easier to determine
the local time with a given degree of exactness than the Greenwich
time; it is something like the difference of reading a clock from
both hands and from the hour hand alone.
There are other difficulties in the case which make the computation
a long and laborious one, and difficult in that sense; but they do
not otherwise affect its practicability.
During this voyage to St. Helena, both when outward bound and when
returning, Maskelyne gave the method of 'lunars' a very thorough
testing, and convinced himself that it was capable of giving the
information required. For by this time the improvement of the
sextant, or quadrant as it then was, by the introduction of a second
mirror, by Hadley, had rendered the actual observation at sea of
lunar distances, and of altitudes generally, a much more exact
operation.
This conclusion he put at once to practical effect, and, in 1763,
he published the _British Mariner's Guide_, a handbook for the
determination of the longitude at sea by the method of lunars.
At the same time, the other method, that by the time-keeper or
chronometer, was practically tested by him. The time-keeper
constructed by John Harrison had been tested by a voyage to Jamaica
in 1761, and now, in 1763, another time-keeper was tested in a
voyage to Barbadoes. Charles Green, the assistant at Greenwich
Observatory, was sent in charge of the chronometer, and Maskelyne
went with him to test its performance, in the capacity of chaplain
to his Majesty's ship Louisa.
[Illustration: HADLEY'S QUADRANT.
(_From an old print._)]
The position which Maskelyne had already won for himself as a
practical astronomer, and the intimate relations into which he
had entered with Bradley and Bliss, made his appointment to the
Astronomer Royalship, on the death of the latter, most suitable.
At once he bent his mind to the completion of the revolution
in nautical astronomy which his _British Mariner's Guide_ had
inaugurated, and in the year after his appointment he published
the first number of the _Nautical Almanac_, together with a volume
entitled, _Tables Requisite to be Used with the Nautical Ephemeris_,
the value of which was so instantly appreciated, that 10,000 copies
were sold at once.
The _Nautical Almanac_ was Maskelyne's greatest work, and it must
be remembered that he carried it on from this time up to the day of
his death--truly a formidable addition to the routine labours of an
Astronomer Royal who had but a single assistant on his staff. The
_Nautical Almanac_ was, however, in the main not computed at the
Observatory; the calculations were effected by computers living in
different parts of the country, the work being done in duplicate, on
the principle which Flamsteed had inaugurated in the preparation of
his _Historia Coelestis_.
Maskelyne's next service to science was almost as important.
He arranged that the regular and systematic publication of the
observations made at Greenwich should be a distinct part of the
duties of an Astronomer Royal, and he procured an arrangement
by which a special fund was set apart by the Royal Society for
printing them. His observations covering the years 1776 to 1811
fill four large folio volumes, and though, as already stated, he
had but one assistant, they are 90,000 in number. Thus it was
Maskelyne who first rendered effective the design which Charles
II. had in the establishment of the Observatory. Flamsteed and
Halley had been too jealous of their own observations to publish;
Bradley's observations--though he himself was entirely free from
this jealousy--were made, after his death, the subject of litigation
by his heirs and representatives, who claimed an absolute property
in them, a claim which the Government finally allowed. None of the
three, however much their work ultimately tended to the improvement
of the art of navigation, made that their first object. Whereas
Maskelyne set this most eminently practical object in the forefront,
and so gave to the Royal Observatory, which under his predecessors
somewhat resembled a private observatory, its distinctive
characteristics of a public institution.
It fell to Maskelyne to have to advise the Government as to the
assignment of their great reward of £20,000 for the discovery of
the longitude at sea. Maskelyne, while reporting favourably of the
behaviour of Harrison's time-keeper, considered that the method
of 'lunars' was far too important to be ignored, and he therefore
recommended that half the sum should be given to Harrison for his
watch, whilst the other half was awarded for the lunar tables
which Mayer, before his death, had sent to the Board of Longitude.
This decision, though there can be no doubt it was the right one,
led to much dissatisfaction on the part of Harrison, who urged
his claim for the whole grant very vigorously; and eventually
the whole £20,000 was paid him. The whole question of rewards to
chronometer-makers must have been one which caused Maskelyne much
vexation. He was made the subject of a bitter and most voluminous
attack by Thomas Mudge, for having preferred the work of Arnold and
Earnshaw to his own.
Otherwise his reign at the Observatory seems to have been a
singularly peaceful one, and there is little to record about it
beyond the patient prosecution, year by year, of an immense amount
of sober, practical work. To Maskelyne, however, we owe the practice
of taking a transit of a star over five wires instead of over one,
and he provided the transit instrument with a sliding eye-piece, to
get over the difficulty of the displacement which might ensue if the
star were observed askew when out of the centre of the field. To
Maskelyne, too, we owe in a pre-eminent degree the orderly form of
recording, reducing, and printing the observations. Much of the work
in this direction which is generally ascribed to Airy was really
due to Maskelyne. Indeed, without a wonderful gift of organization,
it would have been impossible to plan and to carry the _Nautical
Almanac_.
Beside the editing of various works intended for use in nautical
astronomy or in general computation, the chief events of his long
reign at Greenwich were the transit of Venus in 1769, which he
himself observed, and for which he issued instructions in the
_Nautical Almanac_; and his expedition in 1774 to Scotland, where he
measured the deviation of the plumb-line from the vertical caused by
the attraction of the mountain Schiehallion, deducing therefrom the
mean density of the earth to be four and a half times that of water.
[Illustration: JOHN POND.
(_From an old engraving._)]
He died at the Observatory, February 9, 1811, aged 79, leaving but
one child, a daughter, who married Mr. Anthony Mervin Story,
to whom she brought the family estates in Wiltshire, inherited by
Maskelyne on the deaths of his elder brothers, and, in consequence,
Mr. Story added the name of Maskelyne to his own.
Maskelyne's character and policy as Astronomer Royal have been
sufficiently dwelt upon. His private character was mild, amiable,
and generous. 'Every astronomer, every man of learning, found in him
a brother;' and, in particular, when the French Revolution drove
some French astronomers to this country to find a refuge, they
received from the Astronomer Royal the kindest reception and most
delicate assistance.
Maskelyne added no instrument to the Observatory during his reign,
though he improved Bradley's transit materially. He designed the
mural circle, but it was not completed until after his death. His
additions to the Observatory buildings consisted of three new rooms
in the Astronomer Royal's house, and the present transit circle room.
* * * * *
JOHN POND was recommended by Maskelyne as his successor at
Greenwich. At the time of his succession he was forty-four years of
age, having been born in 1767. He was educated at Trinity College,
Cambridge, and then spent some considerable time travelling in
the south of Europe and Egypt. On his return home he settled at
Westbury, where he erected an altazimuth by Troughton, with a
two-and-a-half-foot circle. A born observer, his observations of the
declinations of some of the principal fixed stars showed that the
instrument which Maskelyne was using at Greenwich--the quadrant by
Bird--could no longer be trusted. Maskelyne, in consequence, ordered
a six-foot mural circle from Troughton, but did not live to see it
installed, and in 1816 this was supplemented by Troughton's transit
instrument of five inches aperture and ten feet focal length.
The introduction of these two important instruments, and of other
new instruments, together with new methods of observation, form one
of the chief characteristics of Pond's administration. Under this
head must be specially mentioned the introduction of the mercury
trough, both for determining the position of the vertical, and for
obtaining a check upon the flexure of the mural circle in different
positions; and the use in combination of a pair of mural circles for
determining the declinations of stars.
Another characteristic of his reign was that under him there was the
first attempt to give the Astronomer Royal a salary somewhat higher
than that of a mechanic, and to support him with an adequate staff
of assistants. His salary was fixed at £600 a year, and the single
assistant of Maskelyne was increased to six.
This multiplication of assistants was for the purpose of multiplying
observations, for Pond was the first astronomer to recognize the
importance of greatly increasing the number of all observations upon
which the fundamental data of astronomy were to be based.
In 1833 he finished his standard catalogue of 1113 stars, at that
time the fullest of any catalogue prepared on the same scale of
accuracy. 'It is not too much to say,' was the verdict of the Royal
Astronomical Society, 'that meridian sidereal observation owes more
to him than to all his countrymen put together since the time of
Bradley.'
A yet higher testimony to the exactness of his work is given by his
successor, Airy.
'The points upon which, in my opinion, Mr. Pond's claims to
the gratitude of astronomers are founded, are principally the
following. _First_ and chief, the accuracy which he introduced
into all the principal observations. This is a thing which,
from its nature, it is extremely difficult to estimate now, so
long after the change has been made; and I can only say that,
so far as I can ascertain from books, the change is one of very
great extent; for certainty and accuracy, astronomy is quite a
different thing from what it was, and this is mainly due to Mr.
Pond.'
The same authority eulogizes him further for his laborious working
out of every conceivable cause or indication of error in his
declination instruments, for the system which he introduced in
the observation of transits, for the thoroughness with which he
determined all his fundamental data, and for the regularity which he
infused into the Greenwich observations.
One result of this great increase of accuracy was that Pond was able
at once authoritatively to discard the erroneous stellar parallaxes
that had been announced by Brinkley, Royal Astronomer for Ireland.
But Pond's administration was open, in several particulars, to
serious censure, and the Board of Visitors, which had been for
many years but a committee of the Royal Society, but which had
recently been reconstituted, proved its value and efficiency by
the remonstrances which it addressed to him, and which eventually
brought about his resignation. His personal skill and insight as an
observer were of the highest order; but either from lack of interest
or failing health, he absented himself almost entirely from the
Observatory in later years, visiting it only every ninth or tenth
day. He had caused the staff of assistants to be increased from one
to six, but had stipulated that the men supplied to him should be
'drudges.' His minute on the subject ran--
'I want indefatigable, hard-working, and, above all, obedient
drudges (for so I must call them, although they are drudges
of a superior order), men who will be contented to pass half
their day in using their hands and eyes in the mechanical act
of observing, and the remainder of it in the dull process of
calculation.'
This was a fatal mistake, and one which it is very hard to
understand how any one with a real interest in the science could
have made. Men who had the spirit of 'drudges,' to whom observation
was a mere 'mechanical act,' and calculation a 'dull process,' were
not likely to maintain the honour of the Observatory, particularly
under an absentee Astronomer Royal. Pond tried to overcome the
difficulty by devising rules for their guidance of iron rigidity.
The result was that after his resignation, in 1835, the First Lord
and the Secretary of the Admiralty expressed their feeling to Airy,
Pond's successor, 'that the Observatory had fallen into such a state
of disrepute that the whole establishment should be cleared out.' A
further evil was the excessive development of chronometer business,
so as practically to swamp the real work of the Observatory, whilst
the prices paid for the chronometers at this time were often much
larger than would have been the case under a more business-like
administration.
With all his merits, therefore, as an observer, the administration
of Pond was, in some respects, the least satisfactory of all that
the Observatory has known, and he alone of all the Astronomers Royal
retired under pressure. He did not long survive his resignation,
dying in September, 1836. He was buried by the side of Halley, in
the churchyard at Lee.
Of Pond's instruments, the Observatory retains the fine transit
instrument which was constructed by Troughton at his direction, and
the mural circle, designed by Maskelyne, but which Pond was the
first to use. Both of these have, of course, long been obsolete, and
now hang on the walls of the transit room. The small equatorial,
called, after its donor, the Shuckburgh equatorial, was also added
in Pond's day, and though practically never used, still remains
mounted in its special dome.
CHAPTER IV
AIRY
One hundred and sixty years from the day when Flamsteed laid the
foundation stone of the Observatory, the Royal Warrant under the
sign manual was issued, appointing the seventh and strongest of
the Astronomers Royal, August 11, 1835. He actually entered on
his office in the following October, but did not remove to the
Observatory until the end of the year.
GEORGE BIDDELL AIRY was born at Alnwick, in Northumberland, on
July 27, 1801. His father was William Airy, of Luddington, in
Lincolnshire, a collector of excise; his mother was the daughter
of George Biddell, a well-to-do farmer, of Playford, near Ipswich.
He was educated at the Grammar School, Colchester, and so
distinguished himself there that although his father was at this
time very straitened in his circumstances, it was resolved that
young Airy should go to Cambridge. Here he was entered as sizar
at Trinity College, and his robust, self-reliant character was
seen in the promptness with which he rendered himself independent
of all pecuniary help from his relatives. In 1823 he graduated as
Bachelor of Arts, being Senior Wrangler and Smith's prizeman,
entirely distancing all other men of his year. He had already begun
to pay attention to astronomy, at first from the side of optics,
to the study of which he had been very early attracted; a paper of
his on the achromatism of eye-pieces and microscopes, written in
1824, being one of especial value. In 1826 he attempted to determine
'the diminution of gravity in a deep mine'--that of Dolcoath, in
Cornwall. In the winter of 1823-24 he was invited to London by Mr.
(afterwards Sir) James South, who took him, amongst other places,
to Greenwich Observatory, and gave him his first introduction to
practical astronomy. In 1826 he was appointed Lucasian Professor at
Cambridge, and in 1828, Plumian Professor, with the charge of the
new University Observatory. Prior to his election he had definitely
told the electors that the salary proposed was not sufficient for
him to undertake the responsibility of the Observatory. He followed
this up by a formal application for an increase, which created not a
little commotion at the time, the action being so unprecedented; and
after a delay of a little over a year he obtained what he had asked
for. The delay gave rise, however, to the remark of a local wit,
that the University had given 'to Airy, nothing, a local habitation
and a name.'
[Illustration: GEORGE BIDDELL AIRY.]
The seven years which he spent in the Cambridge Observatory were
the best possible preparation for that greater charge which he was
to assume later. When he entered on his duties the Observatory had
been completed four years, but no observations had been published;
there was no assistant, and the only instruments were a couple of
good clocks and a transit instrument. But Airy set to work at once
with so much energy that the observations for 1828 were published
early in the following year, and he had very quickly worked out
the best methods for correcting and reducing his observations. In
1829 an assistant was granted to him, in 1833 a second, and in the
latter year Mr. Baldrey, the senior assistant, observed about 5000
transits, and Mr. Glaisher, the junior, about the same number of
zenith distances.
A syndicate had been appointed at Cambridge for the purpose of
visiting the Observatory once in each term, and making an annual
report to the senate. A smaller-minded and less acute man than
Airy might have resented such an arrangement. He, on the contrary,
threw himself heartily into it, and made such formal written
reports to the syndicate as best helped them in the performance
of their duty, and at the same time secured for the Observatory
the support and assistance which from time to time it required.
On his appointment to Greenwich, he at once entered into the same
relations to the Board of Visitors of that Observatory, and from
that time forth the friction that had occasionally existed between
the Board and the Astronomer Royal in the past entirely ceased. The
Board was henceforth no longer a body whose chief function was to
reprove, to check, or to quicken the Astronomer Royal, but rather a
company of experts, before whom he might lay the necessities of the
Observatory, that they in turn might present them to the Government.
Such representations were not likely to be in vain. For, as Mr.
Sheepshanks has left on record--
'When Mr. Airy wants to carry anything into effect by Government
assistance, he states, clearly and briefly, why he wants it;
what advantages he expects from it; and what is the probable
expense. He also engages to direct and superintend the
execution, making himself personally responsible, and giving
his labour gratis. When he has obtained permission (which is
very seldom refused), he arranges everything with extraordinary
promptitude and foresight, conquers his difficulties by storm,
and presents his results and his accounts in perfect order,
before men like ... or myself would have made up our minds
about the preliminaries. Now, men in office naturally like
persons of this stamp. There is no trouble, no responsibility,
no delay, no inquiries in the House; the matter is done, paid
for, and published, before the seekers of a grievance can find
an opportunity to be heard. This mode of proceeding is better
relished by busy statesmen than recommendations from influential
noblemen or fashionable ladies.'
His first action towards the Board was, however, a very bold and
independent one. He made strong representations on the subject of
the growth of the chronometer business, which proved displeasing
to the Hydrographer, Captain Beaufort, who was one of the official
visitors, and by his influence the report was not printed. Airy
'kept it, and succeeding reports, safe for three years, and then
the Board of Visitors agreed to print them, and four reports were
printed together, and bound with the Greenwich Observations of 1838.'
With the completion of arrangements which put the chronometer
business in proper subordination to the scientific charge of the
Observatory, Airy was free to push forward its development on the
lines which he had already marked out for himself. To go through
these in detail is simply to describe the Observatory as he left
it. Little by little he entirely renovated the equipment. Greatly
as Pond had improved the instruments of the Observatory, Airy
carried that work much further still. Though he did not observe
much himself, and was not Pond's equal in the actual handling of a
telescope, he had a great mechanical gift, and the detail in its
minutest degree of every telescope set up during his long reign was
his own design.
In the work of reduction he introduced the use of printed skeleton
forms, to which Pond had been a stranger. The publication of the
Greenwich results was carried on with the utmost regularity; and,
in striking contrast to the reluctance of Flamsteed and Halley, he
was always most prompt in communicating any observations to every
applicant who could show cause for his request for them.
It is most difficult to give any adequate impression of his
far-reaching ability and measureless activity. Perhaps the best
idea of these qualities may be obtained from a study of his
autobiography, edited and published some four years after his death
by his son. The book, to any one who was not personally acquainted
with Airy, is heavy and monotonous, chiefly for the reason that
its 400 pages are little but a mere catalogue of the works which
he undertook and carried through; and catalogues, except to the
specialist, are the dullest of reading. To enter into the details of
his work might fill a library.
[Illustration: THE ASTRONOMER ROYAL'S ROOM.]
As Astronomer Royal he seems to have inherited and summed up all
the great qualities of his predecessors: Flamsteed's methodical
habits and unflagging industry; Halley's interest in the lunar
theory; Bradley's devotion to star observation and catalogue
making; Maskelyne's promptitude in publishing, and keen interest
in practical navigation; Pond's refinement of observation. Nor did
he allow this inheritance to be merely metaphorical; he made it
an actual reality. He discussed, reduced, and published, in forms
suitable for use and comparison to-day, the whole vast mass of
planetary and lunar observations made at the Royal Observatory from
the year 1760 to his own accession, a work of prodigious labour,
but of proportionate importance. Airy has been accused--and with
some reason--of being a strong, selfish, aggressive man; yet nothing
can show more clearly than this great work how thoroughly he placed
the fame and usefulness of the Observatory before all personal
considerations. With far less labour he could have carried on a
dozen investigations that would have brought him more fame than this
great enterprise, the purpose of which was to render the work of
his predecessors of the highest possible use. The light in which he
regarded his office may best be expressed in his own words:--
'The Observatory was expressly built for the aid of astronomy
and navigation, for promoting methods of determining longitude
at sea, and (as the circumstances that led to its foundation
show) more especially for determination of the moon's motions.
All these imply, as their first step, the formation of accurate
catalogues of stars, and the determination of the fundamental
elements of the solar system. These objects have been steadily
pursued from the foundation of the Observatory; in one way by
Flamsteed; in another way by Halley, and by Bradley in the
earlier part of his career; in a third form by Bradley in his
later years; by Maskelyne (who contributed most powerfully both
to lunar and to chronometric nautical astronomy), and for a time
by Pond; then with improved instruments by Pond, and by myself
for some years; and subsequently, with the instruments now in
use. It has been invariably my own intention to maintain the
principles of the long-established system in perfect integrity;
varying the instruments, the modes of employing them, and
the modes of utilizing the observations of calculation and
publication, as the progress of science might seem to require.'
The result of this keen appreciation of the essential continuity
of the Astronomer Royalship has been that it is to Airy, more than
to any of his predecessors, or than to all of them put together,
that the high reputation of Greenwich Observatory is due. Professor
Newcomb, the greatest living authority on the subject outside our
own land--and other great foreign astronomers have independently
pronounced the same verdict--has said:--
'The most useful branch of astronomy has hitherto been that
which, treating of the positions and motions of the heavenly
bodies, is practically applied to the determination of
geographical positions on land and at sea. The Greenwich
Observatory has, during the past century, been so far the
largest contributor in this direction as to give rise to the
remark that, if this branch of astronomy were entirely lost, it
could be reconstructed from the Greenwich observations alone.'
Early in 1836 Airy proposed to the Board of Visitors the creation
of the Magnetic and Meteorological department of the Observatory,
and in 1840 a system of regular two-hourly observations was set
on foot. This was the first great enlargement of programme for
the Observatory beyond the original one expressed in Flamsteed's
warrant. It was followed in 1873 with the formation of the Solar
Photographic department, to which the Spectroscope was added a
little later.
Though he had objected strongly on his first coming to the
Observatory to the excessive time devoted to the merely commercial
side of the care of chronometers, yet the perfecting of these
instruments was one that he had much at heart, and many recent
appliances are either of his own invention or are due to suggestions
which he threw out.
Much work lying outside the Observatory, and yet intimately
connected with it, was carried out either by him or in accordance
with his directions. The transit of Venus expeditions of 1874, the
delimitation of the boundary line between Canada and the United
States, and, later, that of the Oregon boundary; the determination
of the longitudes of Valencia, Cambridge, Edinburgh, Brussels,
and Paris; assistance in the determination of the longitude of
Altona--all came under Airy's direction. Nor did he neglect
expeditions in connection with what we would now call the physical
side of astronomy. On three occasions, 1842, 1851, and 1860, he
himself personally took part in successful eclipse expeditions. The
determination of the increase of gravity observable in the descent
of a deep mine was also the subject of another expedition, to the
Harton Colliery, near South Shields.
But with all these, and many other inquiries--for he was the
confidential adviser of the Government in a vast number of subjects:
lighthouses, railways, standard weights and measures, drainage,
bridges--he yet always kept the original objects of the Observatory
in the very first place. It was in order to get more frequent
observations of the moon that he had the altazimuth erected, which
was completed in May, 1847. This was followed, in 1851, by the
transit circle, as he had long felt the need for more powerful
light grasp in the fundamental instrument of the Observatory. The
transit circle took the place both of the old transit instrument and
of the mural circle. Above all, he arranged for the observations
of moon and stars to be carried out with practical continuity. The
observations were made and reduced at once, and published in such a
way that any one wishing to discuss them afresh could for himself go
over every step of the reduction from the commencement, and could
see precisely what had been done.
The greatest addition made to the equipment of the Observatory in
Airy's day was the erection of the 12-3/4-inch Merz equatorial,
which proved of great service when spectroscopy became a department
of the Observatory.
[Illustration: THE SOUTH-EAST TOWER.
(_From a photograph by Mr. Lacey._)]
So strong and gifted a man as Airy was bound to make enemies, and
at different times of his life bitter attacks were made on him from
one quarter or another. One of these, curiously enough, was from
Sir James South, the man who, as he said, first introduced him to
practical astronomy. Later came the discovery of Neptune, and Airy
was subjected to much bitter criticism, since, as it appeared on the
surface, it was owing to his supineness that Adams missed being
held the sole discoverer of the new planet, and narrowly missed
all credit for it altogether. Last of all was the vehement attack
made upon him by Richard Anthony Proctor, in connection with his
preparations for the transit of Venus. All such attacks, however,
simply realized the old fable of the viper and the file. Attacks
which would have agonized Flamsteed's every nerve, and have called
forth full and dignified rejoinders from Maskelyne, were absolutely
and entirely disregarded by Airy. He had done his duty, and in
his own estimation--and, it should be added, in the estimation of
those best qualified to judge--had done it well. He was perfectly
satisfied with himself, and what other people thought or said about
him influenced him no more than the opinions of the inhabitants of
Saturn.
But great as Airy was, he had the defects of his qualities, and
some of these were serious. His love of method and order was often
carried to an absurd extreme, and much of the time of one of the
greatest intellects of the century was often devoted to doing what
a boy at fifteen shillings a week could have done as well, or
better. The story has often been told, and it is exactly typical
of him, that on one occasion he devoted an entire afternoon to
himself labelling a number of wooden cases 'empty,' it so happening
that the routine of the establishment kept every one else engaged
at the time. His friend Dr. Morgan jocularly said that if Airy
wiped his pen on a piece of blotting-paper he would duly endorse
the blotting-paper with the date and particulars of its use, and
file it away amongst his papers. His mind had that consummate grasp
of detail which is characteristic of great organizers, but the
details acquired for him an importance almost equal to the great
principles, and the statement that he had put a new pane of glass
into a window would figure as prominently in his annual report to
the Board of Visitors as the construction of the new transit circle.
His son remarks of him that 'in his last days he seemed to be more
anxious to put letters which he received into their proper place for
reference than even to master their contents,' his system having
grown with him from being a means to an end, to becoming the end
itself.
So, too, his regulation of his subordinates was, especially in his
earlier days, despotic in the extreme--despotic to an extent which
would scarcely be tolerated in the present day, and which was the
cause of not a little serious suffering to some of his staff, whom,
at that time, he looked upon in the true spirit of Pond, as mere
mechanical 'drudges.' For thirty-five years of his administration
the salaries of his assistants remained discreditably low, and
his treatment of the supernumerary members of his staff would now
probably be characterized as 'remorseless sweating.' The unfortunate
boys who carried out the computations of the great lunar reductions
were kept at their desks from eight in the morning till eight
at night, without the slightest intermission, except an hour at
midday. As an example of the extreme detail of the oversight which
he exercised over his assistants, it may be mentioned that he drew
up for each one of those who took part in the Harton Colliery
experiment, instructions, telling them by what trains to travel,
where to change, and so forth, with the same minuteness that one
might for a child who was taking his first journey alone; and he
himself packed up soap and towels with the instruments, lest his
astronomers should find themselves, in Co. Durham, out of reach of
these necessaries of civilization.
A regime so essentially personal may indeed have been necessary
after Pond's administration, and to give the Observatory a
fresh start. But it would not have been to the advantage of
the Observatory, had it become a permanent feature of its
administration, as it militated--was almost avowedly intended to
militate--against the growth of real zeal and intelligence in the
staff, and necessarily occasioned labour and discomfort out of
proportion to the results obtained. Fortunately, in Airy's later
years, the extension of the work of the Observatory, a slight
failing in his own powers, and the efforts he was devoting to the
working out of the lunar theory, compelled him to relax something
of that microscopic imperiousness which had been the chief
characteristic of his rule for so long.
Airy had, in the fullest degree, the true spirit of the public
servant; his sense of duty to the State was very high. He was
always ready to undertake any duty which he felt to be of public
usefulness, and many of these he discharged without fee or reward.
So great an astronomer was necessarily most highly esteemed by
astronomers. He was President of the Royal Society for two years;
he was five times President of the Royal Astronomical Society,
and twice received its gold medal, beside a special testimonial
for his reduction of the Greenwich lunar observations. From the
Royal Society he received the Copley medal and the Royal medal,
beside honorary titles from the Universities of Oxford, Cambridge,
and Edinburgh. So invaluable a public servant, he received the
distinction of a Knight Commandership of the Bath in 1872. He had
been repeatedly offered knighthood before, but had not thought it
well to receive it. He was in the receipt of decorations also from
a great number of foreign countries; for, for many years, he was
looked up to, not only by English astronomers, but by scientific men
in all countries, as the very head and representative of his science.
And he also received a more popular appreciation--and most justly
so. For whilst no one could have less of the arts of the ordinary
popularizer about him, no one has ever given popular lectures on
astronomy which more fully corresponded to the ideal of what such
should be than Airy's six lectures to working men, delivered at
Ipswich. And we may count the bestowal upon him of the honorary
freedom of the City of London, in 1875, as one of the tokens that
his services in this direction had not been unappreciated.
During the last seven years of his official career he undertook
the working out of a lunar theory, and, to allow himself more
leisure for its completion, he resigned his position August 15,
1881, after forty-six years of office. He was now eighty years of
age, and he took up his residence at the White House, just outside
Greenwich Park. He resided there till his death, more than ten years
later--January 2, 1892.
* * * * *
Airy was succeeded in the Astronomer Royalship by the present and
eighth holder of the office, W. H. M. CHRISTIE. He was born at
Woolwich, in 1845, his father having been Professor Samuel Hunter
Christie, F.R.S. He was educated at King's College, London, and
Trinity College, Cambridge, graduating as fourth Wrangler in 1868.
In 1870 he was appointed chief assistant at Greenwich, in succession
to Mr. Stone, who had become her Majesty's astronomer at the Cape,
and in 1881 he succeeded Airy as Astronomer Royal.
[Illustration: W. H. M. CHRISTIE, ASTRONOMER ROYAL.
(_From a photograph by Elliott and Fry._)]
During Mr. Christie's office, the two new departments of the
Astrographic Chart and Double-star observations have come into
being. The following buildings have been erected under his
administration: the great New Observatory in the south ground,
the New Altazimuth, the New Library, nearly opposite to it, the
Transit Pavilion, the porter's lodge, and the Magnetic Pavilion
out in the Park. Whilst in the old buildings the Astrographic dome
has been added, and the Upper and Lower Computing rooms have been
rebuilt and enlarged. As to the instruments, the 28-inch refractor,
the astrographic twin telescope, the new altazimuth, the 26-inch
and 9-inch Thompson photographic refractors, and the 30-inch
reflector are all additions during the present reign. Roughly
speaking, therefore, we may say that three-fourths of the present
Observatory has been added during the nineteen years of the present
Astronomer Royal. One exceedingly important improvement should not
be overlooked. Airy observed little himself whilst at Greenwich,
and had an inadequate idea of the necessity for room in a dome and
breadth in a shutter-opening. With the sole exception, perhaps, of
the transit circle, every instrument set up by Airy was crammed into
too small a dome or looked out through too narrow an opening. The
increase of shutter-opening of the newer domes may be well seen by
contrasting, say, the old altazimuth or the Sheepshanks dome with
that of the astrographic. This reform has had much to do with the
success of later work.
CHAPTER V
THE OBSERVATORY BUILDINGS
Like a living organism, Greenwich Observatory bears the record of
its life-history in its structure. It was not one of those favoured
institutions that have sprung complete and fully equipped from the
liberality of some great king or private millionaire. As we have
seen, it was originally established on the most modest--not to say
meagre--scale, and has been enlarged just as it has been absolutely
necessary. To quote again from Professor Newcomb--
'Whenever any part of it was found insufficient for its purpose,
new rooms were built for the special object in view, and thus
it has been growing from the beginning by a process as natural
and simple as that of the growth of a tree. Even now the very
value of its structure is less than that of several other public
observatories, though it eclipses them all in the results of its
work.'
Entering the courtyard--an enclosure some eighty feet deep by
ninety feet in extreme breadth--by the great gate, we see before
us Flamsteed House, the original building of the Observatory.
Flamsteed's little domain was only some twenty-seven yards wide
by fifty deep, and for buildings comprised little beyond a small
dwelling-house on the ground floor, and one fine room above it. This
room--the original Greenwich Observatory--still remains, and is
used as a council room by the official Board of Visitors, who come
down to the Observatory on the first Saturday in June, to examine
into its condition and to receive the Astronomer Royal's report. The
room is called, from its shape, the Octagon Room, and is well known
to Londoners from the great north window which looks out straight
over the river between the twin domes of the Hospital.
In Bradley's time, about 1749, the first extension of the domains
of the Observatory took place to the south and east of the original
building, the direction in which, on the whole, all subsequent
extensions have taken place, owing to the fact that the original
building was constructed at the extremity of what Sir George Airy
was accustomed to call a 'peninsula'--a projecting spur of the
Blackheath plateau, from which the ground falls away very sharply on
three sides and on part of the fourth.
The Observatory domain at present is fully two hundred yards in
greatest length, with an average breadth of about sixty. Nearly the
whole of this accession took place under the directorates of Pond
and Airy. The present instruments are, therefore, as a rule, the
more modern in direct proportion to their distance from the Octagon
Room--the old original Observatory. There is one notable exception.
The very first extension of the Observatory buildings, made in
the time of Halley, the second Astronomer Royal, consisted in the
setting up of a strong pier, to carry two quadrant telescopes. The
pier still remains, but now forms the base of the support of the
twin telescopes devoted to the photographic survey of the heavens
for the International Chart.
Standing just within the gate of the courtyard, and looking
westward, that is toward Flamsteed House, we have immediately on
our right hand the porter's lodge; a little farther forward, also
on the right, the Transit Pavilion, a small building sheltering
a portable transit instrument; and farther forward, still on the
right, the entrance to the Chronograph Room. Above the Chronograph
Room is a little, inconveniently-placed dome, containing a small
equatorially-mounted telescope, known as the Shuckburgh. Beyond the
Chronograph Room a door opens on to the North Terrace, over which
is seen the great north window of the Octagon Room. Close by the
door of the Chronograph Room a great wooden staircase rises to the
roof of the main building. It is not an attractive-looking ascent,
as the steps overlap inconveniently. Still, there is no record of
an accident upon them, and those who venture on the climb to the
roof, where are placed the anemometers and the turret carrying the
time-ball, which is dropped daily at 1 p.m., will be well repaid by
the splendid view of the river which is there afforded to them.
Passing under this staircase, on the wall by its side is seen the
following inscription:--
CAROLUS II^S REX OPTIMUS
ASTRONOMIÆ ET NAUTICÆ ARTIS
PATRONUS MAXIMUS
SPECULAM HANC IN UTRIUSQUE COMMODUM
FECIT
ANNO D^{NI} MDCLXXVI. REGNI SUI XXVIII.
Curante Iona Moore milite
R. T. S. G.
[Illustration: THE ASTRONOMER ROYAL'S HOUSE.
(_From a photograph by Mr. Lacey._)]
In the extreme angle of the courtyard is the entrance to the mean
solar clock cupboard, and to the staircase leading up to the Octagon
Room. At the head of this staircase in a small closet is the winch
for winding up the time-ball.
Coming back into the courtyard, and crossing the face of the
Astronomer Royal's private house, the range of buildings is reached
which form the left hand or south side of the enclosure. Entering
the first of these, we find ourselves in the Lower Computing Room,
which is devoted to the 'Time Department.' The next room which opens
out of it, as we turn eastwards, was Bradley's Transit Room, but is
now used for the storage of chronometers. Passing through Bradley's
Transit Room, we come to the present Transit Room, which brings
us close to the great gate. The range of buildings is, however,
continued somewhat farther, containing on the ground floor some
small sitting-rooms and a fire-proof room for records.
[Illustration: THE COURTYARD.
(_From a photograph by Mr. Lacey._)]
Turning back to the Lower Computing Room, we notice in it the stone
pier, already alluded to, which was set up by Halley, and formed
the first addition to the original Observatory of Flamsteed. The
Lower Computing Room itself and Bradley's Transit Room were due
to the Astronomer after which the latter is named. An iron spiral
staircase in the middle of the Lower Computing Room leads up to the
Upper Computing Room, and above that to the Astrographic dome, so
called because the twin telescope housed therein is devoted to the
work of the Astrographic Chart--a chart of the entire sky to be made
by eighteen co-operating observatories by means of photography. In
this way it is intended to secure a record of the places of far
more stars than could be done by the ordinary methods, and in this
project Greenwich has necessarily taken a premier place. This is
a work which, whilst it is the legitimate and natural outcome of
the original purpose of the Observatory, is yet pushed beyond what
is necessary for any mere utilitarian assistance to navigation. For
the sailor it will always be sufficient to know the places of a
mere handful of the brightest stars, and the vast majority of those
in the great photographic map will never be visible in the little
portable telescope of the sailor's sextant. But it will be freely
admitted that in the case of an enterprise of this nature, in which
the observatories of so many different nations were uniting, and
which was so precisely on the lines of its original charter, though
an extension of it, it was impossible for Greenwich to hold back on
the plea that the work was not entirely utilitarian.
Descending again to the Lower Computing Room, and passing through
it, not to the east, into Bradley's Transit Room, but through a
little lobby to the south, we come upon an inconvenient wooden
staircase winding round a great stone pillar with three rays. This
pillar is the support of Airy's altazimuth, and very nearly marks
the place where Flamsteed set up his original sextant.
Returning again to the Lower Computing Room, and passing out to the
east, just in front of the Time Superintendent's desk, we enter a
small passage running along the back of Bradley's Transit Room, and
from this passage enter the present Transit Room near its south end.
Just before reaching the Transit Room, however, we pass the Reflex
Zenith Tube, a telescope of a very special kind.
Immediately outside the Transit Room is a staircase leading on the
first floor to two rooms long used as libraries, and to the leads
above them, on which is a small dome containing the Sheepshanks
equatorial. These libraries are over the small sitting-rooms already
referred to. The fire-proof Record Rooms, two stories in height,
terminate this range of buildings.
Beyond the Record Rooms the boundary turns sharply south, where
stands a large octagonal building surmounted by a dome of oriental
appearance, a 'circular versatile roof,' as the Visitors would have
called it a hundred years ago. This dome--which has been likened,
according to the school of æsthetics in which its critics have been
severally trained, to the Taj at Agra, a collapsed balloon, or a
mammoth Spanish onion--houses the largest refractor in England, the
'South-east Equatorial' of twenty-eight inches aperture. But, though
the largest that England possesses, it would appear but as a pigmy
beside some of the great telescopes for which America is famous.
Beyond this dome the hollow devoted to the Astronomer Royal's
private garden reduces the Observatory ground to a mere 'wasp's
waist,' a narrow, inconvenient passage from the old and north
observatory to the younger southern one.
The first building, as the grounds begin to widen out to the south,
contains the New Altazimuth, a transit instrument which can be
turned into any meridian. A library of white brick and a low wooden
cruciform building--the Magnetic Observatory--follow it closely.
This latter building houses the Magnetic Department, a department
which, though it lies aside from the original purposes of the
Observatory, as defined in the warrant given to Flamsteed, is yet
intimately connected with navigation, and was founded by Airy very
early in his period of office. This deals with the observation of
the changes in the force and direction of the earth's magnetism, an
inquiry which the greater delicacy of modern compasses, and, in more
recent times, the use of iron instead of wood in the construction of
ships, has rendered imperative.
Closely associated with the Magnetic Department is the
Meteorological. Weather forecasts, so necessary for the safety
of shipping round our coasts, are not issued from Greenwich
Observatory, any more than the _Nautical Almanac_ is now issued from
it. But just as the Observatory furnishes the astronomical data upon
which the almanac is based, so also a considerable department is set
apart for furnishing observations to be used by the Meteorological
Office at Westminster for their daily predictions.
So far, the development of the Observatory had been along the
central line of assistance to navigation. But the 'Magnetic
Department' led on to a new one, which had but a secondary
connection with it. It had been discovered that the extent of
the daily range of the magnetic needle, and the amount of the
disturbances to which it was subjected, were in close connection
with the numbers and size of the spots on the sun's surface. This
led to the institution of a daily photographic record of the state
of the sun's surface, a record of which Greenwich has now the
complete monopoly.
[Illustration: PLAN OF OBSERVATORY AT PRESENT TIME.
(For key to plan, see p. 135.)]
KEY TO THE PLAN OF THE OBSERVATORY ON PAGE 134.
1. Chronograph Room.
2. Old Altazimuth Dome.
3. Safe Room.
4. Computing Room.
5. Bradley's Transit Room.
6. Transit Circle Room.
7. Assistants' Room.
8. Chief Assistant's Room.
9. Computers' Room.
10. Record Rooms.
11. Chronometer Rooms and South-east Dome.
12. Greenhouse and Outbuildings.
14. New Library.
15. Magnetic Observatory.
16. Offices.
19. Sheds.
23. Winch Room for Time-ball.
24. Porter's Lodge.
25. New Transit Pavilion.
26. New Altazimuth Pavilion.
27. Museum: New Building.
28. South Wing "
29. North Wing "
30. West Wing "
31. East Wing "
F. Rooms built for Flamsteed.
H. Added by Halley.
B. " Bradley.
M. " Maskelyne.
A. " Airy.
F'F'. Flamsteed's boundaries.
M'M'. Maskelyne's " 1790.
P'P'. Pond's " 1814.
A'A'. Airy's " 1837.
A"A". Airy's " 1868.
Beyond the Magnetic Observatory the ground widens out into an area
about equal to that of the northern part, and the new building
just completed, and which is now emphatically 'The Observatory,'
stands clear before us. The transfer to this stately building of the
computing rooms, libraries, and store rooms has been aptly described
as a shift in the latitude of Greenwich Observatory, which still
preserves its longitude. It may be noted that the only two buildings
of any architectural pretensions in the whole range are--Flamsteed's
original observatory, built by Sir Christopher Wren, and containing
little beyond the octagon room, in the extreme north; and this
newest building in the extreme south.
This 'New Observatory,' like the old, and like the great
South-eastern tower, is an octagon in its central portion. But
whilst the two other great buildings are simply octagonal, here the
octagon serves only as the centre from which radiate four great
wings to the four points of the compass. The building is by far the
largest on the ground, but in little accord with the popular idea of
an astronomer as perpetually looking through a telescope, carries
but a single dome; its best rooms being set apart as 'computing
rooms,' for the use of those members of the staff who are employed
in the calculations and other clerical work, which form, after all,
much the greater portion of the Observatory routine.
An observer with the transit instrument, for instance, will take
only three or four minutes to make a complete determination of the
place of a single star. But that observation will furnish work to
the computers for many hours afterwards. Or, to take a photograph of
the sun will occupy about five minutes in setting the instrument,
whilst the actual exposure will take but the one-thousandth part of
a second. But the plate, once exposed, will have to be developed,
fixed, and washed; then measured, and the measures reduced, and, _on
the average_, will provide one person with work for four days before
the final results have been printed and published.
It is easy to see, then, that observing, though the first duty of
the Observatory, makes the smallest demand on its time. The visitor
who comes to the Observatory by day (and none are permitted to do
so by night) finds the official rooms not unlike those of Somerset
House or Whitehall, and its occupants for the most part similarly
engaged in what is, apparently, merely clerical work. An examination
of the big folios would of course show that instead of being ledgers
of sales of stamps, or income-tax schedules, they referred to stars,
planets, and sun-spots; but for one person actively engaged at a
telescope, the visitor would see a dozen writing or computing at a
desk.
The staff, like the building, is the result of a gradual
development, and bears traces of its life history in its
composition. First comes the Astronomer Royal, the representative
and successor of the original 'King's Astronomer,' the Rev. John
Flamsteed. But the 'single surly and clumsy labourer,' which was
all that the 'Merry Monarch' could grant for his assistance, is now
represented by a large and complex body of workers; each varied
class and rank of which is a relic of some stage in the progress of
the Observatory to its present condition.
The following extract from the Annual Report of the Astronomer
Royal to the Board of Visitors, June, 1900, describes the present
_personnel_ of the establishment:--
'The staff at the present time is thus constituted, the names in
each class being arranged in alphabetical order:--
'Chief assistants--Mr. Cowell, Mr. Dyson.
'Assistants--Mr. Hollis, Mr. Lewis, Mr. Maunder, Mr. Nash, Mr.
Thackeray.
'Second-class assistants--Mr. Bryant, Mr. Crommelin.
'Clerical assistant--Mr. Outhwaite.
'Established computers--Mr. Bowyer, Mr. Davidson, Mr. Edney, Mr.
Furner, Mr. Rendell, and one vacancy.
'The two second-class assistants will be replaced by higher
grade established computers as vacancies occur.
'Mr. Dyson and Mr. Cowell have the general superintendence of
all the work of the Observatory. Mr. Maunder is charged with
the heliographic photography and reductions, and with the
preparation of the Library Catalogue. Mr. Lewis has charge
of the time-signals and chronometers, and of the 28-inch
equatorial. Mr. Thackeray superintends the miscellaneous
astronomical computations, including the preparation of
the new Ten-Year Catalogue. Mr. Hollis has charge of the
photographic mapping of the heavens, the measurement of the
plates, and the computations for the Astrographic Catalogue. Mr.
Crommelin undertakes the altazimuth and Sheepshanks equatorial
reductions, and Mr. Bryant the transit and meridian zenith
distance reductions and time-determinations. In the magnetic and
meteorological branch, Mr. Nash has charge of the whole of the
work. Mr. Outhwaite acts as responsible accountant officer; has
charge of the library, records, manuscripts, and stores, and
conducts the official correspondence. As regards the established
computers, Mr. Bowyer, Mr. Furner, Mr. Davidson, and Mr. Rendell
assist Mr. Lewis, Mr. Thackeray, Mr. Hollis, and Mr. Bryant
respectively, and Mr. Edney assists Mr. Nash.
'There are at the present time twenty-four supernumerary
computers employed at the Observatory, ten being attached to
the astronomical branch, two the chronometer branch, six to the
astrographic, one to the heliographic, four to the magnetic and
meteorological, and one to the clerical.
'A foreman of works, with two carpenters, and two labourers;
a skilled mechanic with an assistant; a gate porter, two
messengers, a watchman, a gardener, and a charwoman, are also
attached to the Observatory.
'The whole number of persons regularly employed at the
Observatory is fifty-three.'
The day work, as said before, is by far the greatest in amount,
the 'office hours' being from nine till half-past four, with an
hour's interval. The arrangements for the night watches present some
complications.
For many years the instruments in regular use were two only, the
transit circle and the altazimuth. The arrangements for observing
were simple. Four assistants divided the work between them thus: an
assistant was on duty with the transit circle one day, his watch
beginning about six a.m. or a little later, and ending about three
the following morning; a watch of twenty-one hours in maximum
length. The second day his duties were entirely computational, and
were only two or three hours in length. The third day he had a full
day's work on the calculations, followed by a night duty with the
altazimuth. The latter instrument might give him a very easy watch
or a terribly severe one. If the moon were a young one it was easy,
especially if the night was clear, as in that case an hour was
enough to secure the observations required.
Very different was the case with a full moon, especially in the
long, often cloudy, nights of winter. Then a vigilant watch had to
be kept from sunset to sunrise, so that in case of a short break
in the clouds the moon might yet be observed. Such a watch was the
severest (with one exception) that an assistant had to undergo.
His fourth day would then resemble his second, and with the fifth
day a second cycle of his quartan fever would commence, the symptoms
following each other in the same sequence as before.
Such a routine carried on with iron inflexibility was exceedingly
trying, as it was absolutely impossible for an observer to keep any
regularity in his hours of rest or times for meals.
This routine has been considerably modified by the present
Astronomer Royal, partly because the instruments now in regular
daily use are five instead of two, and partly because a less
stringent system has proved not merely far less wearing to the
observers, but also much more prolific of results. It was impossible
for a man to be at his best for long under the old _régime_, and
from forty-six to forty-seven has been an ordinary age for an
assistant to break down under the strain.
One point in which the observing work has been lightened has been in
the discontinuance of the altazimuth observations at the full of the
moon, another in the shortening of the hours of the transit circle
watch; and a further and most important one in the arrangement that
the observers with the larger instruments should have help at their
work. The net result of these changes has been a most striking
increase in the amount of work achieved. Thus, whilst in the year
ending May 20, 1875, 3780 transits were taken with the transit
circle, and 3636 determinations of north polar distance; in that
ending May 10, 1895, the numbers had risen to 11,240 and 11,006
respectively, the telescope remaining precisely the same.
One principle of Airy's rule still remains. So far as possible no
observer is on duty for two consecutive days, but a long day of desk
work and observing is followed by a short day of desk work without
observing.
It will be readily understood that with five principal telescopes
in constant work and one or two minor ones, some demanding two
observers, others only one, each telescope having its special
programme and its special hours of work, whilst by no means every
member of the staff is authorized to observe with all instruments
indifferently, it becomes a somewhat intricate matter to arrange the
weekly _rota_ in strict accordance with the foregoing principle, and
with the further one, that whilst a considerable amount of Sunday
observing is inevitable, the average duty of an observer should be
three days a week, not seven days a fortnight. There is a story,
received with much reserve at Cambridge, that there was once a man
at that university who had mastered all the colours and combinations
of shades and colours of the various colleges and clubs. If so
gifted a being ever existed, he may be paralleled by the Greenwich
assistant who can predict for any future epoch the sequence of
duties throughout the entire establishment. At any rate, one of the
first items in the week's programme is the preparation of the _rota_
for the week, or rather, to use an ecclesiastical term, for the
'octave,' _i.e._ from the Monday to the Monday following.
The special work to be carried out on any telescope is likewise
a matter of programme. For the transit circle a list of the most
important objects to be observed is supplied for the observer's
use, and the general lines upon which the other stars are to be
selected from a huge 'Working Catalogue' are well understood.
With some of the other telescopes the principles upon which the
objects are to be selected are laid down, but the actual choice
is left to the discretion of the observer at the time. There
is no time for the watcher to spend in what the outsider would
regard as 'discovery'; such as sweeping for comets or asteroids,
hunting for variable stars, sketching planets, and so forth.
Indeed, there is a story current in the Observatory that some fifty
years ago, when the tide of asteroid discovery first set in, Airy
found an assistant, since famous, working with a telescope on his
'off-duty' night. That stern disciplinarian asked what business the
assistant had to be there on his free night, and on being told he
was 'searching for new planets,' he was severely reprimanded and
ordered to discontinue at once. A similar energy would not meet
so gruff a discouragement to-day; but the routine work so fully
occupies both staff and telescopes that an assistant may be most
thoroughly devoted to his science, and yet pass a decade at the
Observatory without ever seeing those 'show places' of the sky which
an amateur would have run over in the first week after receiving his
telescope. For example, there is no refractor in the British Isles
so competent to bring out the vivid green light of the great Orion
nebula--that marvellous mass of glowing, curdling, emerald cloud--or
the indescribable magnificence of the myriad suns that cluster
like swarming bees or the grapes of Eshcol in the constellation of
Hercules; yet probably most of the staff have never seen either
spectacle through it. The professional astronomer who is worth his
salt will find abundance of charm and interest in his work, but he
will not,
'Like a girl,
Valuing the giddy pleasures of the eyes,'
consider the charm to lie mainly in the occasional sight of
wonderful beauty which his work may bring to him, nor the interest
in some chance phenomenon which may make his name known.
It is not every field of astronomy that is cultivated at Greenwich.
The search for comets and for 'pocket planets' forms no part of
its programme; and the occupation so fascinating to those who take
it up, of drawing the details on the surfaces of the moon, Mars,
Jupiter, or Saturn, has been but little followed. Such work is here
incidental, not fundamental, and the same may be said of certain
spectroscopic observations of new or variable stars, and of many
similar subjects. Work such as this is most interesting to the
general public, and is followed with much devotion by many amateur
astronomers. For that very reason it does not form an integral
part of the programme of our State observatory. But work which
is necessary for the general good, or for the advancement of the
science, and which demands observations carried on continuously for
many years, and strict unity of instruments and methods, cannot
possibly be left to chance individual zeal, and is therefore rightly
made the first object at Greenwich.
Those striking discoveries which from time to time appeal strongly
to the popular imagination, and which have rendered so justly
famous some of the great observatories of the sister continent, have
not often been made here.
Its work has, none the less, been not only useful but essential. A
century ago, when we were engaged in the hand-to-hand struggle with
Napoleon, by far the most brilliant part of that naval war which we
waged against the French, and the most productive of prize-money,
was carried on by our cruisers, who captured valuable prizes in
every sea. But a much greater service, indeed an absolutely vital
one, was rendered to the State by those line-of-battle ships which
were told off to watch the harbours wherein the French fleet was
taking refuge. This was a work void of the excitement, interest,
and profit of cruising. It was monotonous, wearing, and almost
inglorious, but absolutely necessary to the very existence of
England. So the continuance for more than two centuries of daily
observations of places of moon, stars, and planets is likewise
'monotonous, wearing, and almost inglorious;' the one compensation
is that it is essential to the life of astronomy.
The eight Astronomers Royal have, as already said, kept the
Observatory strictly on the lines originally laid down for it,
subject, of course, to that enlargement which the growth of
the science has inevitably brought. But had they been inclined
to change its course, the Board of Visitors has been specially
appointed to bring them back to the right way. As already mentioned
in the account of Flamsteed, the Board dates from 1710, when it
practically consisted of the President and Council of the Royal
Society. Its Royal warrant lapsed on the death of Queen Anne, and
was not renewed at the accession of the two following sovereigns;
but in the reign of George III. a new warrant was issued under date
February 22, 1765; and this was renewed at the accession of George
IV. When William IV. came to the throne, the constitution of the
Board was extended, so as to give a representation to the new Royal
Astronomical Society, founded in 1820. The President of the Royal
Society is still chairman of the Board, but the Admiralty, of which
the Observatory is a department, the two Universities of Oxford and
Cambridge, and the Royal Astronomical Society are all represented on
it by _ex officio_ members, and twelve other members are contributed
by the Royal and Royal Astronomical Societies respectively, six
by each. The first Saturday in June is the appointed day for the
annual inspection by the Board, and for the presentation to it
of the Astronomer Royal's Report. To this all-important business
meeting has been added something of a social function, by the
invitation of many well-known astronomers and the leading men of the
allied sciences to inspect the results of the year, and to partake
of the chocolate and cracknels, which have been the traditional
refreshments offered on these occasions for a period 'whereof the
memory of man runneth not to the contrary.'
CHAPTER VI
THE TIME DEPARTMENT
One day two Scotchmen stood just outside the main entrance
of Greenwich Observatory, looking intently at the great
twenty-four-hour clock, which is such an object of attention to the
passers through the Park. 'Jock,' said one of them to the other,
'd'ye ken whaur ye are?' Jock admitted his ignorance. 'Ye are at the
vara ceentre of the airth.'
Geographers tell us that there is a sense in which this statement as
it stands may be accepted as true. For if the surface of the globe
be divided into two hemispheres, so related to each other that the
one contains as much land as possible, and the other as little, then
London will occupy the centre or thereabouts of the hemisphere with
most land.
This was not, however, what the Scotchman meant. He meant to tell
his companion that he was standing on the prime meridian of the
world, the imaginary base line from which all distances, east or
west, are reckoned; in short, that he was on 'Longitude Nought.'
He was not absolutely correct, however, for the great
twenty-four-hour clock does not mark the exact meridian of
Greenwich. To find the instrument which marks it out and defines it
we must step inside the Observatory precincts, and just within the
gate we see before us on the left hand a door which leads through
a little lobby straight into the most important room of the whole
Observatory--the Transit Room.
[Illustration: THE GREAT CLOCK AND PORTER'S LODGE.
(_From a photograph by Mr. Lacey._)]
This room is not well adapted for representation by artist or
photographer. Four broad stone pillars occupy the greater part of
the space, and leave little more than mere passage room beside. Two
of these pillars are tall, as well as broad and massive, and stand
east and west of the centre of the room, carrying between them the
fundamental instrument of the Observatory, the transit circle. The
optical axis of this telescope marks 'Longitude Nought,' which is
further continued by a pair of telescopes, one to the north of it,
the other to the south, mounted on the third and fourth of the
pillars alluded to above.
This room has not always marked the meridian of Greenwich, for it
stands outside the original boundary of the Observatory. But it is
only a few feet to the east of the first transit instrument which
was set up by Halley, the second Astronomer Royal, in the extreme
N.-W. corner of the Observatory domain, a distance equivalent to
very much less than one-tenth of a second of time, an utterly
insensible quantity with the instruments of two hundred years ago.
It would be a long story to tell in detail how the Greenwich transit
room has come to define one of the two fundamental lines that
encircle the earth. The other, the equator, is fixed for us by the
earth itself, and is independent of any political considerations,
or of any effort or enterprise of man. But of all the infinite
number of great circles which could be drawn at right angles to the
equator, and passing through the north and south poles, it was not
easy to select one with such an overwhelming amount of argument in
its favour as to obtain a practically universal acceptance. The
meridians of Jerusalem and of Rome have both been urged, upon what
we may call religious or sentimental grounds; that of the Great
Pyramid at Ghizeh has been pressed in accordance with the fantastic
delusion that the Pyramid was erected under Divine inspiration and
direction; that of Ferro, in the Canaries, as being an oceanic
station, well to the west of the Old World, and as giving a base
line without preference or distinction for one nation rather than
another.
The actual decision has been made upon no such grounds as these.
It has been one of pure practical convenience, and has resulted
from the amazing growth of Great Britain as a naval and commercial
power. Like Tyre of old, she is 'situate at the entry of the sea,
a merchant of the people for many isles,' and 'her merchants are
the great men of the earth.' To tell in full, therefore, the
steps by which the Greenwich meridian has overcome all others is
practically to tell again, from a different standpoint, the story
of the 'expansion of England.' The need for a supreme navy, the
development of our empire beyond the seven seas, the vast increase
of our carrying trade--these have made it necessary that Englishmen
should be well supplied with maps and charts. The hydrographic and
geographic surveys carried on, either officially by this country, or
by Englishmen in their own private capacity, have been so numerous,
complete, and far-reaching as not only to outweigh those of all
other countries put together, but to induce the surveyors and
explorers of not a few other countries to adopt in their work the
same prime meridian as that which they found in the British charts
of regions bordering on those which they were themselves studying.
Naturally, the meridian of Greenwich has not only been adopted
for Great Britain, but also for the British possessions over-sea,
and, from these, for a large number of foreign countries; whilst
our American cousins retain it, an historic relic of their former
political connection with us. The victories of Clive at Arcot and
Plassy, of Nelson at the Nile and Trafalgar, the voyages and surveys
of Cook and Flinders, and many more; the explorations of Bruce,
Park, Livingstone, Speke, Cameron, and Stanley; these are some of
the agencies which have tended to fix 'Longitude Nought' in the
Greenwich Transit Room.
There are two somewhat different senses in which the meridian of
Greenwich is the standard meridian for nearly the entire world. The
first is the sense about which we have already been speaking; it
constitutes the fundamental line whence distances east and west are
measured, just as distances north and south are measured from the
equator. But there is another, though related sense, in which it has
become the standard. _It gives the time to the world._
There are few questions more frequently put than, 'What time is
it?' 'Can you tell me the true time?' A stickler for exactitude
might reply, 'What kind of time do you mean?' 'Do you mean solar
or sidereal time?' 'Apparent time or mean time?' 'Local time or
standard time?' There are all these six kinds of time, but it is
only within the last two generations, within, indeed, the reign of
our Sovereign, Queen Victoria, that the subject of the differences
of most of these kinds of time has become of pressing importance to
any but theorists.
In one of the public gardens of Paris a little cannon is set up
with a burning-glass attached to it in such a manner that the
sun itself fires the cannon as it reaches the meridian. This, of
course, is the time of Paris noon--apparent noon--but it would be
exceedingly imprudent of any traveller through Paris who wished,
say, to catch the one o'clock express, to set his watch by the gun.
For if it happened to be in February, he would find when he reached
the railway station that the station clock was faster than the sun
by nearly a full quarter of an hour, and that his train had gone;
whilst towards the end of October or the beginning of November, he
would find himself as much too soon.
Until machines for accurately measuring time were invented, apparent
time--time, that is to say, given by the sun itself, as by a
sun-dial--was the only time about which men knew or cared. But when
reasonably good clocks and watches were made, it was very soon seen
that at different times in the year there was a marked difference
between sun-dial time and that shown by the clock, the reason being
simply that the apparent rate of motion of the sun across the sky
was not always quite the same, whilst the movement of the clock was,
of course, as regular as it could be made.
This difference between time as shown by the actual sun and by a
perfect clock is known as the 'equation of time.' It is least about
April 15, June 15, August 31, and December 25. It is greatest,
the sun being after the clock, about February 11; and the sun
being before the clock, about November 2. Flamsteed, before he
became Astronomer Royal, investigated the question, and so clearly
demonstrated the existence, cause, and amount of the equation of
time as entirely to put an end to controversy on the subject.
We had thus, early in the century, the two kinds of time in common
use, apparent time and mean time, or clock time. But as the sun can
only be on one particular meridian at any given instant, the time
as shown by the clocks in one particular town will differ from that
of another town several miles to the east or west of it. It is thus
noon at Moscow 1 hr. 36 min. before it is noon at Berlin, and noon
at Berlin 54 min. before it is noon in London.
This was all well enough known, but occasioned no inconvenience
until the introduction of railway travelling; then a curious
difficulty arose. Suppose an express train was running at the rate
of sixty miles an hour from London to Bristol. The guard of the
train sets his watch to London time before he leaves Paddington,
but if the various towns through which the train passes, Reading,
Swindon, etc., each keep their own local time, he will find his
watch apparently fast at each place he reaches; but on his return
journey, if he sets to Bristol time before starting, he will in a
similar way find it apparently slow by the Swindon, Reading, and
Paddington clocks as he reaches them in succession.
It became at once necessary to settle upon one uniform system of
time for use in the railway guides. Apart from this, a passenger
taking train, say, at Swindon, might have been very troubled to
know whether the advertised time of his train was that of Exeter,
the place whence it started, or Swindon, the station where he was
getting in, or London, its destination. 'Railway time,' therefore,
was very early fixed for the whole of Great Britain to be the same
as London time, which is, of course, time as determined at Greenwich
Observatory. At first it was the custom to keep at the various
stations two clocks, one showing local time, the other 'railway,' or
Greenwich time, or else the clocks would be provided with a double
minute hand, one branch of which pointed to the time of the place,
the other to the time of Greenwich.
It was soon found, however, that there was no sufficient reason
for keeping up local time. Even in the extreme West of England the
difference between the two only amounted to twenty-three minutes,
and it was found that no practical inconvenience resulted from
saying that the sun rose at twenty-three minutes past six on March
22, rather than at six o'clock. The hours of work and business were
practically put twenty-three minutes earlier in the day, a change of
which very few people took any notice.
Other countries besides England felt the same difficulty, and solved
it in the same way, each country as a rule taking as its standard
time the time of its own chief city.
There were two countries for which this expedient was not
sufficient--the United States and Canada. The question was of no
importance until the iron road had linked the Atlantic to the
Pacific in both countries. Then it became pressing. No fewer than
seventy different standards prevailed in the United States only some
twenty years ago. The case was a very different one here from that
of England, where east and west differed in local time by only a
little over twenty minutes. In North America, in the extreme case,
the difference amounted to four hours, and it seemed asking too much
of men to call eight o'clock in their morning, or it might be four
o'clock in their afternoon, their noonday.
The device was therefore adopted of keeping the minutes and seconds
the same for all places right across the continent, but of changing
the hour at every 15° of longitude. The question then arose what
longitude should be adopted as the standard. The Americans might
very naturally have taken their standard time from their great
national observatory at Washington, or from that of their chief
city, New York, or of their principal central city, Chicago. But,
guided partly no doubt by a desire to have their standard times
correspond directly to the longitudes of their maps, and partly
from a desire to fall in, if possible, with some universal time
scheme, if such could be brought forward, they fixed upon the
meridian of Greenwich as their ultimate reference line, and defined
their various hour standards as being exactly so many hours slow of
Greenwich mean time.
The decision of the United States and of Canada brought with it
later a similar decision on the part of all the principal States
of Europe; and Greenwich is not only 'Longitude Nought' for the
bulk of the civilized world, but Greenwich mean time, increased or
decreased by an exact number of hours or half-hours, is the standard
time all over the planet.
No; the statement requires correction. Two countries hold out, both
close to our own doors. France, instead of adopting Greenwich time
as such, adopts _Paris time less_ 9 m. 21 s. (that being the precise
difference in longitude between the two national observatories).
Ireland disdains even such a veiled surrender, and Dublin time is
the only one recognized from the Hill of Howth to far Valentia. So
the distressful country preserves her old grievance, that she does
not even get her time until after England has been served.
The alteration in national habits following on the adoption of
this European system has had a very perceptible effect in some
cases. Thus, Switzerland has adopted Mid-European time, one hour
fast of Greenwich; the true local time for Berne being just half
an hour later. The result of putting the working hours this thirty
minutes earlier in the day has had such a noticeable effect on
the consumption of gas, as to lead the gas company to contemplate
agitating for a return to the old system.
Thus, Greenwich time, as well as the Greenwich meridian, has
practically been adopted the world over.
It follows, then, that the determination of time is the most
important duty of the Royal Observatory; and the Time Department,
the one to which is entrusted the duty of determining, keeping, and
distributing the time, calls for the first attention.
Entering the transit room, the first thing that strikes the
visitor is the extreme solidity with which the great telescope is
mounted. It turns but in one plane, that of 'longitude nought,'
and its pivots are supported by the pair of great stone pillars
which we have already spoken of as occupying the principal part of
the transit-room area, and the foundations of which go deep down
under the surface of the hill. On the west side of the telescope,
and rigidly connected with it, is a large wheel some six feet in
diameter, and with a number of wooden handles attached to it,
resembling the steering-wheel of a large steamer. This wheel carries
the setting circle, which is engraved upon a band of silver let
into its face near its circumference, a similar circle being at the
back of the wheel nearer the pillar. Eleven microscopes, of which
only seven are ordinarily used, penetrate through the pier, and are
directed on to this second circle.
The present transit is the fourth which the Observatory has
possessed, and its three predecessors, known as Halley's, Bradley's,
and Troughton's, respectively, are still preserved and hang on
the walls of the transit room, affording by their comparison an
interesting object-lesson in the evolution of a modern astronomical
instrument.
The watcher who wishes to observe the passing of a star must note
two things: he must know in what direction to point his telescope,
and at what time to look for the star. Then, about two minutes
before the appointed time, he takes his place at the eyepiece. As
he looks in he sees a number of vertical lines across his field
of view. These are spider-threads placed in the focus of the
eye-piece. Presently, as he looks, a bright point of silver light,
often surrounded by little flashing, vibrating rays of colour, comes
moving quickly, steadily onward--'swims into his ken,' as the poet
has it. The watcher's hand seeks the side of the telescope till his
finger finds a little button, over which it poises itself to strike.
On comes the star, 'without haste, without rest,' till it reaches
one of the gleaming threads. Tap! The watcher's finger falls sharply
on the button. Some three or four seconds later and the star has
reached another 'wire,' as the spider-threads are commonly called.
Tap! Again the button is struck. Another brief interval and the
third wire is reached, and so on, until ten wires have been passed,
and the transit is over. The intervals are not, however, all the
same, the ten wires being grouped into three sets, two of three
apiece, and the third of four.
[Illustration: THE CHRONOGRAPH.]
Each tap of the observer's finger completed for an instant an
electric circuit, and recorded a mark on the 'chronograph.' This
is a large metal cylinder covered with paper, and turned by a
carefully-regulated clock once in every two minutes. Once in every
two seconds a similar mark was made by a current sent by means of
the standard sidereal clock of the Observatory. The paper cover of
the chronograph after an hour's work shows a spiral trace of little
dots encircling it some thirty times. These dots are at regular
intervals, about an inch apart, and are the marks made by the clock.
Interspersed between them are certain other dots, in sets of ten;
and these are the signals sent from the telescope by the transit
observer. If, then, one of the clock dots and one of the observer's
dots come exactly side by side, we know that the star was on one
of the wires at a given precise second. If the observer's dot comes
between two clock dots, it is easy, by measuring its distance from
them with a divided scale, to tell the instant the star was on
the wire to the tenth of a second, or even to a smaller fraction.
Whilst, since the transit was taken over ten wires, and the distance
of each wire from the centre of the field of view is known, we have
practically ten separate observations, and the average of these
will give a much better determination of the time of transit than a
single one would.
But let the watcher be ever so little too slow in setting his
telescope, or ever so little late in placing himself at his
eye-piece, and the star will have passed the wire, and as it
smoothly, resistlessly moves on its inexorable way, will tell the
tardy watcher in a language there is no mistaking, 'Lost moments can
never be recalled.' The opportunity let slip, not until twenty-four
hours have gone by will another chance come of observing that same
star.
It is the stars that are chiefly used in this determination, partly
because the stars are so many, whilst there is but one sun. If,
therefore, clouds cover the sun at the important moment of transit,
the astronomer may well exclaim, so far as this observation is
concerned, 'I have lost a day!' The chance will not be offered him
again until the following noon. But if one star is lost by cloud,
there are many others, and the chance is by no means utterly gone.
Beside, the sun enables us to tell the time only at noon; the stars
enable us to find it at various times throughout the entire night;
indeed, throughout both day and night, since the brighter stars can
be observed in a large telescope even during the day.
There are two great standard clocks at the Observatory: the mean
solar clock and the sidereal clock. The latter registers twenty-four
hours in the precise time that the earth rotates on its axis. A
'day' in our ordinary use of the term is somewhat longer than this;
it is the average time from one noon to the next, and as the earth
whilst turning round on its axis is also travelling round the sun,
it has to rather more than complete a rotation in order to bring the
sun again on to the same meridian. A solar day is therefore some
four minutes longer than an actual rotation of the earth, _i.e._ a
sidereal day, as it is called, since such rotation brings a star
back again to the same meridian.
The sidereal clock can therefore be readily checked by the
observation of star transits, for the time when the star ought to
be on the meridian is known. If, therefore, the comparison of the
transit taps on the chronograph with the taps of the sidereal clock
show that the clock was not indicating this time at the instant
of the transit, we know the clock must be so much fast or slow.
Similarly, the difference which should be shown between the sidereal
and solar clocks at any moment is known; and hence when the error of
the sidereal clock is known, that of the solar can be readily found.
It is often quite sufficient to know how much a clock is wrong
without actually setting its hands right; but it is not possible
to treat the Greenwich clock so, for it controls a number of other
clocks continually, and sends hourly signals out over the whole
country, by which the clocks and watches all over the kingdom are
set right.
In the lower computing room, below the south window, we find the
Time-Desk, the head-quarters of the Time Department. This is a very
convenient place for the department, since one of the chronometer
rooms, formerly Bradley's transit room, opens out of the lower
computing room; the transit instrument is just beyond; it is
close to the main gate of the Observatory, and so convenient for
chronometer makers or naval officers bringing chronometers or coming
for them, whilst just across the courtyard is the chronograph room,
with the Battery Basement, in which the batteries for the electric
currents are kept, and the Mean Solar Clock lobby, with the winch
for the winding of the time-ball at the head of the stairs above it.
These rooms do not exhaust the territory of the department, since it
owns two other chronometer rooms on the ground floor and first floor
respectively of the S.-E. tower.
At the time-desk means are provided for setting the clock right very
easily and exactly. Just above the desk are a range of little dials
and bright brass knobs, that almost suggest the stops of a great
organ.
Two of these little dials are clock faces, electrically connected
with the solar and sidereal standard clocks, so that, though these
clocks are themselves a good way off, in entirely different parts
of the Observatory, the time superintendent, seated here at the
time-desk, can see at once what they are indicating. Between the
two is a dial labelled 'Commutator.' From this dial a little handle
usually hangs vertically downwards, but it can be turned either
to the right or to the left, and when thus switched hard over, an
electric current is sent through to the mean solar clock. If now
we leave the computing room and cross the courtyard to the extreme
north-west corner, we find the Mean Solar Clock in a little lobby,
carefully guarded by double doors and double windows against rapid
changes of temperature. Opening the door of the clock case, we
see that the pendulum carries on its side a long steel bar, and
that this bar as the pendulum swings passes just over the upper
end of an electro-magnet. When the current is switched on at the
commutator, this electro-magnet attracts or repels the steel bar
according to the direction of the current, and the action of the
clock is accordingly quickened or retarded. To put the commutator
in action for one minute will alter the clock by the tenth of a
second. As the error of the clock is determined twice a day, shortly
before ten o'clock in the morning, and shortly before one o'clock in
the afternoon, its error is always small, usually only one or two
tenths. These two times are chosen because, though time-signals are
sent over the metropolitan area every hour from the Greenwich clock
through the medium of the Post Office, at ten and at one o'clock
signals are also sent to all the great provincial centres. Further,
at one o'clock the time balls at Greenwich and at Deal are dropped,
so that the captains of ships in the docks, on the river, or in the
Downs may check their chronometers.
The Time-Ball is dropped directly by the mean solar clock itself.
It is raised by means of a windlass turned by hand-power to the top
of its mast just before one o'clock. Connected with it is a piston
working in a stout cylinder. When the ball has reached the top of
the mast, the piston is lightly supported by a pair of catches.
These catches are pulled back by the hourly signal current, and
the piston at once falls sharply, bringing the ball with it. But
after a fall of a few feet, the air compressed by the piston acts
as a cushion and checks the fall, the ball then gently and slowly
finishing its descent. The instant of the beginning of the fall is,
of course, the true moment to be noted.
The other dials on the time-desk are for various purposes connected
with the signals. One little needle in a continual state of
agitation shows that the electric current connecting the various
sympathetic clocks of the Observatory is in full action. Another
receives a return signal from various places after the despatch
of the time-signal from Greenwich, and shows that the signal has
been properly received at the distant station, whilst all the many
electric wires within the Observatory or radiating from it are made
to pass through the great key-board, where they can be at once
tested, disconnected, or joined up, as may be required.
[Illustration: THE TIME-DESK.]
The distribution of Greenwich time over the island in this way is
thus a simple matter. The far more important one of the distribution
of Greenwich time to ships at sea is more difficult. The difficulty
lay in the construction of a clock or watch, the rate of which
would not be altered by the uneasy motion of a ship, or by the
changes of temperature which are inevitable on a voyage. Two hundred
years ago it was not deemed possible to construct a watch of
anything like sufficient accuracy. They would not even keep going
whilst they were being wound, and would lose or gain as much as a
minute in the day for a fall or rise of 10° in temperature. This
was owing to the extreme sensitiveness of the balance spring--which
takes the place in a watch of a pendulum in a clock--to the effects
of temperature. The British Government, therefore, in 1714 offered
a prize of the amount of £20,000 for a means of finding the
longitude at sea within half a degree, or, in other words, for a
watch that would keep Greenwich time correct to two minutes in a
voyage across the Atlantic. In 1735, James Harrison, the son of a
Yorkshire carpenter, succeeded in solving the problem. His method
was to attach a sort of automatic regulator to the spring which
should push the regulator over in one direction as the temperature
rose, and bring it back as it fell. This he effected by fastening
together two strips of brass and steel. The brass expanded with heat
more rapidly than the steel, and hence with a rise of temperature
the strip bent over on the steel side. This was the first germ of
the idea of making watches 'compensated for temperature;' watches,
that is, which maintain practically the same rate whether they are
in heat or cold, an idea now brought to great perfection in the
modern chronometer.
[Illustration: HARRISON'S CHRONOMETER.]
The great reward the Government had offered stimulated many men to
endeavour to solve the problem. Of these, Dr. Halley, the second
Astronomer Royal, and Graham, the inventor of the astronomical
clock, were the most celebrated. But when Harrison, then poor and
unknown, came to London in 1735, and laid his invention before them,
with an utter absence of self-seeking, and in the true scientific
spirit, they gave him every assistance.
Harrison's first four time-keepers are still preserved at the
Royal Observatory. He did not, however, receive his reward until
a facsimile of the fourth had been made by his apprentice, Larcum
Kendall. The latter is preserved at the Royal Observatory. There is
a Larcum Kendall at the Royal Institution which is said to have been
used by Captain Cook. Harrison's chronometer was sent on a trial
voyage to Jamaica in 1761, and on its return to Portsmouth in the
following year it was found that its complete variation was under
the two minutes for which the Government had stipulated.
Since Harrison's day the improvement of the chronometer has been
carried on almost to perfection, and now the care and rating of
chronometers for the Royal Navy is one of the most important duties
of the Observatory.
[Illustration: THE CHRONOMETER ROOM.]
A visitor who should make the attempt to compare a single
chronometer with a standard clock would probably feel very
disheartened when, after many minutes of comparison, he had got out
its error to the nearest second, were he told that it was his duty
to compare the entire army here collected, some five hundred or
more, and to do it not to the second, but to the nearest tenth of a
second. Practice and system make, however, the impossible easy, and
one assistant will quietly walk round the room calling out the error
of each chronometer as he passes it, as fast as a second assistant
seated at the table can enter it at his dictation in the chronometer
ledgers. The seconds beat of a clock sympathetic with the solar
standard, rings out loud and clear above the insect-like chatter
of the ticking of the hundreds of chronometers, and wherever the
assistant stands, he has but to lift his eyes to see straight before
him, if not a complete clock-face, at least a seconds dial moving in
exact accordance with the solar standard.
The test to which chronometers are subjected is not merely one of
rate, but one of rate under carefully altered conditions. Thus
they may be tried with the XII pointing in succession to the four
points of the compass, or, in the case of chronometer watches,
they may be laid flat down on the table or hung from the ring or
pendant, or with the ring right or left, as it would be likely to
be when carried in the waistcoat pocket. But the chief test is the
performance of a chronometer when subjected to considerable heat
for a long period. This is a matter of great consequence, since
a chronometer travelling from England to India, Australia, or the
Cape, would necessarily be subjected to very different conditions
of temperature from those to which it would be exposed in England.
They are therefore kept for eight weeks in a closed stove at a
temperature of about 85° or 90°. At one time a cold test was also
applied, and Sir George Airy, the late Astronomer Royal, in one of
his popular lectures, drew a humorous comparison between the unhappy
chronometers thus doomed to trial, now in heat and now in frost,
and the lost spirits whom Dante describes as alternately plunged
in flame and ice. The cold test has, however, been done away with.
It is perfectly easy on the modern ship to keep the chronometer
comfortably warm even on an Arctic expedition. The elaborate cold
testing applied to Sir George Nares' chronometers before he started
on his polar journey was found to have been practically quite
superfluous; the chronometers were, if anything, kept rather too
warm. The exposure of the chronometer in the cooling box, moreover,
was found to be attended with a risk of rusting its springs.
[Illustration: THE CHRONOMETER OVEN.]
Once the determination of the longitude at sea became possible,
it was clearly the next duty to fix with precision the position
of the principal places, cities, ports, capes, islands, the world
over. Of all the work done in this department none has ever been
done better, in proportion to the means at command, than that
accomplished by Captain Cook in his celebrated three voyages. As has
already been pointed out, it is the extent and thoroughness of the
hydrographic surveys of the British Admiralty which have largely
contributed to the honour done to England by the international
selection of the English meridian, and of English standard time, as
in principle those for the whole civilized world. The generosity and
public spirit therefore which led the second Astronomer Royal to
help forward and support his rival, has almost directly led to this
great distinction accruing to the Observatory of which he was the
head.
Three different methods have successively been used in the
determination of longitudes of distant places. In each case the
problem required was to ascertain the time at the standard place,
say Greenwich, at the same time that it was being determined in
the ordinary way at the given station. One method of ascertaining
Greenwich time when at a distance from it was, as stated in Chapter
I., to use the moon, as it were, as the hand of a vast clock, of
which the sky was the face and the stars the dial figures. This is
the method of 'lunar distances,' the distances of the moon from a
certain number of bright stars being given in the _Nautical Almanac_
for every three hours of Greenwich time.
As chronometers were brought to a greater point of perfection, it
was found easier and better in many cases to use 'chronometer runs,'
that is, to carry backwards and forwards between the two stations
a number of good chronometers, and by constant comparison and
re-comparison to get over the errors which might attach to any one
of them.
[Illustration: THE TRANSIT PAVILION.
(_From a photograph by Mr. Lacey._)]
But of late years another method has proved available. Distant
nations are now woven together across thousands of miles of ocean
by the submarine telegraph. The American reads in his morning
paper a summary of the debates of the previous night in the House
of Commons at Westminster. The Londoner watches with interest
the scores of the English cricket team in Australia. It is now
therefore possible for an astronomer in England to record, should
he so desire, the time of the transit of a star across the wires
of his instrument, not only on his own chronograph, but upon that
of another observatory, it may be 2000 miles away. Or, much more
conveniently, each observer may independently determine the error of
his own clock, and then bring his clock into the current, so that
it may send a signal to the chronograph of the other station.
In one way or another this work of the determination of geographical
longitudes has been an important part of the extra-routine work
at Greenwich, part of the work which has built up and sustained
its claim to define 'longitude nought'; and many distinguished
astronomers, especially from the leading observatories of the
Continent, have come here from time to time to obtain more
accurately the longitude of their own cities. The traces of their
visits may be seen here and there about the Observatory grounds in
flat stones which lie level with the surface, and bear a name and
date like the gravestones in some old country churchyard. These are
not, as one might suppose, to mark the burial-places of deceased
astronomers, but record the sites where, on their visits for
longitude purposes, different foreign astronomers have set up their
transit instruments. Now, however, a permanent pier has been erected
in the courtyard, and a neat house--the Transit Pavilion--built over
it, so that in all probability no fresh additions will be made to
these sepulchral-looking little monuments.
It might be asked, What reason is there for a foreign observer to
come over to England for such a purpose? Would it not be sufficient
for the clock signals to be exchanged? But a curious little fact
has come out with the increase of accuracy of transit observation,
and that is, that each observer has his own particular habit or
method of observation. A hundred years ago, Maskelyne, the fifth
Astronomer Royal, was greatly disturbed to find that his assistant,
David Kinnebrook, constantly and regularly observed a star-transit
a little later than he did himself. The offender was scolded,
warned, exhorted, and finally, when all proved useless to bring
his observations into exact agreement with the Astronomer Royal's,
dismissed as an incompetent observer. As a matter of fact, poor
Kinnebrook has a right to be regarded as one of the martyrs of
science, and Maskelyne, by this most natural but mistaken judgment,
missed the chance of making an important discovery, which was not
made until some thirty years later. Astronomers now would be more
cautious of concluding that observations were bad simply because
they differed from what had been expected. They have learnt by
experience that these unexpected differences are the most likely
hunting-ground in which to look for new discoveries.
In a modern transit observation with the use of the chronograph
it will be seen at once that before the observer can register a
star-transit on the chronograph, he has to perceive with his eye
that the star has reached the wire, he has to mentally recognize
the fact, and consciously or unconsciously to exert the effort of
will necessary to bring his finger down on the button. A very slight
knowledge of character will show that this will require different
periods of time for different people. It will be but a fraction
of a second in any case, but there will be a distinct difference,
a constant difference, between the eager, quick, impulsive man
who habitually anticipates, as it were, the instant when he sees
star and wire together, and the phlegmatic, slow-and-sure man
who carefully waits till he is quite sure that the contact has
taken place, and then deliberately and firmly records it. These
differences are so truly personal to the observer that it is quite
possible to correct for them, and after a given observer's habit has
become known, to reduce his transit times to those of some standard
observer. It must, of course, be remembered that this 'personal
equation' is an exceedingly minute quantity, and in most cases is
rather a question of hundredths of seconds than of tenths.
It will be seen from the foregoing description how little of what
may be termed the picturesque or sensational side of astronomy
enters into the routine of the Time Department, the most important
of all the departments of the Observatory. The daily observation of
sun and of many stars--selected from a carefully chosen list of some
hundreds, and known as 'clock stars'--the determination of the error
of the standard clock to the hundredth of a second if possible, and
its correction twice a day, the sending out of time signals to the
General Post Office and other places, whence they are distributed
all over the country; the care, winding, and rating of hundreds of
chronometers and chronometer watches, and from time to time the
determination of the longitude of foreign or colonial cities, make
up a heavy, ceaseless routine in which there is little opportunity
for the realization of an astronomer's life as it is apt to be
popularly conceived.
Yet there is interest enough in the work. There is the charm
which always attaches to work of precision, the delight of using
delicate and exact instruments, and of obtaining results of steadily
increasing perfection. It may be akin to the sporting passion for
record-breaking, but surely it is a noble form of it which has led
the assistants, in recent years, to steadily increase the number of
observations in a normal night's work up to the very limit, taking
care the while that their accuracy has in no degree suffered. In
longitude work also 'the better is the enemy of the good,' and there
is the ambition that each fresh determination shall be markedly
more precise than all that have preceded it. The constant care of
chronometers soon reveals a kind of individuality in them which
forms a fresh source of interest, whilst if a man has but a spark
of imagination, how easily he will wrap them round with a halo of
romance!
Glance through the ledgers, and you will see how some of them have
heard the guns at the siege of Alexandria, others have been carried
far into the frozen north, others have wandered with Livingstone or
Cameron in the trackless forests of equatorial Africa.
More striking still are those pages across which the closing line
has been drawn; never again will the time-keeper there scheduled
return to the kindly inquisition of Flamsteed Hill. This sailed away
in the Wasp, and was swallowed up in the eastern typhoon; that went
down in the sudden squall that smote the Eurydice off the Isle of
Wight; these foundered with the Captain. The last fatal journey of
Sir John Franklin to find the North-West Passage leaves its record
here; the chronometers of the Erebus and Terror will never again
appear on the Greenwich muster roll. Land exploration claims its
victims too. Sturt's ill-fated expedition across Australia, and
Livingstone's last wandering, are represented.
[Illustration: 'LOST IN THE BIRKENHEAD.']
Sometimes an amusing entry interrupts the silent pathos of these
closed pages. 'Lost by Mr. Smith on the coast of Africa,' reads at
first sight like a rather thin attempt of some one to shift the
responsibility of his own carelessness on to the broad shoulders of
Mr. Nobody. In reality it probably gives a hint of the necessary,
dangerous, and exciting work of slave-dhow chasing which gives
employment to our ships on the African coast. 'Mr. Smith' was no
doubt a petty officer who was told off to carry the chronometer for
a boat's crew sent to search for a slave-dhow up some equatorial
estuary. Probably the dhow was found, and the Arabs who manned it
gave so stout a resistance that 'Mr. Smith' and his men had other
things to do than take care of chronometers before they could
overcome them. We may take it that the real story outlined here was
one of courage and hard fighting, not of carelessness and shirking.
Stories of higher valour and nobler courage yet are also hinted:
the calm discipline of the crew of the Victoria as she sank from
the ram of the Camperdown, the yet nobler devotion of the men of
the Birkenhead, as they formed up in line on deck and cheered the
boats that bore away the women and children to safety, whilst they
themselves went down with the ship into the shark-crowded sea.
'There rose no murmur from the ranks, no thought
By shameful strength, unhonoured life to seek;
Our post to quit we were not trained, nor taught
To trample down the weak.
'What followed, why recall? The brave who died
Died without flinching in that bloody surf.
They sleep as well beneath that purple tide
As others under turf.'
CHAPTER VII
THE TRANSIT AND CIRCLE DEPARTMENTS
The determination of time is a duty the importance of which readily
commends itself to the general public. It is easy to see that in any
civilized country it is very necessary to have an accurate standard
of time. Our railways and telegraphs make it quite impossible for
us to be content with the rough-and-ready sun-dial which satisfied
our forefathers. But it should be remembered that it was neither to
establish a 'longitude nought,' nor to create a system of standard
time, that Greenwich Observatory was founded in 1675. It was for
'The Rectifying the Tables of the Motions of the Heavens and the
Places of the Fixed Stars, in order to find out the so-much-desired
Longitude at Sea for the perfecting the Art of Navigation.'
The two related departments, therefore, those of the Transit and the
Circle, which are concerned in the work of making star-catalogues,
come next in order to the Time Department. Though both departments
deal with the same instrument, the transit circle, they are at
present placed at opposite ends of the Observatory domain; the
Circle Department being lodged in the upper computing room of the
old building; the Transit Department in the south wing of the New
Observatory in the south ground.
It may be asked why, if this were the purpose of the Observatory
at its foundation, two and a quarter centuries ago; if, as was
the case, the work was set on foot from the beginning and was
carried out with every possible care, how comes it that it is still
the fundamental work of the Observatory, and, instead of being
completed, has assumed greater proportions at the present day than
ever before?
The answer to this inquiry may be found in a special application of
the old proverb, originally directed against the discontent of man:
'The more he has, the more he wants.' For, however paradoxical it
may seem, it is true that the fuller a star-catalogue is, and the
more accurate the places of the stars that it contains, the greater
is the need for a yet fuller catalogue, with places more accurate
still.
It is worth while following up this paradox in some detail, as
it affords a very instructive example of the way in which a work
started on purely utilitarian grounds extends itself till it crosses
the undefined boundary and enters the region of pure science.
We have no idea who made the earliest census of the sky. It is
written for us in no book; it is not even engraved on any monument.
And yet no small portion of it is in our hands to-day, and,
strangest of all, we are able to fix fairly closely the time at
which it was made, and the latitude in which its compiler lived. The
catalogue is very unlike our star-catalogues of to-day. The places
of the stars are very roughly indicated; and yet this catalogue has
left a more enduring mark than all those that have succeeded it. The
catalogue simply consists of the star names.
An old lady who had attended a University Extension lecture
on astronomy was heard to exclaim: 'What wonderful men these
astronomers are! I can understand how they can find out how far
off the stars are, how big they are, and what they weigh--that is
all easy enough; and I think I can see how they find out what they
are made of. But there is one thing that I can't understand--I
don't know how they can find out what are their names!' This same
difficulty, though with a much deeper meaning than the old lady in
her simplicity was able to grasp, has occurred to many students of
astronomy. Many have wished to know what was the meaning of, and
whence were derived, the sonorous names which are found attached to
all the brighter stars on our celestial globes: Adhara, Alderamin,
Betelgeuse, Denebola, Schedar, Zubeneschamal, and many more. The
explanation lies here. Some 5000 years ago, a man, or college of
men, living in latitude 40° north, in order that they might better
remember the stars, associated certain groups of them with certain
fancied figures, and the individual star names are simply Arabic
words designed to indicate whereabouts in its peculiar figure or
constellation that special star was situated. Thus Adhara means
'back,' and is the name of the bright star in the back of the great
Dog. Alderamin means 'right arm,' and is the brightest star in the
right arm of Cepheus, the king. Betelgeuse is 'giant's shoulder,'
the giant being Orion; Denebola is 'lion's tail.' Schedar is the
star on the 'breast' of Cassiopeia, and Zubeneschamal is 'northern
claw,' that is, of the Scorpion. So far is clear enough. The names
of the stars for the most part explain themselves; but whence the
constellations derived their names, how it was that so many snakes
and fishes and centaurs were pictured out in the sky, is a much more
difficult problem, and one which does not concern us here.
One point, however, these old constellations do tell us, and tell
us plainly. They show that the axis of the earth, which, as the
earth travels round the sun, moves parallel with itself, yet, in
the course of ages, itself rotates so as in a period of some 26,000
years to trace out a circle amongst the stars. This is the cause of
what is called 'precession,' and explains how it is that the star
we call the pole-star to-day was not always the pole-star, nor will
always remain so. We learn this fact from the circumstance that
the old constellations do not cover the entire celestial sphere.
They leave a great circular space of 40° radius unmapped in the
southern heavens. This simply means that the originators of the
constellations lived in 40° north latitude, and stars within 40° of
their south pole never rose above their horizon, and consequently
were never seen, and could not be mapped, by them. In like manner,
the star census taken at Greenwich Observatory does not include
the whole sky, but leaves a space some 52° in radius round our
south pole. Since the latitude of Greenwich is nearly 52° north,
stars within that distance of the south pole do not rise above our
horizon, and are never seen here. But if we compare the vacant space
left by the old original constellations with the vacant space left
by a Greenwich catalogue of to-day, we see that the centre of the
first space, which must have been the south pole of that time, is
a long way from the centre of the second space--our south pole of
to-day. The difference tells us how far the pole has moved since
those old forgotten astronomers did their work. We know the rate
at which the pole appears to move, by comparing our more modern
catalogues one with another; and so we are able to fix pretty nearly
the time when lived those old first census-takers of the stars,
whose names have perished so completely, but whose work has proved
so immortal.
These old workers gave us the constellation groupings and names
which still remain to us, and are still in common, every-day use.
Their work affords us the most striking illustration of the result
of precession, but precession itself was not recognized till nearly
3000 years after their day, when a marvellous genius, Hipparchus,
established the fact, and 'built himself an everlasting name' by
the creation of a catalogue of over 1000 stars prepared on modern
principles. That catalogue formed the basis of one which survives to
us at the present time, and was made some 1750 years ago by Claudius
Ptolemy, the great astronomer of Alexandria, whose work, which still
bears the proud name of _Almagest_, 'The Greatest,' remained for
fourteen centuries the one universal astronomical text-book.
A modern catalogue contains, like that of Ptolemy, four columns
of entry. The first gives the star's designation; the second
an indication of its brightness; the third and fourth the
determinations of its place. These are expressed in two directions,
which, in modern catalogues, not in Ptolemy's, correspond on the
celestial sphere to longitude and latitude on the terrestrial.
Distance north or south of the celestial equator is termed
'declination,' corresponding to terrestrial latitude. Distance in
a direction parallel to the equator is termed 'right ascension,'
corresponding to terrestrial longitude. For geographical purposes
we conceive the earth to be encircled by two imaginary lines at
right angles to each other--the one, the equator, marked out for us
by the earth itself; the other, 'longitude nought,' the meridian
of Greenwich, fixed for us by general consent, after the lapse of
centuries, by a kind of historical evolution. On the celestial globe
in like manner we have two fundamental lines--one, the celestial
equator, marked out for us by nature; the other at right angles to
it, and passing through the poles of the sky, adopted as a matter
of convenience. But a difficulty at once confronts us--Where can
we fix our 'right ascension nought'? What star has the right to be
considered the Greenwich of the sky?
The difficulty is met in the following manner: For six months of
the year, the summer months, the sun is north of the celestial
equator; for the other six months of the year, the months of winter,
it is south of it. It crosses the equator, therefore, twice in
the year--once when moving northward at the spring equinox; once
when moving southward at the equinox of autumn. The point where it
crosses the equator at the first of these times is taken as the
fundamental point of the heavens, and the first sign of the zodiac,
Aries the Ram, is said to begin here, and it is called, therefore,
'the first point of Aries.'
One of the very first facts noticed in the very early days of
astronomy was that, as the stars seemed to move across the sky night
by night, they seemed to move in one solid piece, as if they were
lamps rigidly fixed in one and the same solid vault. Of course it
has long been perfectly understood that this apparent movement was
not in the least due to any motion of the stars, but simply to the
rotation of the earth on its axis. This rotation is the smoothest,
most constant, and regular movement of which we know. It follows,
therefore, that the interval of time between the passage of one star
across the meridian of Greenwich and that of any other given star
is always the same. This interval of time is simply the difference
of their right ascension. If we are able, then, to turn our transit
instrument to the sun, and to a number of stars, each in its proper
turn, and by pressing the tapping-piece on the instrument as the
sun or star comes up to each of the ten wires in succession, to
record the times of its transit on the chronograph, we shall have
practically determined their right ascensions--one of the two
elements of their places.
The other element, that of declination, is found in a different
manner. The celestial equator, like the terrestrial, is 90° from the
pole. The bright star Polaris is not exactly at the north pole, but
describes a small circle round it. Twice in the twenty-four hours
it transits across the meridian--once when going from east to west
it passes above the pole, once when going from west to east below
the pole. The mean between these two altitudes of Polaris above the
horizon gives the position of the true pole.
[Illustration: THE TRANSIT CIRCLE.]
A complete transit observation of a star consists therefore of two
operations. The observer, as we have already described, sees a
star entering the field of the telescope, and as it swims forward,
he presses the galvanic button, which sends a signal to the
chronograph as the star comes up to each of the ten vertical wires
in succession. But, beside the ten wires, there are others. Two
vertical wires lie outside the ten of which we have already spoken,
and there is also a horizontal wire. The latter can be moved by a
graduated screw-head just above the eye-piece, and as the star comes
in succession to these two vertical wires, this horizontal wire is
moved by the screw-head, so as to meet the star at the moment it
is crossing the vertical wire, and the observer presses a second
little button, which records the position of the horizontal wire on
a small paper-covered drum. Then, the transit over, the observer
leaves the telescope and comes round to the outside of the west
pier. Here he finds seven large microscopes, which pierce the whole
thickness of the pier, and are directed towards the circumference
of a large wheel which is rigidly attached to the telescope and
revolves with it. This wheel is six feet in diameter, and has a
silver circle upon both faces. Each circle is divided extremely
carefully into 4320 divisions--these divisions, therefore, being
about the one-twentieth of an inch apart. There are, therefore,
twelve divisions to every degree (12 × 360 = 4320), and each
division equals five minutes of arc. The lowest microscope is the
least powerful, and shows a large part of the circle, enabling the
observer to see at once to what degree and division of a degree
the microscope is pointing. The other six microscopes are very
carefully placed 60° apart--as equally placed as they possibly can
be. These microscopes are all fitted with movable wires--wires moved
by a very fine and delicate screw; the screw-head having divisions
upon it so that the exact amount of its movement can be told. Each
of the six screw-heads will read to the one five-thousandth part
of a division of the circle; in other words, to the one hundred
thousandth part of an inch. Using all six microscopes, and taking
their mean, we are able to _read_ to the one-hundredth of a second
of arc. If, therefore, the observations could be made with perfect
certainty down to the extremest nicety of reading which the
instrument supplies, we should be able to read the declination of a
star to this degree of refinement. It may be added that a halfpenny,
at a distance of three miles, appears to be one second of arc in
diameter; at three hundred miles it would be one-hundredth of a
second. It need scarcely be said that we cannot observe with quite
such refinement of exactness as this would indicate. Nevertheless,
this exactness is one after which the observer is constantly
striving, and tenths, even hundredths, of seconds of arc are
quantities which the astronomer cannot now neglect.
The observer has then to read the heads of all these seven
microscopes on the pier side, and also two positions of the
horizontal wire on the screw-head at the eye-piece. The following
morning he will also read off from the chronograph-sheet the times
when he made the ten taps as the star passed each of the ten
vertical wires. There are, therefore, nine entries to make for one
position of a star in declination, and ten for one position of a
star in right ascension. The observer will also have to read the
barometer to get the pressure of the air at the time of observation,
and one thermometer inside the transit room, and another outside,
to get the temperature of the air. In some cases thermometers at
different heights in the room are also read. A complete observation
of a single star means, therefore, the entry of two-and-twenty
different numbers.
It may be asked, What is the use of reading the barometer and
thermometer? The answer to the question can only be given by
contradicting a statement made above, that the true pole lay midway
between the position of the telescope when pointing to the pole-star
at its upper transit, and its position when pointing to it at its
lower transit. The pole being very high in the heavens in this
country, there are a great number of stars that, like the pole-star,
cross the meridian twice in the twenty-four hours--once when they
pass above the pole, moving from east to west, once when they pass
below it, moving from west to east As the real distance of a star
from the true pole does not alter, it follows that we ought to get
the position of the pole from the mean of the two transits of any of
these stars, and they ought all to exactly agree with each other.
But they do not. So, too, I said that the stars all appeared to move
as in a single piece. If, then, we constructed an instrument with
its axis parallel to the axis of the earth, and fixed a telescope to
it, pointing to any particular star, if we turn the telescope round
as fast from east to west as the earth itself is turning from west
to east--if we built an equatorial, that is to say--we ought to find
that the star once in the centre of the field would remain there. As
a matter of fact, when the star got near the horizon it would soon
be a long way from the centre of the field.
Sir George Airy, the seventh Astronomer Royal, makes, with reference
to this very point, the following remarks:
'Perhaps you may be surprised to hear me say the rule is
established as true, and yet there is a departure from it.
This is the way we go on in science, as in everything else;
we have to make out that something is true, then we find out
under certain circumstances that it is not quite true; and
then we have to consider and find out how the departure can be
explained.'
In this particular case, the disturbing cause is found in the
action of our own atmosphere. The rays of light from the star are
bent out of a perfectly straight course as they pass through the
various layers of that atmosphere, layers which necessarily become
denser the closer we get to the actual surface of the earth. Every
celestial body therefore appears to be a little higher in the sky
than it really is. This action is most noticeable at the horizon,
where it amounts to about half a degree. As both sun and moon are
about half a degree in diameter, it follows that when they have
really just entirely sunk below the horizon they appear to be just
entirely above it. It happens, in consequence, on rare occasions,
that an eclipse of the moon will take place when both sun and moon
are together seen above the horizon.
It was a great matter to discover this effect of refraction. It
was soon seen that it was not constant, that it varied with both
temperature and pressure. It is, indeed, the most troublesome of all
the hindrances to exact observation with which the astronomer has
to contend; partly because of its large amount--half a degree, as
has been already said, in the extreme case--and partly because it is
difficult in many cases to determine its exact effect.
The double observation with the transit circle gives us, then, the
place in the sky where the star _appeared_ to be at the moment of
observation, not its true place; to find that true place we have
to calculate how much refraction had displaced the star at the
particular height in the sky, and at the particular temperature and
atmospheric pressure at which the observation was made.
[Illustration: THE MURAL CIRCLE.]
The transit circle is a comparatively recent instrument. In earlier
times the two observations of right ascension and declination were
entrusted to perfectly separate instruments. The transit instrument
was mounted as the transit circle is, between two solid stone piers,
and moved in precisely the same way. But the great six-foot wheel,
which was made as stiff as it possibly could be, was mounted on
the face of a great stone pier or wall, from which circumstance it
was called the 'mural circle,' and a light telescope was attached
to it which turned about its centre. This arrangement had a double
disadvantage--that the two observations had to be made separately,
and the mural circle, not being a symmetrical instrument, was
liable to small errors which it was difficult to detect. Thus, being
supported on one side only, a flexure or bending outwards of either
telescope or circle, or both, might be feared.
It was for this reason that Pond set up a pair of mural circles,
one on the east side of its supporting pier and the other on
the west.[3] His plan was not only to have each star observed
simultaneously in the two instruments, a plan by which, at the cost
of some additional labour, he would have got rid, to a large extent,
of the individual errors of the two separate instruments, inasmuch
as, on the whole, it might have been expected that the errors of
the two instruments would have been very nearly equal in amount,
but of opposite character. The differences, too, between the two
instruments would have afforded the means for tracing these small
errors to their respective causes, and so ascertaining the laws to
which they were subject.
[3] The second circle was intended for the Cape Observatory, but
Pond obtained leave to retain it. In 1851 it was transferred to the
Observatory of Queen's College, Belfast.
Pond went further still. He added to the mural circle a simple
instrument, the extreme value of which every astronomer recognizes
to-day--the mercury trough. Not only was the star to be observed by
both circles when the two telescopes were pointing directly to it,
it was also to be observed by reflection; the telescopes were to
be pointed down towards a basin of mercury, in which the image of
the star would be seen reflected. The mercury being a liquid, its
surface is perfectly horizontal; and, since the law of reflection
is that the angle of incidence is equal to the angle of reflection,
it follows that the telescope, when pointed down toward the mercury
trough, points just at as great an angle below the horizon as, when
it is set directly on the star, it points above it. If the circle,
therefore, be carefully read at both settings, half the difference
between the two readings will give the angular elevation of the
star above the horizon. A combination, therefore, of all four
observations, that is to say, one reflection and one direct with
each of the telescopes, would give an exceedingly exact value for
the star's altitude. The conception of this method gives a striking
idea of Pond's thoroughness and skill as a practical observer, and
it is a distinct blot upon Airy's justly high reputation in the same
line that he discontinued the system upon his accession to office.
However, in 1851, as already mentioned, Airy substituted for the
two separate instruments--the transit and mural circle--the transit
circle, which, unlike the mural circle, is equally supported on both
sides. This, however, does not free it from the liability to some
minute flexure in the direction of its length, from the weight of
its two ends, and the mercury trough is used for the detection of
such bending, should it exist. The present practice is to observe
a star both by reflection and directly in the course of the same
transit. The observer sets the telescope carefully before ever the
star comes into the field of view, and reads his seven microscopes.
Then he climbs up a narrow wooden staircase and watches the star
transit nearly half across the field. Then comes a rush, the
observer swings himself down the ladder, unclamps the telescope,
turns it rapidly up to the star itself, clamps it again, flings
himself on his back on a bench below the telescope, and does it so
quickly that he is able to observe the star across the second half
of the field. There is no time for dawdling, no room for making any
mistakes; the stars never forgive; 'they haste not, they rest not;'
and if the unfortunate observer is too slow, or makes some slip in
his second setting, the star, cold and inexorable, takes no pity,
and moves regardless on.
It will be seen that a considerable amount of work is involved
in taking a single observation of a star-place. But in making a
star-catalogue it is always deemed necessary to obtain at least
three observations of each star; and many are observed much more
frequently.
A modern star-catalogue contains, like Ptolemy's, four columns. It
contains also several more. Of these the principal are devoted to
the effect of precession. As precession is caused by a movement
of the earth's axis making the pole of the sky seem to describe a
circle in the heavens, it follows that the celestial poles, and the
celestial equator with them are slowly, but continually, changing
their place with respect to the stars, and therefore that the
declinations of the stars are always undergoing change, and as the
equator changes, the point where the sun crosses it in spring--the
first point of Aries--changes also, and with it the stars' right
ascensions.
To make one determination of a star's place comparable with
another made at another time, it is clear that we must correct for
the effects of precession in the interval of time between the two
observations, and for the effects of refraction. But observations
made with the transit circle must also be corrected for errors
in the instrument itself. The astronomer will see to it that his
instrument is made and is set up as perfectly as possible. The
pivots on which it turns must be exactly on the same level; they
must point exactly east and west, and the axis of the telescope
must be exactly at right angles to the line joining the pivots
in all positions of the instrument. These conditions are very
nearly fulfilled, but never absolutely. Day by day, therefore, the
astronomer has to ascertain just how much his instrument is in error
in each of these three matters. Were his instrument absolutely
without error to-day, he could not assume that it would remain so,
nor, if he had measured the amount of its errors yesterday, would it
be safe to assume that those errors would not change to-day.
In the examination of these sources of error the mercury trough
comes again into use. The transit circle is turned directly
downwards, and the mercury trough brought below it. A light is
so arranged as to illuminate the field of the telescope, and the
observer, looking in, sees the ten transit wires and the one
declination wire, and also sees their images reflected from the
surface of the mercury. If the telescope be pointing _exactly_
down towards the surface of the mercury, then the image of the
declination wire will fall exactly on the declination wire
itself, and by reading the circle we can tell where the zenith
point of the circle is. Similarly, if the pivots of the telescope
are precisely on the same level, the centre wire of the right
ascension series would coincide with its reflected image. A third
point is determined by looking through the eye-piece of the north
collimator telescope--that is to say, the telescope mounted in a
horizontal position at the north end of the room--at the spider
lines in the focus of the south collimator. In order to get this
view, the transit telescope has either to be lifted up out of its
usual position, or else the middle of the tube has to be opened.
The spider lines in the north collimator are then made to coincide
with the image of the wires of the south collimator. The transit
telescope is then turned first to one collimator, then to the other,
and the central wire of the right ascension series is turned till it
coincides with the wire of the collimator; the amount by which it
has to be moved giving an index of the error of collimation; that is
to say, of the deviation of the optical axis of the telescope from
perpendicularity to the line joining the pivots.
I have said enough to show that the making of an observation is
a small matter as compared with those corrections which have to
be made to it afterwards, before it is available for use. But I
have only mentioned some of the reductions and corrections which
have to be made. There are several more, and it is a just pride of
Greenwich that her third ruler, Bradley, as has been already told
in the notice of his life, discovered two of the most important.
The one, aberration, is due to the fact that light, though it moves
so swiftly--186,000 miles per second--yet does not move with an
infinitely greater velocity than that of the earth. The other,
nutation, might be called a correction to precession, inasmuch as,
moved by the moon's attraction, the earth's axis does not swing
round smoothly, but with a slight nodding or staggering motion.
But when these observations of the places of a star have been made,
and have been properly 'reduced,' even then we do not find an
exact correspondence between two different determinations. Little
differences still remain. Some of these are to be accounted for by
changes in the actual crust of the earth, which, solid and stable as
we think it, is yet always in motion. Professor Milne, our greatest
authority on earth movements, says, 'The earth is so elastic that
a comparatively small impetus will set it vibrating; why, even two
hills tip together when there is a heavy load of moisture in the
valley between them. And then, when the moisture evaporates in a
hot sun, they tip away from each other.' So there is a perceptible
rocking to and fro even of the huge stone piers of a transit
circle, as seasons of rain and drought, heat and cold, follow each
other. More than that, the earth is so sensitive to pressure that
it was found, at the Oxford University Observatory, that there was
a distinct swaying shown by a horizontal pendulum when the whole
of a party of seventy-six undergraduates stood on one side of the
instrument and close up to it, from the position it had when the
party stood ninety feet away. More wonderful still, a comparison of
the star-places, obtained at a number of observatories, including
Greenwich, has shown that the earth is continually changing her axis
of rotation. And so the star-places determined at Greenwich have
shown that the north pole of the earth, 2600 miles away, moves about
in an irregular curve about thirty feet in radius.
Nothing is stable, nothing is immovable, nothing is constant. The
astronomer even finds that his own presence near the instrument is
sufficient to disturb it.
The great interest attaching to transit-circle work is this
striving after ever greater and greater precision, with the result
of bringing out fresh little discordances, which, at first sight,
appear purely accidental, but which, under further scrutiny, show
themselves to be subject to some law. Then comes the hunt for this
new unknown law. Its discovery follows. It explains much, but when
it is allowed for, though the observations now come much closer
together, little deviations still remain, to form the subject of a
fresh inquiry. Astronomy has well been called the exact science, and
yet exactitude ever eludes its pursuer.
If it be asked, 'What is the use of this ever-increasing refinement
of observation?' no better answer can be given than the words of
Sir John Herschel in one of his Presidential addresses to the Royal
Astronomical Society:--
'If we ask to what end magnificent establishments are maintained
by States and sovereigns, furnished with masterpieces of art,
and placed under the direction of men of first-rate talent and
high-minded enthusiasm, sought out for those qualities among the
foremost in the ranks of science, if we demand, _cui bono?_
for what good a Bradley has toiled, or a Maskelyne or a Piazzi
has worn out his venerable age in watching?--the answer is,
Not to settle mere speculative points in the doctrine of the
universe; not to cater for the pride of man by refined inquiries
into the remoter mysteries of nature; not to trace the path
of our system through space, or its history through past and
future eternities. These, indeed, are noble ends, and which I
am far from any thought of depreciating; the mind swells in
their contemplation, and attains in their pursuit an expansion
and a hardihood which fit it for the boldest enterprise. But
the direct practical utility of such labours is fully worthy of
their speculative grandeur. The stars are the landmarks of the
universe; and, amidst the endless and complicated fluctuations
of our system, seem placed by its Creator as guides and records,
not merely to elevate our minds by the contemplation of what
is vast, but to teach us to direct our actions by reference to
what is immutable in His works. It is, indeed, hardly possible
to over-appreciate their value in this point of view. Every
well-determined star, from the moment its place is registered,
becomes to the astronomer, the geographer, the navigator, the
surveyor, a point of departure which can never deceive or fail
him, the same for ever and in all places; of a delicacy so
extreme as to be a test for every instrument yet invented by
man, yet equally adapted for the most ordinary purposes; as
available for regulating a town clock as for conducting a navy
to the Indies; as effective for mapping down the intricacies of
a petty barony as for adjusting the boundaries of Transatlantic
empires. When once its place has been thoroughly ascertained
and carefully recorded, the brazen circle with which that
useful work was done may moulder, the marble pillar may totter
on its base, and the astronomer himself survive only in the
gratitude of posterity; but the record remains, and transfuses
all its own exactness into every determination which takes it
for a groundwork, giving to inferior instruments--nay, even to
temporary contrivances, and to the observations of a few weeks
or days--all the precision attained originally at the cost of so
much time, labour, and expense.'
But for these strictly utilitarian purposes a comparatively small
number of stars would meet our every requisite. In the narrow sense
of which Sir John Herschel is here speaking, we have no use for
anything beyond the smallest of catalogues; and if the question
before us is, 'Why are we continually extending our catalogues?' the
following words of a more recent writer[4] on the subject will set
forth the real explanation:--
'A word in conclusion, suggested by the history of
star-catalogues. We have no difficulty in understanding that it
is necessary to study the planets, and a reasonable number of
the brighter stars, for the purpose of determining the figure
of the earth, and the positions of points upon its surface; but
the use for a catalogue of ten thousand stars, such as La Caille
compiled, is not just so apparent. Nay, what did Ptolemy want
with a thousand stars, or Tamerlane's grandson, born, reared,
and destined to die amidst a horde of savages, however splendid
in their trappings? There is not, and there never was, any
real, practical use for the great volumes of star-catalogues
that weigh down the shelves of our libraries. The navigator
and explorer need never see them at all. Why, then, were these
pages compiled? Why have astronomers, from Hipparchus's time
to the present, spent their lives in the weary routine-work of
observing the places of tiny points in the stellar depths? Does
it not seem that there is something in the mind of man that
impels him to seek after knowledge--truly--for its own sake?
something heaven-born, heaven-nurtured, God-given ... that there
is something in man common to him and his Creator, and therefore
eternal ... in beautiful accord with the plain statement that
"God made man in His own image?"'
[4] Mr. Thomas Lindsay, _Transactions of the Astronomical and
Physical Society of Toronto_, 1899, p. 17.
CHAPTER VIII
THE ALTAZIMUTH DEPARTMENT
The determining of the places of the fixed stars which Flamsteed
carried out so efficiently in his _British Catalogue of Stars_--the
first 'Census of the Sky' made with the aid of a telescope--was but
half of the work imposed upon him. The other half, equally necessary
for the solution of the problem of the longitude at sea, was the
'Rectifying the Tables of the Motions of the Heavens.'
This second duty was not less necessary than the other, for, if we
may again use the old simile of the clock-face, the fixed stars
may be taken to represent the figures on the vast dial of the sky,
whilst the moon, as it moves amongst them, corresponds to the moving
hand of the timepiece. To know the places of the stars, then,
without being able to predict the place of the moon, would be much
like having a clock without its hands. But if not less necessary,
it was certainly more difficult. The secret of the movements of the
moon and planets had not then been grasped, and the only tables
which had been calculated were based upon observations made before
the days of telescopes.
It is one of the most fortunate and remarkable coincidences in the
whole history of science, that at the very time that Greenwich
Observatory was being called into existence, the greatest of
all astronomers was working out his demonstration of the great
fundamental law of the material universe--the law that every
particle of matter attracts every other particle with a force which
varies directly with the mass and inversely with the square of the
distance.
Several other of the great minds of that time, in particular
Dr. Hooke, the Gresham Professor of Astronomy, had seen that
it was possible that some such law might supply the secret of
planetary motion; but it is one thing to make a suggestion, and
a very different matter indeed to be able to demonstrate it; and
the latter was in Newton's power alone. He did much more than
demonstrate it--he brought out a whole series of most important and
far-reaching consequences. He showed that the ebb and flow of the
tides was due to the attraction of both sun and moon, especially
the latter, upon the waters of our oceans. He pointed out certain
irregularities which must take place in the motion of our moon, due
to the influence of the sun upon it. He showed, too, what was the
cause of that swinging of the axis of the earth which gives rise to
precession. He deduced the relative weights of the earth, the sun,
and of Jupiter and Saturn, the planets with satellites. He proved
also that comets, which had seemed hitherto to men as perfectly
lawless wanderers, obeyed in their orbits the self-same law which
governed the moon and planets. The whole vast system of celestial
movements, which had long seemed to men irregular and uncontrolled,
now fell, every one of them, into its place, as but the necessary
manifestations of one grand, simple order.
This great discovery gave a new and additional importance to the
regular observation of the moon and planets. They were needed now,
not only to assist in the practical work of navigation, but for
the development of questions of pure science. Halley, the second
Astronomer Royal, and Maskelyne, the fifth, devoted themselves
chiefly to this department of work, to the partial neglect of the
observation of the places of stars. Airy, the seventh, whilst making
catalogue-work a part of the regular routine of the Observatory,
developed the observation of the members of the solar system, and
especially of the moon, in a most marked degree, and collected and
completely reduced the vast mass of material which the industry of
his predecessors had gathered. It is pre-eminently of the work of
Airy that the memorable words quoted before of Professor Newcomb,
the great American mathematician and astronomer, are applicable:
'that if this branch of astronomy were entirely lost, it could be
reconstructed from the Greenwich observations alone.'
A most important step taken by Airy was the construction of an
altazimuth. An altazimuth is practically a theodolite on a large
scale. Its purpose is to determine, not the declination and right
ascension of some celestial body, as is the case with the transit
circle, but its altitude, _i.e._ its height above the horizon, and
its azimuth, _i.e._ the angle measured on the horizontal plane from
the north point. The altazimuth, then, like the transit circle,
consists of a telescope revolving on a horizontal axis, but, unlike
the transit circle, both the telescope and the piers which carry its
pivots can be rotated so as to point not merely due north and south,
but in any direction whatsoever.
[Illustration: AIRY'S ALTAZIMUTH.]
The observations with the altazimuth are rather more complicated
than those with the transit circle. Looking in the telescope, the
observer sees a double set of spider threads or 'wires'; and when a
star or other heavenly body enters the field, it will generally be
observed to move obliquely across both sets of wires. The observer
usually determines to make an observation either in altitude or
azimuth. In the former case he presses the little contact button,
which, as in the transit circle, is provided close to the eyepiece,
as the star reaches each of the horizontal wires in succession. If
in azimuth, it is the times of crossing the vertical wires that are
in like manner telegraphed to the chronograph. The transit over,
the appropriate circle is read; for the telescope itself is rigidly
attached to a vertical wheel having a carefully engraved circle on
its face and read by four microscopes, whilst the entire instrument
carries another set of microscopes, pointing to a fixed horizontal
circle, and upon which the azimuth can be read. A complete
observation involves four such transits and sets of circle readings,
two of altitude, and two of azimuth; for after one of altitude
and one of azimuth the telescope is turned round, and a second
observation is taken in each element.
The observation gives us the altitude and azimuth of the star. These
particulars are of no direct value to us. But it is a mere matter
of computation, though a long and laborious one, to convert these
elements into right ascension and declination.
The usefulness of the altazimuth will be seen at once. It will be
remembered that with the transit circle any particular object can
only be observed as it crosses the meridian. If the weather should
be cloudy, or the observer late, the chance of observation is
lost for four and twenty hours, and in the case of the moon, for
which the altazimuth is specially used, it is on the meridian only
in broad daylight during that part of the month which immediately
precedes and follows new moon. At such times it is practically
impossible to observe it with the transit circle; with the
altazimuth it may be caught in the twilight before sunrise or after
sunset; and at other times in the month, if lost on the meridian in
the transit circle, the altazimuth still gives the observer a chance
of catching it any time before it sets. But for this instrument, our
observations of the moon would have been practically impossible over
at least one-fourth of its orbit.
Airy's altazimuth was but a small instrument of three and
three-quarter inches aperture, mounted in a high tower built on
the site of Flamsteed's mural arc; and, after a life history of
about half a century, has been succeeded by a far more powerful
instrument. The 'New Altazimuth' has an aperture of eight inches,
and is housed in a very solidly constructed building of striking
appearance, the connection of the Observatory with navigation being
suggested by a row of circular lights which strongly recall a ship's
portholes. This building is at the southern end of the narrow
passage, 'the wasp's waist,' which connects the older Observatory
domain with the newer. It is the first building we come to in the
south ground. The computations of the department are carried on in
the south wing of the new Observatory.
It will be seen from the photograph that the instrument is much
larger, heavier, and less easy to move in azimuth than the old
altazimuth. It is, therefore, not often moved in azimuth, but is set
in some particular direction, not necessarily north and south, in
which it is used practically as a transit circle.
[Illustration: NEW ALTAZIMUTH BUILDING.]
There is quite another way of determining the place of the moon,
which is sometimes available, and which offers one of the prettiest
of observations to the astronomer. As the moon travels across the
sky, moving amongst the stars from west to east, it necessarily
passes in front of some of them, and hides them from us for a time.
Such a passage, or 'occultation,' offers two observations: the
'disappearance,' as the moon comes up to the star and covers it; the
'reappearance,' as it leaves it again, and so discloses it.
[Illustration: THE NEW ALTAZIMUTH.
(_From a photograph by Mr. Lacey._)]
Except at the exact time of full moon, we do not see the entire face
of our satellite; one edge or 'limb' is in darkness. As the moon
therefore passes over the star, either the limb at which the star
disappears, or that at which it reappears, is invisible to us. To
watch an occultation at the bright limb is pretty; the moon, with
its shining craters and black hollows, its mountain ranges in bright
relief, like a model in frosted silver, slowly, surely, inevitably
comes nearer and nearer to the little brilliant which it is going to
eclipse. The movement is most regular, most smooth, yet not rapid.
The observer glances at his clock, and marks the minute as the two
heavenly bodies come closer and closer to each other. Then he counts
the clock beats: 'five, six, seven,' it may be, as the star has
been all but reached by the advancing moon. 'Eight,' it is still
clear; ere the beat of the clock rings to the 'nine,' perhaps the
little diamond point has been touched by the wide arch of the moon's
limb, and has gone! Less easy to exactly time is a reappearance at
the bright limb. In this case the observer must ascertain from the
_Nautical Almanac_ precisely where the star will reappear; then
a little before the predicted time he takes his place at the
telescope, watches intently the moon's circumference at the point
indicated, and, listening for the clock-beats, counts the seconds as
they fly. Suddenly, without warning, a pin-point of light flashes
out at the edge of the moon, and at once draws away from it. The
star has 'reappeared.'
Far more striking is a disappearance or reappearance at the 'dark
limb.' In this case the limb of the moon is absolutely invisible,
and it may be that no part of the moon is visible in the field
of the telescope. In this case the observer sees a star shining
brightly and alone in the middle of the field of his telescope. He
takes the time from his faithful clock, counting beat after beat,
when suddenly the star is gone! So sudden is the disappearance that
the novice feels almost as astonished as if he had received a slap
in the face, and not unfrequently he loses all count or recollection
of the clock beats. The reappearance at the dark limb is quite as
startling; with a bright star it is almost as if a shell had burst
in his very face, and it would require no very great imagination to
make him think that he had heard the explosion. One moment nothing
was visible; now a great star is shining down serenely on the
watcher. A little practice soon enables the observer to accustom
himself to these effects, and an old hand finds no more difficulty
in observing an occultation of any kind than in taking a transit.
Such an observation is useful for more purposes than one. If the
position of the star occulted is known--and it can be determined at
leisure afterwards--we necessarily know where the limb of the moon
was at the time of the observation. Then the time which the moon
took to pass over the star enables us to calculate the diameter of
our satellite; the different positions of the moon relative to the
star, as seen from different observatories, enable us to calculate
its distance.
But if the disappearance takes place at the bright limb, the
reappearance usually takes place at the dark, and _vice versâ_;
and the two observations are not quite comparable. There is one
occasion, however, when both observations are made under similar
circumstances, namely, at the full. And if the moon happens also
to be totally eclipsed, the occultations of quite faint stars can
be successfully observed, much fainter than can ordinarily be
seen close up to the moon. Total eclipses of the moon, therefore,
have recently come to be looked upon as important events for the
astronomer, and observatories the world over usually co-operate in
watching them. October 4, 1884, was the first occasion when such an
organised observation was made; there have been several since, and
on these nights every available telescope and observer at Greenwich
is called into action.
It may be asked why these different modes of observing the moon
are still kept up, year in and year out. 'Do we not know the
moon's orbit sufficiently well, especially since the discovery of
gravitation?' No; we do not. This simple and beautiful law--simple
enough in itself, gives rise to the most amazing complexity of
calculation. If the earth and moon were the only two bodies in the
universe, the problem would be a simple one. But the earth, sun,
and moon are members of a triple system, each of which is always
acting on both of the others. More, the planets, too, have an
appreciable influence, and the net result is a problem so intricate
that our very greatest mathematicians have not thoroughly worked it
out. Our calculations of the moon's motions need, therefore, to be
continually compared with observation, need even to be continually
corrected by it.
There is a further reason for this continual observation, not only
in the case of the sun, which is our great standard star, since
from it we derive the right ascensions of the stars, and it is also
our great timekeeper; not only in that of the moon, but also in the
case of the planets. Their places as computed need continually to
be compared with their places as observed, and the discordances,
if any, inquired into. The great triumph which resulted to science
from following this course--to pure science, since Uranus is too
faint a planet to be any help to the sailor in navigation--is well
known. The observed movements of Uranus proved not to be in accord
with computation, and from the discordances between calculation and
observation Adams and Leverrier were able to predicate the existence
of a hitherto unseen planet beyond--
'To see it, as Columbus saw America from Spain. Its movements
were felt by them trembling along the far-reaching line of their
analysis, with a certainty hardly inferior to that of ocular
demonstration.'[5]
[5] From Sir John Herschel's address to the British Association,
September 10, 1846, thirteen days before Galle's first observation
of the planet.
The discovery of Neptune was not made at Greenwich, and Airy has
been often and bitterly attacked because he did not start on the
search for the predicted planet the moment Adams addressed his
first communication to him, and so allowed the French astronomer
to engross so much of the honour of the exploit. The controversy
has been argued over and over again, and we may be content to leave
it alone here. There is one point, however, which is hardly ever
mentioned, which must have had much effect in determining Airy's
conduct. In 1845, the year in which Adams sent his provisional
elements of the unseen disturbing planet to Airy, the largest
telescope available for the search at Greenwich was an equatorial of
only six and three-quarter inches aperture, provided with small and
insufficient circles for determining positions, and housed in a very
small and inconvenient dome; whilst at Cambridge, within a mile or
so of Adams' own college, was the 'Northumberland' equatorial, of
nearly twelve inches aperture, under the charge of the University
Professor of Astronomy, Professor Challis, and which was then much
the largest, best mounted and housed equatorial in the entire
country. The 'Northumberland' had been begun from Airy's designs and
under his own superintendence, when he was Professor of Astronomy at
Cambridge. Naturally, then, knowing how much superior the Cambridge
telescope was to any which he had under his care, he thought the
search should be made with it. He had no reason to believe that his
own instrument was competent for the work.
[Illustration: THE NEW OBSERVATORY AS SEEN FROM FLAMSTEED'S
OBSERVATORY.]
On the other hand, it is hard for the ordinary man to understand
how it was that Adams not only left unnoticed and unanswered for
three-quarters of a year, an inquiry of Airy's with respect to his
calculations, but also never took the trouble to visit Challis,
whom he knew well, and who was so near at hand, to stir him up
to the search. But, in truth, the whole interest of the matter
for Adams rested in the mathematical problem. The irregularities
in the motion of Uranus were interesting to him simply for the
splendid opportunity which they gave him for their analysis. A
purely imaginary case would have served his purpose nearly as well.
The actuality of the planet which he predicted was of very little
moment; the _éclat_ and popular reputation of the discovery was less
than nothing; the problem itself and the mental exercise in its
solution, were what he prized.
But it was not creditable to the nation that the Royal Observatory
should have been so ill-provided with powerful telescopes; and a
few years later Airy obtained the sanction of the Government for
the erection of an equatorial larger than the 'Northumberland,'
but on the same general plan and in a much more ample dome. This
was for thirty-four years the 'Great' or 'South-East' equatorial,
and the mounting still remains and bears the old name, though the
original telescope has been removed elsewhere. The object-glass had
an aperture of twelve and three-quarter inches and a focal length of
eighteen feet, and was made by Merz of Munich, the engineering work
by Ransomes and Sims of Ipswich, and the graduations and general
optical work by Simms, now of Charlton, Kent. The mounting was so
massive and stable that the present Astronomer Royal has found it
quite practicable and safe to place upon it a telescope (with its
counterpoises) of many times the weight, one made by Sir Howard
Grubb, of Dublin, of twenty-eight inches aperture and twenty-eight
feet focal length, the largest refractor in the British Empire,
though surpassed by several American and Continental instruments.
The stability of the mounting was intended to render the telescope
suitable for a special work. This was the observation of 'minor
planets.' On the first day of the present century the first of these
little bodies was discovered by Piazzi at Palermo. Three more were
discovered at no great interval afterwards, and then there was an
interval of thirty-eight years without any addition to their number.
But from December 8, 1845, up to the present time, the work of
picking up fresh individuals of these 'pocket planets' has gone on
without interruption, until now more than 400 are known. Most of
these are of no interest to us, but a few come sufficiently near to
the earth for their distance to be very accurately determined; and
when the distance of one member of the solar system is determined,
those of all the others can be calculated from the relations which
the law of gravitation reveals to us. It is a matter of importance,
therefore, to continue the work of discovery, since we may at any
time come across an interesting or useful member of the family; and
that we may be able to distinguish between minor planets already
discovered and new ones, their orbits must be determined as they
are discovered, and some sort of watch kept on their movements.
A striking example of the scientific prizes which we may light upon
in the process of the rather dreary and most laborious work which
the minor planets cause, has been recently supplied by the discovery
of Eros. On August 13, 1898, Herr Witt, of the Urania Observatory,
Berlin, discovered a very small planet that was moving much faster
in the sky than is common with these small bodies. The great
majority are very much farther from the sun than the planet Mars,
many of them twice as far, and hence, since the time of a planet's
revolution round the sun increases, in accordance with Kepler's law,
more rapidly than does its distance, it follows that they move much
more slowly than Mars. But this new object was moving at almost the
same speed as Mars; it must, therefore, be most unusually near to
us. Further observations soon proved that this was the case, and
Eros, as the little stranger has been called, comes nearer to us
than any other body of which we are aware except the moon. Venus
when in transit is 24-1/2 millions of miles from us, Mars at its
nearest is 34-1/2 millions, Eros at its nearest approach is but
little over 13 millions.
The use of such a body to us is, of course, quite apart from any
purpose of navigation, except very indirectly. But it promises to
be of the greatest value in the solution of a question in which
astronomers must always feel an interest, the determination of
the distance of the earth from the sun. We know the _relative_
distances of the different planets, and, consequently if we could
determine the absolute distance of any one, we should know the
distances of all. As it is practically impossible to measure our
distance from the sun directly, several attempts have been made
to determine the distances of Venus, Mars, or such of the minor
planets as come the nearest to us. Three of these in particular, the
little planets Iris, Victoria, and Sappho, have given us the most
accurate determinations of the sun's distance (92,874,000 miles)
which we have yet obtained; but Eros at its nearest approach will
be six times as near to us as either of the three mentioned above,
and therefore should give us a value with only one-sixth of the
uncertainty attaching to that just mentioned.
The discovery of minor planets has lain outside the scope of
Greenwich work, but their observation has formed an integral part of
it. The general public is apt to lay stress rather on the first than
on the second, and to think it rather a reproach to Greenwich that
it has taken no part in such explorations. Experience has, however,
shown that they may be safely left to amateur activity, whilst the
monotonous drudgery of the observation of minor planets can only be
properly carried out in a permanent institution.
The observation of these minute bodies with the transit circle
and altazimuth is attended with some difficulties; but precise
observations of various objects may be made with an equatorial;
indeed, comets are usually observed by its means.
The most ordinary way of observing a comet with an equatorial is as
follows: Two bars are placed in the eye-piece of the telescope, at
right angles to each other, and at an angle of forty-five degrees
to the direction of the apparent daily motion of the stars. The
telescope is turned to the neighbourhood of the comet, and moved
about until it is detected. The telescope is then put a little in
front of the comet, and very firmly fixed. The observer soon sees
the comet entering his field, and by pressing the contact button he
telegraphs to the chronograph the time when the comet is exactly
bisected by each of the bars successively. He then waits until a
bright star, or it may be two or three, have entered the telescope
and been observed in like manner. The telescope is then unclamped,
and moved forward until it is again ahead of the comet, and the
observations are repeated; and this is done as often as is thought
desirable. The places of the stars have, of course, to be found out
from catalogues, or have to be observed with the transit circle, but
when they are known the position of the comet or minor planet can
easily be inferred.
Next to the glory of having been the means of bringing about the
publication of Newton's _Principia_, the greatest achievement of
Halley, the second Astronomer Royal, was that he was the first to
predict the return of a comet. Newton had shown that comets were
no lawless wanderers, but were as obedient to gravitation as were
the planets themselves, and he also showed how the orbit of a comet
could be determined from observations on three different dates.
Following these principles, Halley computed the orbits of no fewer
than twenty-four comets, and found that three of them, visible at
intervals of about seventy-five years, pursued practically the same
path. He concluded, therefore, that these were really different
appearances of the same object, and, searching old records, he
found reason to believe that it had been observed frequently
earlier still. It seems, in fact, to have been the comet which is
recorded to have been seen in 1066 in England at the time of the
Norman invasion; in A.D. 66, shortly before the commencement of
that war which ended in the destruction of Jerusalem by Titus;
and earlier still, so far back as B.C. 12. Halley, however,
experienced a difficulty in his investigation. The period of the
comet's revolution was not always the same. This, he concluded,
must be due to the attraction of the planets near which the comet
might chance to travel. In the summer of 1681 it had passed very
close to Jupiter, for instance, and in consequence he expected
that instead of returning in August 1757, seventy-five years after
its last appearance, it would not return until the end of 1758 or
the beginning of 1759. It has returned twice since Halley's day,
a triumphant verification of the law of gravitation; and we are
looking for it now for a third return some ten years hence, in 1910.
Halley's comet, therefore, is an integral member of our solar
system, as much so as the earth or Neptune, though it is utterly
unlike them in appearance and constitution, and though its path is
so utterly unlike theirs that it approaches the sun nearer than our
earth, and recedes farther than Neptune. But there are other comets,
which are not permanent members of our system, but only passing
visitors. From the unfathomed depths of space they come, to those
depths they go. They obey the law of gravitation so far as our sight
can follow them, but what happens to them beyond? Do they come under
some other law, or, perchance, in outermost space is there still a
region reserved to primeval Chaos, the 'Anarch old,' where no law
at all prevails? Gravitation is the bond of the solar system; is it
also the bond of the Universe?
CHAPTER IX
THE MAGNETIC AND METEOROLOGICAL DEPARTMENTS
Passing out of the south door of the new altazimuth building, we
come to a white cruciform erection, constructed entirely of wood.
This is the Magnet House or Magnetic Observatory, the home of a
double Department, the Magnetic and Meteorological.
This department does not, indeed, lie within the original purpose
of the Observatory as that was defined in the warrant given to
Flamsteed, and yet is so intimately connected with it, through its
bearing on navigation, that there can be no question as to its
suitability at Greenwich. Indeed, its creation is a striking example
of the thorough grasp which Airy had upon the essential principles
which should govern the great national observatory of an essentially
naval race, and of the keen insight with which he watched the new
development of science. The Magnetic Observatory, therefore, the
purpose of which was to deal with the observation of the changes in
the force and direction of the earth's magnetism--an inquiry which
the greater delicacy of modern compasses, and, in more recent times,
the use of iron instead of wood in the construction of ships has
rendered imperative--was suggested by Airy on the first possible
occasion after he entered on his office, and was sanctioned in 1837.
The Meteorological Department has a double bearing on the purpose of
the Observatory. On the one side, a knowledge of the temperature and
pressure of the atmosphere is, as we have already seen, necessary
in order to correct astronomical observations for the effect of
refraction. On the other hand, meteorology proper, the study of
the movements of the atmosphere, the elucidation of the laws which
regulate those movements, leading to accurate forecasts of storms,
are of the very first necessity for the safety of our shipping.
It is true that weather forecasts are not issued from Greenwich
Observatory, any more than the _Nautical Almanac_ is now issued from
it; but just as the Observatory furnishes the astronomical data upon
which the Almanac is based, so also it takes its part in furnishing
observations to be used by the Meteorological Office at Westminster
for its daily predictions.
Those predictions are often made the subject of much cheap ridicule;
but, however far short they may fall of the exact and accurate
predictions which we would like to have, yet they mark an enormous
advance upon the weather-lore of our immediate forefathers.
'He that is weather wise
Is seldom other wise,'
says the proverb, and the saying is not without a shrewd amount of
truth. For perhaps nowhere can we find a more striking combination
of imperfect observation and inconsequent deduction than in the
saws which form the stock-in-trade of the ordinary would-be weather
prophet. How common it is to find men full of the conviction that
the weather must change at the co-called 'changes of the moon,'
forgetful that
'If we'd no moon at all--
And that may seem strange--
We still should have weather
That's subject to change.'
They will say, truly enough, no doubt, that they have known the
weather to change at 'new' or 'full,' as the case may be, and they
argue that it, therefore, must always do so. But, in fact, they have
only noted a few chance coincidences, and have let the great number
of discordances pass by unnoticed.
But observations of this kind seem scientific and respectable
compared with those numerous weather proverbs which are based upon
the mere jingle of a rhyme, as
'If the ash is out before the oak,
You may expect a thorough soak'--
a proverb which is deftly inverted in some districts by making 'oak'
rhyme to 'choke.'
Others, again, are based upon a mere childish fancy, as, for
example, when the young moon 'lying on her back' is supposed to bode
a spell of dry weather, because it looks like a cup, and so might be
thought of as able to hold the water.
During the present reign, however, a very different method of
weather study has come into action, and the foundations of a
true weather wisdom have been laid. These have been based, not
on fancied analogies or old wives' rhymes, or a few forechosen
coincidences, but upon observations carried on for long periods of
time and over wide areas of country, and discussed in their entirety
without selection and bias. Above all, mathematical analysis has
been applied to the motions of the air, and ideas, ever gaining
in precision and exactness, have been formulated of the general
circulation of the atmosphere.
As compared with its sister science, astronomy, meteorology appears
to be still in a very undeveloped state. There is such a difference
between the power of the astronomer to foretell the precise position
of sun, moon, and planets for years, even for centuries, beforehand,
and the failure of the meteorologist to predict the weather for a
single season ahead, that the impression has been widely spread that
there is yet no true meteorological science at all. It is forgotten
that astronomy offered us, in the movements of the heavenly bodies,
the very simplest and easiest problem of related motion. Yet for how
many thousands of years did men watch the planets, and speculate
concerning their motions, before the labours of Tycho, Kepler, and
Newton culminated in the revelation of their meaning? For countless
generations it was supposed that their movements regulated the
lives, characters, and private fortunes of individual men; just as
quite recently it was fancied that a new moon falling on a Saturday,
or two full moons coming within the same calendar month, brought bad
weather!
It is still impossible to foresee the course of weather change for
long ahead; but the difference between the modern navigator, surely
and confidently making a 'bee-line' across thousands of miles of
ocean to his destination, and the timid sailor of old, creeping from
point to point of land, is hardly greater than the contrast between
the same two men, the one watching his barometer, the other trusting
in the old wives' rhymes which afforded him his only indication as
to coming storms.
It is still impossible to foresee the weather change for long ahead,
but in our own and in many other countries, especially the United
States, it has been found possible to predict the weather of the
coming four-and-twenty hours with very considerable exactness, and
often to forecast the coming of a great storm several days ahead.
This is the chief purpose of the two great observatories of the
storm-swept Indian and Chinese seas, Hong Kong and Mauritius; and
the value of the work which they have done in preventing the loss
of ships, and the consequent loss of lives and property, has been
beyond all estimate.
The Royal Observatory, Greenwich, is a meteorological as well as an
astronomical observatory, but, as remarked above, it does not itself
issue any weather forecasts. Just as the Greenwich observations
of the places of the moon and stars are sent to the _Nautical
Almanac_ Office, for use in the preparation of that ephemeris; just
as the Greenwich determinations of time are used for the issue of
signals to the Post Office, whence they are distributed over the
kingdom, so the Greenwich observations of weather are sent to the
Meteorological Office, there to be combined with similar records
from every part of the British Isles, to form the basis of the
daily forecasts which the latter office publishes. To each of these
three offices, therefore, the Royal Observatory, Greenwich, stands
in the relation of purveyor. It supplies them with the original
observations more or less in reduced and corrected form, without
which they could not carry on most important portions of their work.
Let it be noted how closely these three several departments,
the _Nautical Almanac_ Office, the Time Department, and the
Meteorological Office, are related to practical navigation. Whatever
questions of pure science--of knowledge, that is, apart from its
useful applications--may arise out of the following up of these
several inquiries, yet the first thought, the first principle of
each, is to render navigation more sure and more safe.
The first of all meteorological instruments is the barometer, which,
under its two chief forms of mercurial and aneroid, is simply a
means of measuring the pressure exerted by the atmosphere.
There are two important corrections to which its readings are
subject. The first is for the height of the station above the
level of the sea; the second is for the effect of temperature upon
the mercury in the barometer itself, lengthening the column. To
overcome these, the height of the standard barometer at Greenwich
above sea-level has been most carefully ascertained, and the
heights relative to it of the other barometers of the Observatory,
particularly those in rooms occupied by fundamental telescopes,
have also been determined, whilst the self-recording barometer is
mounted in a basement, where it is almost completely protected from
changes of temperature.
Next in importance to the barometer as a meteorological instrument
comes the thermometer. The great difficulty in the Observatory
use of the thermometer is to secure a perfectly unexceptionable
exposure, so that the thermometer may be in free and perfect contact
with the air, and yet completely sheltered from any direct ray from
the sun. This is secured in the great thermometer shed at Greenwich
by a double series of 'louvre' boards, on the east, south, and west
sides of the shed, the north side being open. The shed itself is
made a very roomy one, in order to give access to a greater body of
air.
A most important use of the thermometer is in the measurement
of the amount of moisture in the air. To obtain this, a pair of
thermometers are mounted close together, the bulb of one being
covered by damp muslin, and the other being freely exposed. If
the air is completely saturated with moisture, no evaporation
can take place from the damp muslin, and consequently the two
thermometers will read the same. But if the air be comparatively
dry, more or less evaporation will take place from the wet bulb,
and its temperature will sink to that at which the air would be
fully saturated with the moisture which it already contained. For
the higher the temperature, the greater is its power of containing
moisture. The difference of the reading of the two thermometers is,
therefore, an index of humidity. The greater the difference, the
greater the power of absorbing moisture, or, in other words, the
dryness of the air. The great shed already alluded to is devoted to
these companion thermometers.
[Illustration: THE SELF-REGISTERING THERMOMETERS.]
Very closely connected with atmospheric pressure, as shown us by the
barometer, is the study of the direction of winds. If we take a map
of the British Isles and the neighbouring countries, and put down
upon them the barometer readings from a great number of observing
stations, and then join together the different places which show the
same barometric pressure, we shall find that these lines of equal
pressure--technically called 'isobars'--are apt to run much nearer
together in some places than in others. Clearly, where the isobars
are close together it means that in a very short distance of country
we have a great difference of atmospheric pressure. In this case
we are likely to get a very strong wind blowing from the region of
high pressure to the region of low pressure, in order to restore the
balance.
If, further, we had information from these various observing
stations of the direction in which the wind was blowing, we should
soon perceive other relationships. For instance, if we found that
the barometer read about the same in a line across the country from
east to west, but that it was higher in the north of the islands
than in the south, we should then have a general set of winds from
the east, and a similar relation would hold good if the barometer
were highest in some other quarter; that is, the prevailing wind
will come from a quarter at right angles to the region of highest
barometer, or, as it is expressed in what is known as 'Buys Ballot's
law,' 'stand with your back to the wind, and the barometer will be
lower on your left hand than on your right.' This law holds good for
the northern hemisphere generally, except near to the equator; in
the southern hemisphere the right hand is the side of low barometer.
The instruments for wind observation are of two classes: vanes
to show its direction, and anemometers to show its speed and its
pressure. These may be regarded as two different modes in which the
strength of the wind manifests itself. Pressure anemometers are
usually of two forms: one in which a heavy plate is allowed to swing
by its upper edge in a position fronting the wind, the amount of its
deviation from the vertical being measured; and the other in which
the plate is supported by springs, the degree of compression of the
springs being the quantity registered in that case. Of the speed
anemometers, the best known form is the 'Robinson,' in which four
hemispherical cups are carried at the extremities of a couple of
cross bars.
For the mounting of these wind instruments the old original
Observatory, known as the Octagon Room, has proved an excellent
site, with its flat roof surmounted by two turrets in the north-east
and north-west corners, and raised some two hundred feet above
high-water mark.
[Illustration: THE ANEMOMETER ROOM, NORTH-WEST TURRET.]
The two chief remaining instruments are those for measuring the
amount of rainfall and of full sunshine. The rain gauge consists
essentially of a funnel to collect the rain, and a graduated glass
to measure it. The sunshine recorder usually consists of a large
glass globe arranged to throw an image of the sun on a piece of
specially prepared paper. This image, as the sun moves in the sky,
moves along the paper, charring it as it moves, and at the end of
the day it is easy to see, from the broken, burnt trace, at what
hours the sun was shining clear, and when it was hidden by cloud.
An amusing difficulty was encountered in an attempt to set on
foot another inquiry. The Superintendent of the Meteorological
Department at the time wished to have a measure of the rate at which
evaporation took place, and therefore exposed carefully measured
quantities of water in the open air in a shallow vessel. For a few
days the record seemed quite satisfactory. Then the evaporation
showed a sudden increase, and developed in the most erratic
and inexplicable manner, until it was found that some sparrows
had come to the conclusion that the saucer full of water was a
kindly provision for their morning 'tub,' and had made use of it
accordingly.
A large proportion of the meteorological instruments at
Greenwich and other first-class observatories are arranged to be
self-recording. It was early felt that it was necessary that the
records of the barometer and thermometer should be as nearly as
possible continuous; and at one time, within the memory of members
of the staff still living, it was the duty of the observer to read
a certain set of instruments at regular two-hour intervals during
the whole of the day and night--a work probably the most monotonous,
trying, and distasteful of any that the Observatory had to show.
The two-hour record was no doubt practically equivalent to a
continuous one, but it entailed a heavy amount of labour. Automatic
registers were, therefore, introduced whenever they were available.
The earliest of these were mechanical, and several still make their
records in this manner.
On the roof of the Octagon Room we find, beside the two turrets
already referred to, a small wooden cabin built on a platform
several feet above the roof level. This cabin and the north-western
turret contain the wind-registering instruments. Opening the turret
door, we find ourselves in a tiny room which is nearly filled by
a small table. Upon this table lies a graduated sheet of paper in
a metal frame, and as we look at it, we see that a clock set up
close to the table is slowly drawing the paper across it. Three
little pencils rest lightly on the face of the paper at different
points. One of these, and usually the most restless, is connected
with a spindle which comes down into the turret from the roof, and
which is, in fact, the spindle of the wind vane. The gearing is so
contrived that the motion on a pivot of the vane is turned into
motion in a straight line at right angles to the direction in which
the paper is drawn by the clock. A second pencil is connected with
the wind-pressure anemometer. The third pencil indicates the amount
of rain that has fallen since the last setting, the pencil being
moved by a float in the receiver of the rain gauge.
[Illustration: THE ANEMOMETER TRACE.]
An objection to all the mechanical methods of continuous
registration is that, however carefully the gearing between the
instrument itself and the pencil is contrived, however lightly
the pencil moves over the paper, yet some friction enters in
and affects the record: this is of no great moment in wind
registration, when we are dealing with so powerful an agent as
the wind, but it becomes a serious matter when the barometer is
considered, since its variations require to be registered with the
greatest minuteness. When photography, therefore, was invented,
meteorologists were very prompt to take advantage of this new ally.
A beam of light passing over the head of the column of mercury in a
thermometer or barometer could easily be made to fall upon a drum
revolving once in the twenty-four hours, and covered with a sheet
of photographic paper. In this case, when the sensitive paper is
developed, we find its upper half blackened, the lower edge of the
blackened part showing an irregular curve according as the mercury
in the thermometer or barometer rose or fell, and admitted less or
more light through the space above it.
Here we have a very perfect means of registration: the passage of
the light exercises no friction or check on the free motion of the
mercury in the tube, or on the turning of the cylinder covered by
the sensitive paper, whilst it is easy to obtain a time scale on the
register by cutting off the light for an instant--say at each hour.
In this way the wet and dry bulb thermometers in the great shed make
their registers.
The supply of material to the Meteorological Office is not the only
use of the Greenwich meteorological observations. Two elements of
meteorology, the temperature and the pressure of the atmosphere,
have the very directest bearing upon astronomical work. And this
in two ways. An instrument is sensible to heat and cold, and
undergoes changes of form, size, or scale, which, however absolutely
minute, yet become, with the increased delicacy of modern work, not
merely appreciable, but important. So too with the density of the
atmosphere: the light from a distant star, entering our atmosphere,
suffers refraction; and being thus bent out of its path, the star
appears higher in the heavens than it really is. The amount of this
bending varies with the density of the layers of air through which
the light has to pass. The two great meteorological instruments, the
thermometer and barometer, are therefore astronomical instruments as
well.
In the arrangements at Greenwich the Magnetic Department is closely
connected with the Meteorological, and it is because the two
departments have been associated together that the building devoted
to both is constructed of wood, not brick, since ordinary bricks are
made of clay which is apt to be more or less ferruginous. Copper
nails have alone been employed in the construction of the buildings.
The fire-grates, coal-scuttles, and fire-irons are all of the same
metal.
The growth of the Observatory has, however, made it necessary to
set up some of the new telescopes, into the mounting of which much
iron enters, very close to the magnetic building. The present
Astronomer-Royal has therefore erected a Magnetic Pavilion right out
in the park at an ample distance from these disturbing causes.
The double department is, therefore, the most widely scattered in
the whole Observatory. It is located for computing purposes in the
west wing of the New Observatory; many of its magnetic instruments
are in the old Magnet House, others are across the park in the new
Magnetic Pavilion; the anemometers are on the roof of the Octagon
Room, Flamsteed's original observatory, and the self-registering
thermometers are in the south ground between the old Magnet House
and the New Observatory.
[Illustration: MAGNETIC PAVILION--EXTERIOR.
(_From a photograph by Mr. Lacey._)]
The object of the Magnetic Observatory is to study the movements of
the magnetic needle. The quaintest answer that I ever received in an
examination was in reply to the question, 'What is meant by magnetic
inclination and declination?' The examinee replied:
'To make a magnet, you take a needle, and rub it on a lodestone.
If it refuses or _declines_ to become a magnet, that is magnetic
declination; if it is easily made a magnet, or is _inclined_ to
become one, that is magnetic inclination.'
One greatly regretted that it was necessary to mark the reply
according to its ignorance, and not, as one would have wished, in
proportion to its ingenuity. Magnetic declination, however, as
everybody knows, measures the deviation of the 'needle' from the
true geographical north and south direction; the inclination or dip
is the angle which a 'needle' makes with the horizon.
At one time the only method of watching the movements of the
magnetic needles was by direct observation, just precisely as it was
wont to be in the case of the barometer and thermometer. But the
same agent that has been called in to help in their case has enabled
the magnets also to give us a direct and continuous record of their
movements. In principle the arrangement is as follows: A small light
mirror is attached to the magnetic needle, and a beam of light is
arranged to fall upon the mirror, and is reflected away from it to a
drum covered with sensitive paper. If, then, the needle is perfectly
at rest, a spot of light falls on the drum and blackens the paper at
one particular point. The drum is made to revolve by clockwork once
in twenty-four hours, and the black dot is therefore lengthened out
into a straight line encircling the drum. If, however, the needle
moves, then the spot of light travels up or down, as the case may
be.
Now, if we look at one of these sheets of photographic paper after
it has been taken from the drum, we shall see that the north pole of
the magnet has moved a little, a very little, towards the west in
the early part of the day, say from sunrise to 2 p.m., and has swung
backwards from that hour till about 10 p.m., remaining fairly quiet
during the night. The extent of this daily swing is but small, but
it is greater in summer than in winter, and it varies also from year
to year.
[Illustration: MAGNETIC PAVILION--INTERIOR.
(_From a photograph by Mr. Lacey._)]
Besides this daily swing, there occasionally happen what are called
'magnetic storms;' great convulsive twitchings of the needle, as
if some unseen operator were endeavouring, whilst in a state of
intense excitement, to telegraph a message of vast importance, so
rapid and so sharp are the movements of the needle to and fro. These
great storms are felt, so far as we know, simultaneously over the
whole earth, and the more characteristic begin with a single sharp
twitch of the needle towards the east.
Besides the movements of the magnetic needle, the intensity of the
currents of electricity which are always passing through the crust
of the earth are also determined at Greenwich; but this work has
been rendered practically useless for the last few years by the
construction of the electric railway from Stockwell to the City.
Since it was opened, the photographic register of earth currents
has shown a broad blurring from the moment of the starting of the
first train in the morning to the stopping of the last train at
night. As an indication of the delicacy of modern instruments, it
may be mentioned that distinct indications of the current from this
railway have been detected as far off as North Walsham, in Norfolk,
a distance of more than a hundred miles. A further illustration of
the delicacy of the magnetic needles was afforded shortly after
the opening of the railway referred to. On one occasion the then
Superintendent of the Magnetic Department visited the Generating
Station at Stockwell, and on his return it was noticed day after day
that the traces from the magnets showed a curious deflection from 9
a.m. to 3 p.m., the hours of his attendance. This gave rise to some
speculation, as it did not seem possible that the gentleman could
himself have become magnetized. Eventually, the happy accident
of a fine day solved the mystery. That morning the Superintendent
left his umbrella at home, and the magnets were undisturbed. The
secret was out. The umbrella had become a permanent magnet, and its
presence in the lobby of the magnetic house had been sufficient to
influence the needles.
CHAPTER X
THE HELIOGRAPHIC DEPARTMENT
So far the development of the Observatory had been along the central
line of assistance to navigation. But the Magnetic Department led on
to one which had but a very secondary connection with it.
A greatly enhanced interest was given to the observations of earth
magnetism, when it was found that the intensity and frequency of
its disturbances were in close accord with changes that were in
progress many millions of miles away. That the surface of the sun
was occasionally diversified by the presence of dark spots, had been
known almost from the first invention of the telescope; but it was
not until the middle of the present century that any connection was
established between these solar changes and the changes which took
place in the magnetism of the earth. Then two observers, the one
interesting himself entirely with the spots on the sun, the other as
wholly devoted to the study of the movements of the magnetic needle,
independently found that the particular phenomenon which each was
watching was one which varied in a more or less regular cycle. And
further, when the cycles were compared, they proved to be the same.
Whatever the secret of the connection, it is now beyond dispute that
as the spots on the sun become more and more numerous, so the daily
swing of the magnetic needle becomes stronger; and, on the other
hand, as the spots diminish, so the magnetic needle moves more and
more feebly.
This discovery has given a greatly increased significance to the
study of the earth's magnetism. The daily swing, the occasional
'storms,' are seen to be something more than matters of merely local
interest; they have the closest connection with changes going on in
the vast universe beyond; they have an astronomical importance.
And it was soon felt to be necessary to supplement the Magnetic
Observatory at Greenwich by one devoted to the direct study of the
solar surface; and here again that invaluable servant of modern
science, photography, was ready to lend its help. Just as, by the
means of photography, the magnets recorded their own movements, so
even more directly the sun himself makes register of his changes by
the same agency, and gives us at once his portrait and his autograph.
This new department was again due to Airy, and in 1873 the 'Kew'
photo-heliograph, which had been designed by De la Rue for this
work, was installed at Greenwich.
[Illustration: THE DALLMEYER PHOTO-HELIOGRAPH.]
In order to photograph so bright a body as the sun, it is not in
the least necessary to have a very large telescope. The one in
common use at Greenwich from 1875 to 1897, is only four inches in
aperture and even that is usually diminished by a cap to three
inches, and its focal length is but five feet. This is not very much
larger than what is commonly called a 'student's telescope,' but it
is amply sufficient for its work.
This 'Dallmeyer' telescope, so called from the name of its maker,
is one of five identical instruments which were made for use
in the observation of the transit of Venus of 1874, and which,
since they are designed for photographing the sun, are called
'photo-heliographs.'
The image of the sun in the principal focus of this telescope is
about six-tenths of an inch in diameter; but a magnifying lens
is used, so that the photograph actually obtained is about eight
inches. Even with this great enlargement, the light of the sun is
so intense that with the slowest photographic plates that are made
the exposure has to be for only a very small fraction of a second.
This is managed by arranging a very narrow slit in a strip of brass.
The strip is made to run in a groove across the principal focus.
Before the exposure, it is fastened up so as to cut off all light
from entering the camera part of the telescope. When all is ready,
it is released and drawn down very rapidly by a powerful spring, and
the slit, flying across the image of the sun, gives exposure to the
plate for a very minute fraction of a second--in midsummer for less
than a thousandth of a second.
Two of these photographs are taken every fine day at Greenwich;
occasionally more, if anything specially interesting appears to
be going on. But in our cloudy climate at least one day in three
gives no good opportunity for taking photographs of the sun,
and in the winter time long weeks may pass without a chance. The
present Astronomer-Royal, Mr. Christie, has therefore arranged that
photographs with precisely similar instruments should be taken in
India and in the Mauritius, and these are sent over to Greenwich as
they are required, to fill up the gaps in the Greenwich series. We
have therefore at Greenwich, from one source or another, practically
a daily record of the state of the sun's surface.
More recently the 'Dallmeyer' photo-heliograph, though still
retained for occasional use, has been superseded generally by the
'Thompson'; a photographic refractor of nine inches aperture, and
nearly nine feet focal length, presented to the Observatory by Sir
Henry Thompson. The image of the sun obtained after enlargement in
the telescope, with this instrument, is seven and a half inches in
diameter. The 'Thompson' is mounted below the great twenty-six-inch
photographic refractor,--also presented to the Observatory by Sir
Henry Thompson,--in the dome which crowns the centre of the New
Observatory.
A photograph of the sun taken, it has next to be measured, the four
following particulars being determined for each spot: First, its
distance from the centre of the image of the sun; next, the angle
between it and the north point; thirdly, the size of the spot; and
fourthly, the size of the umbra of the spot, that is to say, of
its dark central portion. The size or area of the spot is measured
by placing a thin piece of glass, on which a number of cross-lines
have been ruled one-hundredth of an inch apart, in contact with the
photograph. These cross-lines make up a number of small squares,
each the ten-thousandth (1/10000 in.) part of a square inch in
area. When the photograph and the little engraved glass plate are
nearly in contact, the photograph is examined with a magnifying
glass, and the number of little squares covered by a given spot are
counted. It will give some idea of the vast scale of the sun when
it is stated that a tiny spot, so small that it only just covers
one of these little squares, and which is only one-millionth of the
visible hemisphere of the sun in area, yet covers in actual extent
considerably more than one million of square miles.
The dark spots are not the only objects on the sun's surface. Here
and there, and especially near the edge of the sun, are bright
marks, generally in long branching lines, so bright as to appear
bright even against the dazzling background of the sun itself. These
are called 'faculæ,' and they, like the spots, have their times of
great abundance and of scarcity, changing on the whole at the same
time as the spots.
After the solar photographs have been measured, the measures must be
'reduced,' and the positions of the spots as expressed in longitude
and latitude on the sun computed. There is no difficulty in doing
this, for the position of the sun's equator and poles have long been
known approximately, the sun revolving on its axis in a little more
than twenty-five days, and carrying of course the spots and faculæ
round with him.
There are few studies in astronomy more engrossing than the watch on
the growth and changes of the solar spots. Their strange shapes,
their rapid movements, and striking alterations afford an unfailing
interest. For example, the amazing spectacle is continually being
afforded of a spot, some two, three, or four hundred millions of
square miles in area, moving over the solar surface at a speed
of three hundred miles an hour, whilst other spots in the same
group are remaining stationary. But a higher interest attaches to
the behaviour of the sun as a whole than to the changes of any
particular single spot; and the curious fact has been brought
to light, that not only do the spots increase and diminish in a
regular cycle of about eleven years in length, but they also affect
different regions of the sun at different points of the cycle.
At the time when spots are most numerous and largest, they are
found occupying two broad belts, the one with its centre about 15°
north of the equator, the other about as far south, the equator
itself being very nearly free from them. But as the spots begin to
diminish, so they appear continually in lower and lower latitudes,
until instead of having two zones of spots there is only one, and
this one lies along the equator. By this time the spots have become
both few and small. The next stage is that a very few small spots
are seen from time to time in one hemisphere or the other at a great
distance from the equator, much farther than any were seen at the
time of greatest activity. There are then for a little time three
sun-spot belts, but the equatorial one soon dies out. The two belts
in high latitude, on the other hand, continually increase; but as
they increase, so do they move downwards in latitude, until at
length they are again found in about latitude 15° north or south,
when the spots have attained their greatest development.
[Illustration: PHOTOGRAPH OF A GROUP OF SUN-SPOTS.
(_From a photograph taken at the Royal Observatory, Greenwich,
April, 1882, 20 d. 10 h. 6 m._)]
The clearest connection between the magnetic movements and the
sun-spot changes is seen when we take the mean values of either for
considerable periods of time, as, for instance, year by year. But
occasionally we have much more special instances of this connection.
Some three or four times within the last twenty years an enormous
spot has broken out on the sun, a spot so vast that worlds as great
as our own could lie in it like peas in a breakfast saucer, and in
each case there has been an immediate and a threefold answer from
the earth. One of the most remarkable of these occurred in November,
1882. A great spot was then seen covering an area of more than three
thousand millions of square miles. The weather in London happened
to be somewhat foggy, and the sun loomed, a dull red ball, through
the haze, a ball it was perfectly easy to look at without specially
shading the eyes. So large a spot under such circumstances was quite
visible to the naked eye, and it caught the attention of a great
number of people, many of whom knew nothing about the existence of
spots on the sun.
This great disturbance, evidently something of the nature of a storm
in the solar atmosphere, stretched over one hundred thousand miles
on the surface of the sun. The disturbance extended farther still,
even to nearly one hundred millions of miles. For simultaneously
with the appearance of the spot the magnetic needles at Greenwich
began to suffer from a strange excitement, an excitement which grew
from day to day until it had passed half-way across the sun's disc.
As the twitchings of the magnetic needle increased in frequency and
violence, other symptoms were noticed throughout the length of the
British Isles. Telegraphic communication was greatly interfered
with. The telegraph lines had other messages to carry more urgent
than those of men. The needles in the telegraph instruments
twitched to and fro. The signal bells on many of the railway lines
were rung, and some of the operators received shocks from their
instruments. Lastly, on November 17, a superb aurora was witnessed,
the culminating feature of which was the appearance, at about six
o'clock in the evening, of a mysterious beam of greenish light, in
shape something like a cigar, and many degrees in length, which rose
in the east and crossed the sky at a pace much quicker than but
nearly as even as that of sun, moon, or stars, till it set in the
west two minutes after its rising.
So far we have been dealing only with effects. Their causes still
rest hidden from us. There is clearly a connection between the solar
activity as shown by the spots and the agitation of the magnetic
needles. But many great spots find no answer in any magnetic
vibration, and not a few considerable magnetic storms occur when we
can detect no great solar changes to correspond.
Thus even in the simplest case before us we have still very much
to explain. Two far more difficult problems are still offered us
for solution. What is the cause of these mysterious solar spots?
and have they any traceable connection with the fitful vagaries of
earthly weather? It was early suggested that probably the first
problem might find an answer in the ever-varying combinations and
configurations of the various planets, and that the sun-spots in
their turn might hold the key of our meteorology. Both ideas were
eagerly followed up--not that there was much to support either,
but because they seemed to offer the only possible hope of our
being able to foretell the general current of weather change for
any long period in advance. So far, however, the first idea may be
considered as completely discredited. As to the second, there would
appear to be, in the case of certain great tropical and continental
countries like India, some slight but by no means conclusive
evidence of a connection between the changes in the annual rainfall
and the changes in the spotted surface of the sun. Dr. Meldrum, the
late veteran Director of the great Meteorological Observatory in
Mauritius, has expressed himself as confident that the years of most
spots are the years of most violent cyclones in the Indian Ocean.
But this is about as far as real progress has been made, and it may
be taken as certain that many years more of observation will be
required, and the labours of many skilful investigators, before we
can hope to carry much farther our knowledge as to any connection
between storm and sun.
A further relation of great interest has come to light within the
last few years. The year 1868 opened a new epoch in the study of
eclipses of the sun. These, perhaps, scarcely lie within the scope
of a book on the Royal Observatory, since Greenwich has seen but
one in all its history. That fell in the year 1715; for the next
it must wait many centuries. Yet, as the late Astronomer Royal
conducted three expeditions to see total eclipses, and as the
present Astronomer Royal has undertaken a like number, and members
of the staff have been sent on other occasions, it may not be deemed
quite a digression to refer to one feature which they have brought
to light.
When the dark body of the moon has entirely hidden the sun,
we have revealed to us, there and then only, that strange and
beautiful surrounding of the sun which we call the corona. The
earlier observations of the corona seem to reveal it as a body of
the most weird and intricate form, a form which seemed to change
quite lawlessly from one eclipse to another. But latterly it has
been abundantly clear that the forms which it assumes may be
grouped under a few well-defined types. In 1878 the corona was of
a particularly simple and striking character. Two great wings shot
out east and west in the direction of the sun's equator; round
either pole was a cluster of beautiful radiating 'plumes.' It was
then recollected that the corona of 1867 had been of precisely the
same character, both years being years when sun-spots were at their
fewest. The coronæ, on the other hand, seen at times when sun-spots
are more abundant, were of an altogether different character, the
streamers being irregularly distributed all round the sun. Other
types also have been recognized, and it is perfectly apparent
that the corona changes its shape in close accordance with the
eleven-year period. The eclipses of 1889 and 1900, for example,
showed coronæ that bore the very closest resemblance to those of
1878 and 1866, the interval of eleven years bringing a return to the
same form.
The further problem, therefore, now confronts us: Does the corona
produce the sun-spots, or do the sun-spots produce the corona, or
are both the result of some mysterious magnetic action of the sun,
an action powerful enough on occasion to thrill through and through
this world of ours, ninety-three millions of miles away?
CHAPTER XI
THE SPECTROSCOPIC DEPARTMENT
Another department was set on foot by Airy at the same time as
the Heliographic Department, and in connection with it; and it is
the department which has the greatest of interest for the general
public. This deals with astronomical physics, or astrophysics, as
it is sometimes more shortly called; the astronomy, that is, which
treats of the constitution and condition of the heavenly bodies, not
with their movements.
The older astronomy, on the other hand, confined itself to the
movements of the heavens so entirely that Bessel, the man whose
practical genius revolutionized the science of observation,
and whose influence may be traced throughout in Airy's great
reconstitution of Greenwich Observatory, denied that anything but
the study of the celestial movements had a right to the title of
astronomy at all. Hardly more than sixty years ago he wrote:
'What astronomy is expected to accomplish is evidently at all
times the same. It may lay down rules by which the movements
of the celestial bodies, as they appear to us upon the earth,
can be computed. All else which we may learn respecting these
bodies, as, for example, their appearance, and the character of
their surfaces, is, indeed, not undeserving of attention, but
possesses no proper astronomical interest. Whether the mountains
of the moon are arranged in this way or in that is no further
an object of interest to astronomers than is a knowledge of the
mountains of the earth to others. Whether Jupiter appears with
dark stripes upon its surface, or is uniformly illuminated,
pertains as little to the inquiries of the astronomer; and its
four moons are interesting to him only for the motions they
have. To learn so perfectly the motions of the celestial bodies
that for any specified time an accurate computation of these can
be given--that was, and is, the problem which astronomy has to
solve.'
There is a curious irony of progress which seems to delight in
falsifying the predictions of even master minds as to the limits
beyond which it cannot advance. Bessel laid down his dictum as to
the true subjects of astronomical inquiry, Comte declared that
we could never learn what were the elements of which the stars
were composed, at the very time that the first steps were being
taken towards the creation of a research which should begin by
demonstrating the existence in the heavenly bodies of the elements
with which we are familiar on the earth, and should go on to prove
itself a true astronomy, even in Bessel's restricted sense, by
supplying the means for determining motion in a direction which he
would have thought impossible--that is to say, directly to or from
us.
The years that followed Kirchhoff's application of the spectroscope
to the study of the sun, and his demonstration that sodium and iron
existed in the solar atmosphere, were crowded with a succession
of brilliant discoveries in the same field. Kirchhoff, Bunsen,
Angström, Thalèn, added element after element to the list of those
recognized in the sun. Huggins and Miller carried the same research
into a far more difficult field, and showed us the same elements
in the stars. Rutherfurd and Secchi grouped the stars according to
the types of their spectra, and so laid the foundations of what
may be termed stellar comparative anatomy. Huggins discovered true
gaseous nebulæ, and so revived the nebular theory, which had been
supposed crushed when the great telescope of Lord Rosse appeared to
have resolved several portions of the Orion nebula into separate
stars. The great riddle of 'new stars'--which still remains a
riddle--was at least attacked, and glowing hydrogen was seen to be a
feature in their constitution. Glowing hydrogen, again, was, in the
observation of total eclipses, seen to be a principal constituent
of those surroundings of our own sun which we now call prominences
and chromosphere. Then the method was discovered of observing the
prominences without an eclipse, and they were found to wax and wane
in more or less sympathy with the solar spots. Sun-spots, planets,
comets, meteors, variable stars, all were studied with the new
instrument, and all yielded to it fresh and valuable, and often
unexpected, information.
[Illustration: THE GREAT NEBULA IN ORION.
(_From a photograph taken at the Royal Observatory Greenwich,
December 1, 1899, with an exposure of 2-1/4 hours._)]
In this activity Greenwich Observatory practically took no part.
Airy, ever mindful of the original purpose of the Observatory, and
deeply imbued with views similar to those which we have quoted from
Bessel, considered that the new science lay outside the scope of his
duties, until in Mr., now Sir William, Huggins's skilful hands
the spectroscope showed itself not only as a means for determining
the condition and constitution of the stars, but also their
movements--until, in short, it had shown itself as an astronomical
instrument even within Bessel's narrow definition.
The principle of this inquiry is as follows: If a source of light
is approaching us very rapidly, then the waves of light coming from
it necessarily appear a little shorter than they really are, or, in
other words, that light appears to be slightly more blue--the blue
waves being shorter than the red--than it really is. A similar thing
with regard to the waves of sound is often noticed in connection
with a railway train. If an express train, the whistle of which is
blowing the whole time, dashes past us at full speed, there is a
perceptible drop in the note of the whistle after it has gone by.
The sound waves as it was coming were a little shortened, and the
whistle therefore appeared to have a sharper note than it had in
reality. And in the same way, when it had gone by, the sound waves
were a little lengthened, making the note of the whistle appear a
very little flatter.
Such a change of colour in a star could never have been detected
without the spectroscope; but since when light passes through a
prism the shorter waves are refracted more strongly, that is to
say, are more turned out of their course than the longer, the
spectroscope affords us the means of detecting and measuring this
change. Let us suppose that the lines of hydrogen are recognized
in a given star. If we compare the spectrum of this star with the
spectrum of a tube containing hydrogen and through which the
electric spark is passing, we shall be able to see whether any
particular hydrogen line occupies the same place as shown by the two
spectra. If the line from the star is a little to the red of the
line from the tube, the star must be receding from us; if to the
blue, approaching us. The amount of displacement may be measured by
a delicate micrometer, and the rate of motion concluded from it.
[Illustration: THE HALF-PRISM SPECTROSCOPE ON THE SOUTH-EAST
EQUATORIAL.]
The principle is clear enough. The actual working out of the
observation was one of very great difficulty. The movements of the
stars towards us, or away from us, are, in general, extremely slow
as compared with the speed of light itself; and hence the apparent
shift in the position of a line is only perceptible when a very
powerful spectroscope is used. This means that the feeble light of
a star has to be spread out into a great length of spectrum, and
a very powerful telescope is necessary. The work of observing the
motions of stars in the line of sight was started at Greenwich in
1875, the 'Great Equatorial' being devoted to it. This telescope, of
12-3/4 inches aperture, was not powerful enough to do much more than
afford a general indication of the direction in which the principal
stars were moving, and to confirm in a general way the inference
which various astronomers had found, from discussing the proper
motions of stars, that the sun and the solar system were moving
towards that part of the heavens where the constellations Hercules
and Lyra are placed. In 1891, therefore, the work was discontinued,
and as already mentioned, the 12-3/4 telescope by Merz was removed
to make room for the present much larger instrument by Sir Howard
Grubb, upon the same mounting. The new telescope being much larger
than the one for which mounting and observing room were originally
built, it was not possible to put the spectroscope in the usual
position, in the same straight line as the great telescope. It was
therefore mounted under it, and parallel to it, and the light of
the star was brought into it after two reflections. The observer
therefore stood with his back to the object and looked down into
the spectroscope. It had, however, become apparent by this time
that this most delicate field of work was one for which photography
possessed several advantages, and as Sir Henry Thompson had made
the munificent gift to the Observatory of a great photographic
equatorial, it was resolved to devote the 28-inch telescope chiefly
to double-star work, and to transfer the spectroscope to the 'New
Building.'
The 'New Observatory' in the south ground is crowned indeed with
the dome devoted to the great Thompson photographic refractor, but
this is not its chief purpose. Its principal floor contains four
fine rooms which are used as 'computing rooms'--for the office
work, that is to say, of the Observatory. Of these the principal
is in the north wing, where the main entrance is placed, and is
occupied by the Astronomer Royal and the two chief assistants. The
basement contains the libraries and the workshops of the mechanics
and carpenters. The upper floor will eventually be used for the
storage of photographs and manuscripts, and the terrace roofs of
the four wings will be exceedingly convenient for occasional
observations, as, for example, of meteor showers. The central dome,
which rises high above the level of the terraces, is the only room
in the building devoted to telescopic work. As in the New Altazimuth
building, a ring of circular lights just below the coping of the
wall recalls the portholes of a ship, and again reminds us of the
connection of the Observatory with navigation.
[Illustration: THE WORKSHOP.]
Here the spectroscope is now placed, but not, as it happens, on
the Thompson refractor. The equatorial mounting in this new dome
is a modification of what is usually called the 'German' form of
mounting--that is to say, there is but one pier to support the
telescope, and the telescope rides on one side of the pier and a
counterpoise balances it on the other The 'Great Equatorial,' on
the other hand, is an example of the English mounting, and has two
piers, one north and the other south, whilst the telescope swings
in a frame between them. In the new dome three telescopes are found
rigidly connected with each other on one side of the pier, the
telescopes being (1) the great Thompson photographic telescope,
double the aperture and double the focal length of the standard
astrographic telescope used for the International Photographic
Survey; (2) the 12-3/4 telescope by Merz, that used to be in the
great South-East dome, but which is now rigidly connected with the
Thompson refractor as a guide telescope; and (3) a photographic
telescope of 9 inches aperture, already described as the 'Thompson'
photo-heliograph, and used for photographing the sun or in eclipse
expeditions. The counterpoise to this collection of instruments is
not a mere mass of lead, but a powerful reflector of 30 inches'
aperture, and it is to this telescope that the spectroscope is now
attached. At the present time, however (August, 1900), regular work
has not been commenced with it.
[Illustration: THE 30-INCH REFLECTOR WITH THE NEW SPECTROSCOPE
ATTACHED.]
Beside this attempt to determine the motions of the stars as they
approach us or retreat from us, on rare occasions the spectroscope
has been turned on the planets. As these shine by reflected light,
their spectra are normally the same as that of the sun. Mars
appeared to the writer, as to Huggins and others, to show some
slight indication of the presence of water vapour in its atmosphere.
Jupiter and Saturn show that their atmospheres contain some
absorbing vapour unknown to ours. And Uranus and Neptune, faint and
distant as they are, not only show the same dark band given by the
two nearer planets, but several others. More attractive has been
the examination of the spectra of the brighter comets that have
visited us. The years 1881 and 1882 were especially rich in these.
The two principal comets of 1881 were called after their respective
discoverers, Tebbutt's and Schaeberle's. They were not bright
enough to attract popular attention, though they could be seen with
the naked eye, and both gave clear indications of the presence of
carbon, their spectra closely resembling that of the blue part of a
gas or candle flame. There was nothing particularly novel in these
observations, since comets usually show this carbon spectrum, though
why they should is still a matter for inquiry; but the two comets of
the following year were much more interesting. Both comets came very
near indeed to the sun. The earlier one, called from its discoverer
Comet Wells, as it drew near to the sun, began to grow more and more
yellow, until in the first week of June it looked as full an orange
as even the so-called red planet, Mars. The spectroscope showed the
reason of this at a glance. The comet had been rich in sodium. So
long as it was far from the sun the sodium made no sign, but as it
came close to it the sodium was turned into glowing vapour under the
fierce solar heat. And as the writer saw it in the early dawn of
June 7, the comet itself was a disc of much the same colour as Mars,
whilst its spectrum resembled that of a spirit lamp that has been
plentifully fed with carbonate of soda or common salt. The 'Great
Comet' of the autumn of the same year, and which was so brilliant
an object in the early morning, came yet nearer to the sun, and the
heating process went on further. The sodium lines blazed up as they
had done with Comet Wells, but under the fiercer stress of heat to
which the Great Comet was subjected, the lines of iron also flashed
out, a significant indication of the tremendous temperature to which
it was exposed.
There are two other departments of spectroscopic work which it was
attempted for a time to carry on as part of the Greenwich routine.
These were the daily mapping of the prominences round the sun, and
the detailed examination of the spectra of sun-spots. Both are
almost necessary complements of the work done in the heliographic
department--that is to say, the work of photographing the appearance
of the sun day by day, and of measuring the positions and areas of
the spots. For the spots afford but one index out of several, of
the changes in the sun's activity. The prominences afford another,
nor can we at the present moment say authoritatively which is the
more significant. Then again, with regard to the spots themselves,
it is not certain that either their extent or their changes of
appearance are the features which it is most important for us to
study. We want, if possible, to get down to the soul of the spot,
to find out what makes one spot differ from another; and here the
spectroscope can help us. Great sun-spots are often connected with
violent agitation of the magnetic needles, and with displays of
auroræ. But they are not always so, and the inquiry, 'What makes
them to differ?' has been made again and again, without as yet
receiving any unmistakable answer. The great spot of November, 1882,
which was connected with so remarkable an aurora and so violent a
magnetic storm, was as singular in its spectrum as in its earthly
effects. The sun was only seen through much fog, and the spectrum
was therefore very faint, but shooting up from almost every part of
its area, except the very darkest, were great masses of intensely
brilliant hydrogen, evidently under great pressure. The sodium
lines were extremely broadened, and on November 20 a broad bright
flame of hydrogen was seen shooting up at an immense speed from one
edge of the nucleus. A similar effect--an outburst of intensely
luminous hydrogen--has often been observed in spots which have
been accompanied by great magnetic storms; and it may even be that
it is this violent eruption of intensely heated gas which has the
directest connection with the magnetic and auroral disturbances here
upon earth.
This sun-spot work was not carried on for very long, as only one
assistant could be spared for the entire solar work of whatever
character. Yet in that time an interesting discovery was made by the
writer--namely, that in the green part of the spectrum of certain
spots a number of broad diffused lines or narrow bands made their
appearance from time to time, and especially when sun-spots were
increasing in number, or were at their greatest development.
The prominence work had also to be dropped, partly for the same
reason, but chiefly because the atmospheric conditions at Greenwich
are not suitable for these delicate astrophysical researches. When
the Observatory was founded 'in the golden days' of Charles II.,
Greenwich was a little country town far enough removed from the
great capital, and no interference from its smoke and dust had to be
feared or was dreamt of. Now the 'great wen,' as Cobbett called it,
has spread far around and beyond it, and the days when the sky is
sufficiently pure round the sun for successful spectrum work on the
spots or prominences are few indeed.
Whether in the future it will be thought advisable for the Royal
Observatory to enter into serious competition in inquiries of
this description with the great 'astrophysical' observatories of
the Continent and of America--Potsdam, Meudon, the Lick, and the
Yerkes--we cannot say. That would involve a very considerable
departure from its original programme, and probably also a departure
from its original site. For the conditions at Greenwich tend to
become steadily less favourable for such work, and it would most
probably be found that full efficiency could only be secured by
setting up a branch or branches far from the monster town.
With the older work it is otherwise. So long as Greenwich Park
and Blackheath are kept--as it is to be hoped they always will
be--sacred from the invasion of the builder; so long as no
new railways burrow their tunnels in the neighbourhood of the
Observatory, so long the fundamental duties laid upon Flamsteed,
'of Rectifying the Tables of the Motions of the Heavens and the
Places of the Fixed Stars,' will be carried out by his successors on
Flamsteed Hill.
CHAPTER XII
THE ASTROGRAPHIC DEPARTMENT
The two last departments mentioned, the heliographic and
spectroscopic, lie clearly and unmistakably outside the terms of
the original warrant of the Observatory, though the progress of
science has led naturally and inevitably to their being included in
the Greenwich programme. But the Astrographic Department, though
it could no more have been conceived in the days of Charles II.
than the spectroscopic, does come within the terms of the warrant,
and is but an expansion of that work of 'Rectifying the Places of
the Fixed Stars,' which formed part of the programme enjoined upon
Flamsteed, the first Astronomer Royal, at the first foundation of
the Observatory, and which was so diligently carried out by him, the
first Greenwich catalogue, containing about 3000 stars, being due to
his labours.
[Illustration: 'CHART PLATE' OF THE PLEIADES.
(_From a photograph taken at the Royal Observatory, Greenwich, with
an exposure of forty minutes._)]
His immediate successors did much less in this field, though
Bradley's observations were published, long after his death, as a
catalogue of 3222 stars, in some aspects the most important ever
issued. Pond, the sixth Astronomer Royal, restored catalogue-making
to a prominent place in the Greenwich routine, and his precedent
is sedulously followed to-day. But each of these was confined to
about 3000 stars. The necessity has long been felt for a much
ampler census, and Argelander, at the Bonn Observatory, brought
out a catalogue of 324,000 stars north of South declination 2°, a
work which has been completed by Schönfeld, who carried the census
down to South declination 23°, and by the two great astronomers of
Cordoba, South America, Dr. Gould and Dr. Thome, by whom it was
extended to the South Pole.
These last three catalogues embrace stars of all magnitudes down to
the 9th or 10th; but certain astronomers had endeavoured to go much
lower, and to make charts of limited portions of the sky down to
even the 14th magnitude.
From the very earliest days that men observed the stars, they
could not help noticing that 'one star differeth from another star
in glory,' and consequently they divided them into six classes,
according to their brightness--classes which are commonly spoken
of now as magnitudes. The ordinary 6th magnitude star is one which
can be clearly seen by average sight on a good night, and it gives
us about one-hundredth the light of an average 1st magnitude star.
Sirius, the brightest of all the fixed stars, is called a 1st
magnitude star, but is really some six or seven times as bright as
the average. It would take, therefore, more than two and a half
million stars of the 14th magnitude to give as much light as Sirius.
It is evident that so searching a census as to embrace stars of the
14th magnitude would involve a most gigantic chart. But the work
went on in more than one Observatory for a considerable time, until
at last the observers entered on to the region of the Milky Way.
Here the numbers of the stars presented to them were so great as to
baffle all ordinary means of observation. What could be done?
Just at this time immense interest was caused in the astronomical
world by the appearance of the great comet of 1882. It was watched
and observed and sketched by countless admirers, but more important
still, it was photographed, and some of its photographs, taken at
the Royal Observatory, Cape of Good Hope, showed not only the comet
with marvellous beauty of detail, but also thousands of stars,
and the success of these photographs suggested to her Majesty's
Astronomer at the Cape, Dr. Gill, that in photography we possessed
the means for making a complete sky census even to the 14th
magnitude.
The project was thought over in all its bearings, and in 1887
a great conference of astronomers at Paris resolved upon an
international scheme for photographing the entire heavens. The
work was to be divided between eighteen Observatories of different
nationalities. It was to result in a photographic chart extending to
the 14th magnitude, and probably embracing some forty million stars,
and a catalogue made from measures of the photographs down to the
11th magnitude, which would probably include between two and three
million stars.
[Illustration: THE CONTROL PENDULUM AND THE BASE OF THE THOMPSON
TELESCOPE.]
The eighteen Observatories all undertook to use instruments of the
same capacity. This was to be a photographic refractor, with an
object-glass of 13 inches aperture and 11 feet focus. At Greenwich
this telescope is mounted equatorially--that is, so as to follow the
stars in their courses--and is mounted on the top of the pier that
once supported Halley's quadrant. The telescope is driven by a most
efficient clock, whose motive power is a heavy weight. The rate of
the weight in falling is regulated by an ingenious governor, which
brings its speed very nearly indeed to that of the star, and any
little irregularities in its motion are corrected by the following
device. A seconds pendulum is mounted in a glass case on the wall
of the Observatory, and a needle at the lower end of the pendulum
passes at each swing through a globule of mercury. On one of the
wheels of the clock are arranged a number of little brass points,
at such intervals apart that the wheel, when going at the proper
rate, takes exactly one second to move through the distance between
any pair. A little spring is arranged above the wheel, so that
these points touch it as they pass. If this occurs exactly as the
pendulum point passes through the mercury nothing happens, but if
the clock is ever so little late or early, the electric current from
the pendulum brings into action a second wheel, which accelerates
or retards the driving of the clock, as the case may be. The total
motion, therefore, is most beautifully even.
[Illustration: THE ASTROGRAPHIC TELESCOPE.
(_Reproduced from 'Engineering' by permission._)]
But even this is not quite sufficient, especially as the plates
for the great chart have to be exposed for at least forty minutes.
Rigidly united with the 13-inch refractor, so that the two look
like the two barrels of a huge double-barrelled gun, is a second
telescope for the use of the observer. In its eyepiece are fixed
two pairs of cross spider lines, commonly called wires, and a
bright star, as near as possible to the centre of the field to be
photographed, is brought to the junction of two wires. Should the
star appear to move away from the wire, the observer has but to
press one of two buttons on a little plate which he carries in his
hand, and which is connected by an electric wire with the driving
clock, to bring it back to its position.
The photographs taken with this instrument are of two kinds. Those
for the great chart have but a single exposure, but this lasts for
forty minutes. Those for the great catalogue have three exposures on
them, the three images of a star being some 20 seconds of arc apart.
These exposures are of six minutes', three minutes', and twenty
seconds' duration, and the last exposure is given as a test, since,
if stars of the 9th magnitude are visible with an exposure of twenty
seconds, stars of the 11th magnitude should be visible with three
minutes' exposure.
Thus it will be seen that in three minutes an impression is got of
many scores of stars, whose places it would require many hours to
determine at the transit instrument. But the positions of these
stars on the plate still remain to be measured. For this purpose a
net-work of lines, at right angles to each other, is printed on the
photograph before its development, and, after it has been developed,
washed and dried, the distances of the stars from their nearest
cross-lines are measured in the measuring machine.
[Illustration: THE DRIVING CLOCK OF THE ASTROGRAPHIC TELESCOPE.
(_Reproduced from 'Engineering' by permission._)]
The measuring machine is constructed to hold two plates, one half
its breadth higher than the other. In fact, in each of the two
series of photographs the whole sky is taken twice, but the two
photographs of any region are not simply duplicates of each other.
The centre of each plate is at a corner of four other plates, and
in the micrometer the stars on the quarter common to two plates are
measured simultaneously.
In this way will be carried out a great census of the sky that will
exceed Flamsteed's ten thousand fold. And just as Flamsteed's was
but the first of many similar catalogues, so, no doubt, will this be
followed by others--not superseded, for its value will increase with
its age and the number of those that follow it, by comparison with
which it will prove an inexhaustible mine of information concerning
the motions of the stars and the structure of the universe.
There is a great difference between the work of the observer with
the 'Astrographic Telescope,' as this great twin photographic
instrument is called, and the work of the transit observer. The
latter sees the star gliding past him, and telegraphs the instant
that the star threads itself on each of the ten vertical wires in
succession. The astrographic observer, on the other hand, sees his
star shining almost immovably in the centre of his field, threaded
on the two cross wires placed there, for the driving-clock moves
the telescope so as to almost exactly compensate for the rotation
movement of the earth. The observer's duty in this case is to
telegraph to his driving-clock, when it has in the least come short
of or exceeded its duty, and so to bring back the 'guiding star' to
its exact proper place on the cross wires.
So far, the work of the Astrographic Department has been, as
mentioned above, a development on an extraordinary scale, but a
development still, of the original programme of the Observatory.
But the munificent gift of Sir Henry Thompson has put it within
the power of the Astronomer Royal to push this work of sidereal
photography a stage further. Sir Henry Thompson gave to the
Observatory, not merely the photographic refractor of 9 inches'
aperture, now used for solar photography, and known as the 'Thompson
photo-heliograph,' but also one of 26 inches' aperture and 22-1/2
feet focal length. This instrument was specially designed of exactly
double the dimensions of the standard astrographic telescope used
for the International Photographic Survey, the idea being that,
in the case of a field of special interest and importance, a
photograph could be obtained with the larger instrument on exactly
double the scale given by the smaller. It has rather, however,
found its usefulness in a slightly different field. The observation
of the satellites of Jupiter was suggested by Galileo as a means
of determining the longitude at sea. As already pointed out, the
suggestion did not prove to be a practical one for that purpose, but
observations of the satellites have been made none the less with
a view simply to improving our knowledge of their movements, and
of the mass of Jupiter. The utilitarian motive for the work having
fallen through, it has been carried on as a matter of pure science.
And the work has not stopped with the satellites of Jupiter; eight
satellites were in due time discovered to Saturn, four to Uranus,
and two to Mars; and though these could give not the remotest
assistance to navigation, they too have been made the subjects of
observation for precisely the same reason as those of Jupiter have
been.
[Illustration: THE THOMPSON TELESCOPE IN THE NEW DOME.]
In just the same way, when the discovery of Neptune was followed by
that of a solitary companion to it, this also had to be followed.
The difficulties in the way of observing the fainter of all these
satellites were considerable, and the work has been mostly confined
to two or three observatories possessing very large telescopes. As
the largest telescope at Greenwich was only 7 inches in aperture up
to 1859, and only 12-3/4 inches up to 1893, it is only very recently
that it has been able to take any very substantial part in satellite
measures. But since the Thompson photographic telescope was set up,
it has been found that a photograph of Neptune and its satellite can
be taken in considerably less time than a complete set of direct
measures can be made, whilst the photograph, which can be measured
at leisure during the day, gives distinctly the more accurate
results.
So, too, the places of the minor planets can be got more accurately
and quickly by means of photographs with this great telescope than
by direct observation, and photographs of the most interesting
of them all, the little planet Eros, have been very successfully
obtained. So that, though doing nothing directly to improve the
art of navigation, or to find the longitude at sea, the great
photographic refractor takes its share in the work of 'Rectifying
the Tables of the Planets.'
[Illustration: THE NEBULÆ OF THE PLEIADES.
(_From a photograph taken at the Royal Observatory, Greenwich,
December 3, 1899, with an exposure of three hours._)]
The reflector of 30 inches' aperture, which acts as a counterpoise
to the sheaf of telescopes of the Thompson, is intended for use with
the spectroscope, the quality which mirrors possess of bringing
all rays, whatever their colour, to the same focus being of great
importance for spectroscopic work. But the experiments which have
been made with it in celestial photography have proved so extremely
successful as to cause the postponement of the recommencement of
the spectroscopic researches. Chief amongst these photographs are
some good ones of the moon, and more recently some exceedingly fine
photographs of the principal nebulæ.
In no department of astronomy has photography brought us such
striking results as in regard to the nebulæ. Dr. Roberts' photograph
of the great nebula in Andromeda converted the two or three
meaningless rifts--which some of the best drawings had shown--into
the divisions between concentric rings; and what had appeared a
mere shapeless cloud was seen to be a vast symmetrical structure, a
great sidereal system in the making. The great nebula in Orion has
grown in successive photographs in detail and extent, until we have
a large part of the constellation bound together in the convolutions
of a single nebula of the most exquisite detail and most amazing
complexity. The group of the Pleiades has had a more wonderful
record still. Manifestly a single system even to the naked eye,
and showing some faint indications of nebulosity in the telescope,
the photographs have revealed its principal stars shining out from
nebulous masses, in appearance like carded wool, and have shown
smaller stars threaded on nebulous lines like pearls upon a string.
Such photographs are, of course, of no utilitarian value, and at
present they lead us to no definite scientific conclusions. They
lie, therefore, doubly outside the limits of the purely practical,
but they attract us by their extreme beauty, and by the amazing
difficulty of the problems they suggest. How are these weird masses
of gas retained in such complex form over distances which must
be reckoned by millions of millions of miles? By what agency are
they made to glow so as to be visible to us here? What conceivable
condition threads together suns on a line of nebula? What universes
are here in the making, or perhaps it may be falling into ruin and
decay?
CHAPTER XIII
THE DOUBLE-STAR DEPARTMENT
The foregoing chapters will have shown that though the original
purpose of the Observatory has always been kept in view, yet the
progress of science has caused many researches to be undertaken
which overstep its boundaries. Thus in the present transit room,
beside the successive transit instruments we find upon the wall two
long thin tubes, labelled respectively Alpha Aquilæ and Alpha Cygni.
These were two telescopes set up by Pond for a special purpose.
Dr. Brinkley, Royal Astronomer for Ireland, had announced that he
had found that several stars shifted their apparent place in the
sky in the course of a year, due to the change in the position of
the earth from which we view them, by an amount which would show
that they were only about six to nine billions of miles distant
from us; or, in other words, they showed a parallax of from two to
three seconds of arc. Pond was not able to confirm these parallaxes
from his observations, and to decide the point he set up these two
telescopes, the Alpha Aquilæ telescope being rigidly fixed on the
west side of the pier of Troughton's mural circles; the Alpha Cygni
telescope on another pier, the one which now forms the base of the
pier of the astrographic telescope. Pond's method was to compare
the position of these two stars with that of a star almost exactly
the same distance from the pole, but at a great distance from it in
time of crossing the meridian; in other words, of almost the same
declination, but widely different right ascension. The result proved
that Brinkley was wrong, and vindicated the delicacy and accuracy of
Pond's observations.
These two telescopes, therefore, had their day and ceased to be.
Others have followed them. An ingenious telescope was set up by
Sir George Airy in order to ascertain if the speed of light were
different when passing through water than when passing through
air. Or, in other words, if the aberration of light would give the
same value as at present if we observed through water. The water
telescope, as it was called, is kept on the ground floor of the
central octagon of the new observatory. The observations obtained
with it were hardly quite satisfactory, but gave on the whole a
negative result.
Turning back to the transit room, and leaving it by the south-west
door, we come into the little passage which leads at the back of
Bradley's transit room into the lower computing room. Just inside
this passage, on the left-hand side, there is a little room of
a most curious shape, the 'reflex zenith room.' Here is fixed a
telescope pointing straight upwards, the eye-piece being fixed
by the side of the object-glass. The light from a star--the star
Gamma Draconis--which passes exactly over the zenith of Greenwich,
enters the object-glass, passes downwards to a basin of mercury, and
is reflected upwards from the surface of the mercury to a little
prism placed over the centre of the object-glass, from which it
is reflected again into the eye-piece. By means of this telescope
the distance of the star Gamma Draconis from the zenith could
be measured very exactly, and, consequently, the changes in the
apparent position of the star due to aberration, parallax, and other
causes could be very exactly followed, and the corrections to be
applied on account of these causes precisely determined.
This particular telescope was devised by Airy, and the observations
with it were continued to the end of his reign. The germ of the idea
may be traced back, however, to the time of Flamsteed, who would
seem to have occasionally observed Gamma Draconis from the bottom
of a deep well; the precise position of the well is not, however,
now known. Later, Bradley set up his celebrated 12-1/2-foot zenith
sector, still preserved in the transit room, first at Wanstead
and then at Greenwich, for the determination of the amount of
aberration. Later, a zenith tube by Troughton, of 25 feet focus, was
used by Pond in conjunction with the mural circle for observations
of Gamma Draconis in order to determine the zenith point of the
latter instrument.
These telescopes for special purposes have passed out of use.
Observations with the spectroscope have been suspended for some
years. The work of the Astrographic Department will come to an end,
in the ordinary course of events, when the programme assigned to
Greenwich in the International Scheme is completed.
Within the last few years a new department has come into being at
Greenwich--a department which has been steadily worked at many
foreign public observatories, but only recently here.
This is the Department of Double-Star Observation. The first double
star, Zeta Ursæ Majoris, was discovered 250 years ago. Bradley
discovered two exceedingly famous double stars whilst still a young
man observing with his uncle at Wanstead--Gamma Virginis and Castor.
Bradley made also other discoveries of double stars after his
appointment to Greenwich, and Maskelyne succeeded him in the same
line, but the great foundation of double-star astronomy was laid by
Sir William Herschel.
At first it was supposed that double stars were double only in
appearance; one star comparatively near us 'happened' to lie in
almost exactly the same direction as another star much further
off. It was, indeed, in the very expectation that this would
prove to be the case, that the elder Herschel first took up their
study. But he was soon convinced that many of the objects were
true double stars--members of the same system of which the smaller
revolved round the larger--not merely apparently double, one star
appearing by chance to be close to another with which it had no
connection--but real double stars. The discovery of these has led to
the establishment of a new department of astronomy, again scientific
rather than utilitarian.
[Illustration: DOUBLE-STAR OBSERVATION WITH THE SOUTH-EAST
EQUATORIAL.
(_From a photograph by Mr. Edney._)]
As mentioned above, it is only recently that Greenwich has
taken any appreciable part in this work. Under Airy, the largest
equatorial of the time had been furnished with a good micrometer,
and observations of one or two double stars been made now and
again; but Airy's programme of work was far too rigid, and kept the
staff too closely engaged for such observations to be anything but
extremely rare. And, indeed, when the micrometers of the equatorials
were brought into use, they were far more generally devoted to the
satellites of Saturn than to the companions of stars. In the main,
double-star astronomy has been in the hands of amateurs, at least
in England. But the discovery in recent years of many pairs so
close that a telescope of the largest size is required for their
successful observation, has put an important section of double stars
beyond the reach of most private observers, and therefore the great
telescope at Greenwich is now mainly devoted to their study. The
Astronomer Royal, therefore, soon after the completion of the great
equatorial of 28-inches aperture placed in the south-east dome,
added this work to the Observatory programme.
The 28-inch equatorial is a remarkable-looking instrument, its
mounting being of an entirely different kind to that of the other
equatorials in the Observatory, with the solitary exception of the
Shuckburgh, which is set up in a little dome over the chronograph
room. The Shuckburgh was presented to the Observatory in the year
1811, by Sir G. Shuckburgh. It was first intended to be mounted as
an altazimuth, but proved to be unsteady in that position, and was
then converted into an equatorial without clockwork, and mounted in
its present position. The position is about as hopelessly bad a one
as a telescope could well have, completely overshadowed as it is by
the trees and buildings close at hand. The dome is a small one, and
the arrangements for the shutters and for turning the dome are as
bad as they could possibly be. It has practically been useless for
the last forty years.
Its only interest is that the method of mounting employed is a small
scale model of that of the great telescope in the S.-E. dome. In the
German or Fraunhofer form of mounting for an equatorial there is but
a single pillar, which carries a comparatively short polar axis. At
the upper end of the polar axis we find the declination axis, and
at one end of the declination axis is the telescope, whilst at the
other end is a heavy weight to counterpoise it. The German mounting
has the advantage that the telescope can easily point to the pole
of the heavens; its drawbacks are that, except in certain special
forms, the telescope cannot travel very far when it is on the
same side of the meridian as the star to which it is pointed, the
end of the telescope coming into contact under such circumstances
with the central pier, whilst the introduction of mere deadweight
as the necessary counterpoise, is not economical. It has been
already pointed out that the present Astronomer Royal has not only
considerably modified the German mounting in the great collection of
telescopes in the Thompson dome, but has used a powerful reflector
as a counterpoise to the sheaf of refractors at the other end of the
declination axis.
The English equatorial requires two piers. Between these two piers
is a long polar axis. Both in the little Shuckburgh and in the
great 28-inch equatorial the frame of the polar axis consists of
six parallel rods disposed in two equilateral triangles, with their
bases parallel to each other, the telescope swinging in the space
between the two bases. The construction of this form of equatorial,
therefore, is expensive, as it requires two piers. It takes much
more room than the German form, and the telescope cannot be directed
precisely to the pole. But the instrument is symmetrical, there is
no deadweight, and the telescope can follow a star from rising to
setting without having to be reversed on crossing the meridian.
The great stability of the English form of mounting, therefore,
commended it very highly to Airy, and he designed the great
Northumberland equatorial of the Cambridge Observatory on that plan,
as well as one for the Liverpool Observatory at Bidston, and in 1858
the S.-E. equatorial at Greenwich.
The telescope at first mounted upon it had an object-glass of 12-3/4
inches' aperture, and 18 feet focal length. That was dismounted
in 1891, and is now used as the guiding telescope of the Thompson
26-inch photographic refractor. Its place was taken by an immensely
heavier instrument, the present refractor of 28 inches' aperture,
and 28 feet focal length; and that this change was effected safely
was an eloquent testimony to the solidity of the original mounting.
The clock that drives this great instrument, so that it can follow
a star or other celestial object in its apparent daily motion
across the sky, is in the basement of the S.-E. tower. It is a very
simple looking instrument, a conical pendulum in a glass case. The
pendulum makes a complete revolution once in two seconds. Below
it in a closed case is a water turbine. A cistern on the roof of
the staircase supplies this turbine with water, having a fall of
about thirty feet. The water rushing out of the arms of the turbine
forces it backward, and the turbine spins rapidly round, driving a
spindle which runs up into the dome, and gears through one or two
intermediate wheels with the great circle of the telescope; the
extremely rapid rotation of the spindle, four times in a second,
being converted by these intermediate wheels into the exceedingly
slow one of once in twenty-four hours. Just above the centre of
motion of the turbine is a set of three small wheels, all of exactly
the same size, and of the same number of teeth. Of these the bottom
wheel is horizontal, and is turned by the turbine. The top wheel
is also horizontal, and is turned by the pendulum. The third wheel
gears into both these, and is vertical. If the top and bottom wheels
are moving exactly at the same rate, the intermediate wheel simply
turns on its axis, but does not travel; but if the turbine and
pendulum are moving at different rates, then the vertical wheel is
forced to run in one direction or the other, and, doing so, it opens
or closes a throttle valve, which controls the supply of water to
the turbine, and so speedily brings the turbine into accord with
the pendulum. The control of the motion of the great telescope is
therefore almost as perfect as that of the astrographic and Thompson
equatorials, though the principle employed is very different. And
the control needs to be perfect, for, as said above, the great
telescope is mostly devoted to the observation of double stars, and
there can be no greater hindrance to this work than a telescope
which does not move accurately with the star.
There is a striking contrast between the great telescope and all the
massive machinery for its direction and movement, and the objects
on which it is directed--two little points of light separated by a
delicate hair of darkness.
The observation is very unlike those of which we have hitherto
spoken. The object is not to ascertain the actual position in the
sky of the two stars, but their relative position to each other.
A spider's thread of the finest strands is moved from one star to
the other by turning an exquisitely fine screw; this enables us
to measure their distance apart. Another spider thread at right
angles to the first is laid through the centres of both stars, and
a divided circle enables us to read the angle which this line makes
to the true east and west direction. Such observations repeated
year after year on many stars have enabled the orbits of not a few
to be laid down with remarkable precision; and we find that their
movements are completely consistent with the law of gravitation.
Further, just as Neptune was pre-recognized and discovered
from noting the irregularities in the motion of Uranus, so the
discordances in the place of Sirius led to the belief that it was
attracted by a then unseen companion, whose position with respect to
the brighter star was predicted and afterwards seen.
[Illustration: THE SOUTH-EAST DOME WITH THE SHUTTER OPEN.]
Gravitation thus appears, indeed, to be the Bond of the Universe,
yet it leaves us with several weighty problems. The observation of
the positions of stars shows that though we call them fixed they
really have motions of their own. Of these motions, a great part
consists of a drift away from one portion of the heavens towards a
point diametrically opposite to it, a drift such as must be due, not
to a true motion of the individual stars, but to a motion through
space of our sun and its attendant system. The elder Herschel was
the first to discover this mysterious solar motion. Sir George Airy
and Mr. Edwin Dunkin, for forty-six years a member of the Greenwich
staff, and from 1881-1884 the Chief Assistant, contributed important
determinations of its direction.
What is the cause of this motion, what is the law of this motion,
is at present beyond our power to find out. Many years ago a
German astronomer made the random suggestion that possibly we were
revolving in an orbit round the Pleiades as a centre. The suggestion
was entirely baseless, but unfortunately has found its way into many
popular works, and still sometimes is brought forward as if it were
one of the established truths of astronomy. We can at present only
say that this solar motion is a mystery.
There is a greater mystery still. The stars have their own
individual motions, and in the case of a few these are of the most
amazing swiftness. The earth in its motion round the sun travels
nearly nineteen miles in a second, say one thousand times faster
than the quickest rush of an express train. The sun's rate of motion
is probably not quite so swift, but Arcturus, a sun far larger than
our own, has a pace some twenty times as swift as the orbital motion
of the earth. This is not a motion that we can conceive of as being
brought about by gravitation, for if there were some unseen body so
vast as to draw Arcturus with this swiftness, other stars too would
be hurtling across the sky as quickly. Such 'runaway stars' afford a
problem to which we have as yet no key, and, like Job of old, we are
speechless when the question comes to us from heaven, 'Canst thou
guide Arcturus and his sons?'
It will be seen then that, fundamentally, Greenwich Observatory was
founded and has been maintained for distinctly practical purposes,
chiefly for the improvement of the eminently practical science
of navigation. Other inquiries relating to navigation, as, for
instance, terrestrial magnetism and meteorology, have been added
since. The pursuit of these objects has of necessity meant that the
Observatory was equipped with powerful and accurate instruments, and
the possession of these again has led to their use in fields which
lay outside the domain of the purely utilitarian, fields from which
the only harvest that could be reaped was that of the increase of
our knowledge. So we have been led step by step from the mere desire
to help the mariner to find his way across the trackless ocean, to
the establishment of the secret law which rules the movements of
every body of the universe, till at length we stand face to face
with the mysteries of vast systems in the making, with the intimate
structure of the stellar universe, with the apparently aimless,
causeless wanderings of vast suns in lightning flight; with problems
that we cannot solve, nor hope to solve, yet cannot cease from
attempting, problems to which the only answer we can give is the
confession of the magicians of Egypt--'This is the finger of God.'
INDEX
Aberration of light, 79
Adams, John C., his discovery of Neptune, 217
Adhara, 183
Airy, George Biddell, seventh Astronomer Royal, his early
life, 102;
his work at Cambridge, 105;
comes to Greenwich, 105;
his relations with the Visitors, 106;
his autobiography, 108;
his character, 111;
his labours, 113;
attacks on, 114;
his distinctions, 118;
his resignation, 119;
his death, 120;
anecdote of, 142;
his conduct _re_ Adams, 217;
his water telescope, 304
Alderamin, 183
_Almagest_, 185
Almanac making, 29
Alpha Aquilæ, telescope for, 303
---- Cygni, telescope for, 303
Altazimuth the, 114;
description and work of, 207, _et seq._
Altazimuth Department, 205, _et seq._
American time, 153
Andromeda nebula, 301
Anemometer, use of, 238;
trace of, 242
Angström, 268
Anson, Commodore, 17
Apparent time, 152
Arcturus, motion of, 315
Argelander, star catalogue of, 287
_Art of Dialling_, the, 28
Assistants, position of the, 98, 100, 117, 137
Astrographic chart, 128
---- Department, 284, _et seq._
---- dome, 128
---- telescope, 289, _et seq._
Astronomers Royal, the, 25
Astrophysical researches, 282
Auroræ, 281
Automatic register, 241
Axis of the earth, precession of, 184
Ball, Time, 162
Barometer, use of the, 192, 233
Battery basement, 161
Beaufort, Captain, 107
Bessel quoted, 266
Betelgeuse, 184
Birkenhead, wreck of the, 180
Bliss, Nathaniel, fourth Astronomer Royal, history of, 82
Bradley, James, third Astronomer Royal, his life, 73;
his ordination, 74;
Vicar of Bridstow, 74;
Savilian Professor of Astronomy, 75;
discovers Aberration of Light, 75, _et seq._;
becomes Astronomer Royal, 79;
labours of, 80;
character of, 81
Bradley's transit room, 128
Brinkley, Dr., 303
_British Mariner's Guide_, the, 90
Bunsen, 268
Buys Ballot's law, 237
Canadian time, 153
Castor, 74, 306
Catalogues, star, 182, 185, _et seq._, 198, 284
Cepheus, 183
Charles II., warrants of, 39, 40
Christie, W. H. M., eighth Astronomer Royal, work of, 120
Chromosphere of the sun, 268
Chronograph, the, 157
---- room, 126
Chronometer business, 101, 107
Chronometers, Harrison's improvements in, 165, _et seq._;
tests of, 169;
'runs' of, 173;
romance of, 178
Circle Department, 181, _et seq._
Clock, Astrographic driving, 290;
driving 28-inch telescope, 312
Clocks, standard, 160
Columbus, aim of voyage of, 18
Comet, appearance of a, 28
---- Wells, 280
Comets, observation of, 224;
spectra of, 280
Commutator, the, 162
Comte, assertion of, 267
Constant of Aberration, 79
Cook, Captain, work of, 170
Copper, use of in Observatory, 245
Corona of the sun, 264
Crabtree, James, 31
Crosthwait, Joseph, 57
Dallmeyer telescope, 252
Declination, 186, _et seq._
Denebola, 184
Distances of planets, 223;
of sun, 224
Double-Star Department, 303, _et seq._
Double Stars, 306
Dublin time, 155
Dunkin, Edwin, 315
Earth, the, movements of, 201
Eclipses of the moon, 216;
of the sun, July 25, 1748...85;
other eclipses of the sun, 263, _et seq._
Electric Railway, influence of, 249
Equation of Time, the, 29, 151
Equatorial, Shuckburgh's, 101
----, the great 28-inch, 221
----, the Merz, 12-3/4-inch, 114
----, 28-inch, driving clock of, 309;
use of, 313
----, clock-driven, 74
Eros, discovery of, 223;
photographs of, 298
Errors in observations, noting of, 199, _et seq._
Evaporation, 241
Faculæ of the sun, 257
Flamsteed, John, his report on Saint-Pierre's proposal, 23, 32;
appointed first Astronomer Royal, 23, 34;
his autobiography, 26;
his studies, 29;
his almanac, 29;
sent to London, 30;
enters Jesus College, Cambridge, 31;
completes his observatory, 31;
acquaintance with Newton, 31;
takes his degree, 32;
his work, 34;
warrant for his salary, 39;
position of, 42;
his ordination, 45;
his pupils, 45;
his trouble with Newton, 46, _et seq._;
his catalogue, 53;
his letter to Sharp, 54;
his death, 56;
his labours, 57
Flamsteed House, 126
Fraunhofer mounting, 310
French time, 155
Galileo, his discovery of Jupiter's satellites, 19
Gamma Draconis, 75, 304
---- Virginis, 306
Gascoigne, William, 31
Gemma Frisius, plan of, 22
George of Denmark, Prince, 50
German mounting, 276, 310
Gould, Dr., 287
Graham, 166
Gravitation, the bond of the universe, 313
Great comet of 1882, the, 280, 288
Greatrackes, Valentine, 29
Green, Charles, 91
Greenwich time, 153;
distribution of, 163
Halley, Edmund, his life, 60;
his early work, 60;
his catalogue of stars, 63;
elected F.R.S., 63;
his work on Kepler's laws, 64;
becomes captain, 65;
Savilian Professor of Geometry, 66;
Astronomer Royal, 66;
observations on saros of the moon, 67;
pressed by Newton, 68;
his death, 68;
his services to science, 68;
his pay, 70;
nominates his successor, 73;
his transit instrument, 73
Halley's comet, 225
Harrison, James, timekeepers of, 86, 91, 93, 165
Heineken, Rev. N. S., 59
Heineken quadrant, 59
Heliographic Department, 251, _et seq._
Herschel, Caroline, 57
Hipparchus, catalogue of, 185
Hodgson, Mr., 50
Hooke, Robert, 75, 206
Horrox, Jeremiah, 31
Huggins, Sir W., his use of spectroscope, 268
Inscription, an, 126
International Photographic Survey, 296
Ireis, 224
Iron quadrant, 73
Isobars, 237
Jupiter, satellites of, 19, 296;
atmosphere of, 279
Keill, John, 74
Kendall, Larcum, 166
Kepler, laws of, 64
Kew, photo-heliograph, the, 252
Kinnebrook, David, 176
Kirchhoff's use of spectroscope, 267
Latitude, finding the, 18
Ledgers, chronometer, romance of, 176
Leverrier, his discovery of Neptune, 217
Libraries, 132
Linacre, G., 28
Lindsay, Thomas, quoted, 204
Litchford, W., 28
Local apparent time, 22
Longitude, finding the, 18;
at sea, problem of, 86;
determination of, 173
Longitude nought, 148
Lower computing room, 128
Lunars, method of, 86
Magnetic Department, work of, 133;
description of, 228, _et seq._
Magnetic inclination and declination, 246
---- needles, movements of, 247, 262
---- observatory, 132
---- pavilion, 245
---- storms, 248, 262
Mars, distance of, 223;
atmosphere of, 279;
satellites of, 296
Maskelyne, Nevil, fifth Astronomer Royal, 85;
practical work of, 86;
Astronomer Royal, 91;
his work, 92;
his publications, 92;
his observations and work, 92, _et seq._;
his death, 94;
his character, 97;
recommends his successor, 97;
his mural circle, 101
Mean solar clock, 160
Mean time, 152
Meldrum, Dr., on sun spots, 263
Meridian, the, 149
Merz telescope, 279
Meteorological Department, work of, 133;
description of, 228, _et seq._
Micrometers, use of, 309
Microscopes, use of, 188
Milky Way, 288
Miller, Professor, 268
Milne, Professor, on earth movements, 201
Minor planets, 222
Molyneux, Samuel, 75
Moon, observation of the, 212, _et seq._;
eclipses of, 266
Moore, Sir Jonas, 30;
death of, 42
Morin, 33
Mounting telescopes, modes of, 310
Mudge, Thomas, 94
Mural arc, 7-feet, 46
Mural circles, 101, 196
Names of stars, origin of, 183
Nares, Sir George, 170
_Nautical Almanac_, the, 22, 23, 92
Navigation, state of primitive, 17
Neptune, discovery of, 217;
atmosphere of, 280;
satellite of, 298
New altazimuth, the, 132, 210
New Observatory, the, 136, 275
New stars, 268
Newcomb, Professor, on growth of Observatory, 124;
on Greenwich observations, 207
Newton, Sir I., his absent-mindedness, 31;
his trouble with Flamsteed, 46, _et seq._;
on Kepler's laws, 65;
his _Principia_, 65;
his pressure on Halley, 68;
his discovery of gravitation, 206
North terrace, the, 126
Northumberland equatorial, 218
Nutation of the earth, 80
Observation, modes of, 156, 176, 188;
by reflection, 196;
of comets, 224
Observatory, Greenwich, work of, 13;
foundation of, 23;
warrant for building, 40;
position of, 41;
foundation stone laid, 42;
condition of, 79;
enlargement of, 112;
recent extensions of, 120;
description of, 124, _et seq._;
staff of, 137;
work of, 139, _et seq._;
visitors to, 175;
new altazimuth building, 211;
magnet house, 228;
magnetic pavilion, 245;
new Observatory, 275;
future of, 283;
reflex zenith room, 304;
objects of, 316
Occultations by the moon, 212, _et seq._
Octagon room, 125, 238, 242
Oldenburg, Mr., 30
Orion nebula, 268, 301
Parallax of stars, 303
Paramour, the, 65
Paris, conference at, 288
----, noon at, 151
Philip III., offer of, 19
Photographic registration, 244, 247, 252, 255;
refractors, 288
Photographs, star, 290
Photo-heliographs, 252, _et seq._, 279
Piazzi, discovery of, 222
Pleiades, the, 301
Polar plumes of the corona, 264
Polaris, 188
Pole-star, variation of, 184
Pond, John, sixth Astronomer Royal, his life, 97;
his reign, 98;
his salary, 98;
his assistants, 98;
his observations, 99;
censured by Visitors, 99;
his observations of stars, 303
Pound, James, 73
Precession of earth's axis, 184
_Principia_, publication of, 65
Proctor, R. A., attack of, 116
Ptolemy, Claudius, catalogue of, 185
Publication, the problem of, 48, 92
Quadrant, Heineken, 59
----, the iron, 73
Railway time, 152
Rain gauge, 238
Record rooms, 132
Reflection, observation by, 196
Reflex zenith room, 304
---- ---- tube, 131
Refraction, effects of, 194
Right ascension, 186, _et seq._
Roberts, Dr. Isaac, 301
Römer, discovery of, 78
Rosse, Lord, 268
Royal Society and Flamsteed, 46, _et seq._
Saint-Pierre, Le Sieur de, proposal of, 23, 32
Sappho, 224
Saros of the moon, 67
Satellites, discovery of, 296
Saturn, atmosphere of, 279;
satellites of, 296
Schaeberle's comet, 280
Schedar, 184
Schiehallion, attraction of, 94
Schönfeld, 287
Scotchmen, anecdote of, 146
Sharp, Abraham, 46
Sheepshanks, Rev. James, on Airy, 107
Shuckburgh equatorial, 309
Sidereal clock, 160
Sirius, 287
Sloane, Dr., 50
'Smith, Mr.,' his chronometer, 179
Solar photographs, 257
---- storms, 261, 282
Sound waves, 271
South, Sir James, 105, 114
South-east equatorial, the, 132, 221
Spectroscope, use of, 267
Spectroscopic Department, 266, _et seq._
Spots, sun, 251, _et seq._, 281
Staff of Observatory, 137;
work of, 139, _et seq._
Standard time, 21
Stars, observations of, 156, 176, 188;
origin of names of, 183;
movements of, 187;
catalogues of, 198, 284, _et seq._;
composition of, 268, _et seq._;
colour of, 271;
classes of, 287;
census of, 287;
photographs of, 288, _et seq._;
motions of, 303, 315
Story, Mr. A. M., 97
Sun, distance of the, 74, 224;
spots on, 251, _et seq._, 281;
eclipses of, 263, _et seq._;
chromosphere of, 268;
motions of, 315
Sunshine recorder, 238
Swiss time, 155
Tebb, Mr. W., 58
Tebbutt's comet, 280
Telescope, the great transit, 156
----, 28-inch, 275
----, astrographic, 289
----, Shuckburgh, 309
----, Thompson, 256, 279, 296
Thalèn, 268
Thermometer, use of, 192, 234
Thome, Dr., 287
Thompson photo-heliograph, 256, 279, 296
Time ball, 162
---- Department, the, 146, _et seq._
---- desk, 161
----, foreign, 153
---- signals, 162
---- standard, 21
Transit, Halley's, 73
Transit circle, the, 114;
mode of observation with, 188, _et seq._
Transit circle, Troughton's, 98
---- Department, 181, _et seq._
---- observations, number of, 140
---- pavilion, 126, 175
---- room, 128, 147
Troughton's transit circle, 98
Uranus, discovery of, 217;
atmosphere of, 279;
satellites of, 296
Vanes, use of, 238
Venus, distance of, 223
Victoria, 224
Visitors, the Board of, 53;
censures Pond, 99;
work of, 106;
constitution of, 144
Visitors to Observatory, 175
Warrant for Flamsteed's salary, 39
Water telescope, 304
Weather predictions, 229, _et seq._
Winds, study of, 237
Witt, Herr, discovery of, 223
Working Catalogue, the, 142
Zenith sector, 82, 305
---- tube, 75, 305
Zeta Ursæ Majoris, 306
Zubeneschamal, 184
THE END
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