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Title: The Moon: A Popular Treatise
Author: Garrett P. Serviss
Release Date: October 10, 2021 [eBook #66510]
Language: English
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*** START OF THE PROJECT GUTENBERG EBOOK THE MOON: A POPULAR TREATISE ***
THE MOON
------------------------------------------------------------------------
[Illustration:
COPERNICUS AND THE CARPATHIAN MOUNTAINS.
]
------------------------------------------------------------------------
+THE MOON+
+A POPULAR TREATISE+
_By_
GARRETT P. SERVISS
[Illustration]
+D. APPLETON AND COMPANY
NEW YORK+
1907
------------------------------------------------------------------------
COPYRIGHT, 1907, BY
D. APPLETON AND COMPANY
_Published October, 1907_
------------------------------------------------------------------------
PREFACE
THE reader familiar with astronomical literature will doubtless remark a
certain resemblance between the plan on which this book is written and
that of Fontenelle’s “Conversations on the Plurality of Worlds,” a
French classic of the eighteenth century. The author freely acknowledges
that it was the recollection of the pleasure which the reading of
Fontenelle’s book gave him, years ago, that led to the adoption of a
somewhat similar plan for this description of the moon. But, except that
in both cases the conversational method is employed, no great likeness
will be found between what is here presented and the work of the witty
Frenchman.
Having been invited by the Messrs. Appleton & Co. to prepare a small
volume, to be based on a series of lunar photographs representing the
moon as it appears on successive evenings during an entire lunation, the
author felt that the work should be made as entertaining as possible. He
has, therefore, avoided technicalities, while endeavoring to present all
the most essential facts known about our satellite. What he has written
is intended for the general reader, who desires to learn the results of
the great advances in astronomy without being too much troubled with the
scientific methods by whose aid those results have been reached.
This is the first time, as far as the author is aware, that a series of
lunar photographs, showing our satellite in its varying aspects from New
to Old Moon, has been presented in a book, accompanied with a
description of the mountains, plains, volcanoes, and other formations
shown in each successive photograph. The reader is enabled to place
himself, as it were, in an observatory of the first rank, provided with
the most powerful apparatus of the astronomer, and, during an entire
month, view the moon in her changing phases.
The photographs here reproduced were made at the Yerkes Observatory, and
the most grateful acknowledgments are tendered to Prof. Edwin B. Frost,
its director, for generously consenting to their use for this purpose.
He could only have been induced to do so by his desire to see the fruits
of the admirable work accomplished by his associates enjoyed by an
ever-widening circle.
The series of photographs representing the moon on successive evenings
were taken with the 12-inch telescope of the Yerkes Observatory by Mr.
James Wallace, who employed a color filter that he constructed specially
for this telescope, which possesses a visual and not a photographic
objective. The larger scale photographs, representing certain selected
regions on the moon, were taken by Mr. Ritchey, now of the Carnegie
Solar Observatory at Mount Wilson, California, with the great 40-inch
telescope of the Yerkes Observatory. It is unnecessary to speak of the
extraordinary quality of these photographs, which have been admired by
astronomers in all lands.
It should, perhaps, be added that while the director of the Yerkes
Observatory has shown confidence in the author by intrusting to him the
use of these photographs, yet, neither Professor Frost, nor Messrs.
Wallace and Ritchey are in any way responsible for the statements made
in this book. The author has taken pains to be accurate, but if any
errors of fact or opinion have crept in, he alone must be blamed for
them.
GARRETT P. SERVISS.
CHÂTEAU D’ARCEAU,
CÔTE D’OR, FRANCE, June, 1907.
------------------------------------------------------------------------
CONTENTS
PAGE
INTRODUCTION 3
I. —NEW MOON TO FIRST QUARTER 47
II. —FIRST QUARTER TO FULL MOON 83
III. —FULL MOON TO OLD MOON 131
IV. —GREAT SCENES ON THE MOON 181
APPENDIX 239
INDEX 243
------------------------------------------------------------------------
LIST OF ILLUSTRATIONS
FULL-PAGE ILLUSTRATIONS
Copernicus and the Carpathian Mountains _Frontispiece_
PHOTOGRAPHS OF THE MOON SHOWING PHASES OF CHANGE
PAGE
No. 1, February 19, 1904; Moon’s Age 3.85 Days 50
No. 2, September 24, 1903; Moon’s Age 3.87 Days 56
No. 3, July 29, 1903; Moon’s Age 5.54 Days 66
No. 4, November 24, 1903; Moon’s Age 5.74 Days 72
No. 5, July 1, 1903; Moon’s Age 6.24 Days 74
No. 6, November 26, 1903; Moon’s Age 7.75 Days 78
No. 7, July 2, 1903; Moon’s Age 7.24 Days 90
No. 8, August 31, 1903; Moon’s Age 9.22 Days 94
No. 9, August 2, 1903; Moon’s Age 8.97 Days 102
No. 10, November 30, 1903; Moon’s Age 11.78 Days 106
No. 11, December 1, 1903; Moon’s Age 12.98 Days 112
No. 12, September 4, 1903; Moon’s Age 13.27 Days 118
No. 13, September 5, 1903; Moon’s Age 14.40 Days 122
No. 14, August 26, 1904; Moon’s Age 15.65 Days 132
No. 15, August 28, 1904; Moon’s Age 17.41 Days 134
No. 16, August 29, 1904; Moon’s Age 18.62 Days 138
No. 17, October 10, 1903; Moon’s Age 20.06 Days 142
No. 18, September 29, 1904; Moon’s Age 20.50 Days 144
No. 19, August 16, 1903; Moon’s Age 23.81 Days 150
No. 20, August 17, 1903; Moon’s Age 24.84 Days 158
No. 21, August 19, 1903; Moon’s Age 26.89 Days 168
PHOTOGRAPHS OF THE MOON SHOWING SIGNIFICANT FEATURES
PAGE
Bullialdus and the _Mare Nubium_ 182
Tycho, Clavius, and their Surroundings 192
The Great Southwest on the Moon 198
The Giant Ring Mountain Theophilus and its 206
Neighbors.
Two Great Lunar “Seas”—the _Mare Serenitatis_ and 218
a Part of the _Mare Imbrium_
DIAGRAMS IN TEXT
PAGE
Phases and Rotation of the Moon 14
The Moon’s Path with Respect to the Sun and the 17
Earth
The Moon and the Tides 23
Effect of Moon, Varying Velocity in Orbit 58
Producing Libration in Longitude
Lunar Volcano, in Section 85
Terrestrial Volcano, in Section 85
Diagram Showing Why the Winter Moon Runs High 170
Diagram Showing Why Moon Rises Later Every Night 173
Diagram Illustrating the Harvest Moon 174
------------------------------------------------------------------------
INTRODUCTION
------------------------------------------------------------------------
INTRODUCTION
ONE serene evening, when the full moon, rising slowly above the tree
tops, began to spread over the landscape that peculiar radiance which,
by half revealing and half concealing, by softening all outlines, and by
imparting a certain mystery to the most familiar objects, fascinates at
once the eye and the imagination, I was walking with a friend, a lady of
charming intelligence, in a private park adjoining an old mansion in one
of the most beautiful districts of central New York. For a long time we
both remained silent, admiring the scene before us, so different in
every aspect from its appearance in the glare of daylight—each occupied
with the thoughts that such a spectacle suggests. Suddenly my friend
turned to me and said:
“Tell me—for, like so many thousand others, I am virtually ignorant of
these mysteries of the sky—tell me, what is that moon? What do
astronomers really know about it?”
“But,” I replied, “you certainly exaggerate your ignorance. You must
have read what so many books have told about the moon.”
“Not a word,” was the reply, “or at least, what I have read has made
little impression upon my mind. I read few books of science; generally
they repel me. But face to face with that marvelous moon, I find it
irresistible, and my desire for knowledge concerning it becomes intense.
I remember something about eclipses, and something about tides, with
which, I believe, the moon is concerned. I recall the statement that the
moon has no atmosphere, but does possess great mountains and volcanoes.
Yet these things are so jumbled in my memory with technical statements
which failed to interest me, that really my ignorance remains profound.
But I have heard that many surprising discoveries have been made lately
concerning the moon, and that astronomers have succeeded in taking
wonderful photographs of scenes in the lunar world. I have, indeed, seen
copies of some of these photographs, but beyond awaking curiosity by
their _bizarre_ effects of light and shadow, they impressed me little,
for lack, I suppose, of information as to their meaning. I beg you,
then, to tell me what is really known about the world of the moon. There
it is; I see it; I experience the delightful impressions which its light
produces—but, after all, what is it, and what should we behold if we
could go there? I once read Jules Verne’s romance of a trip to the moon,
but unfortunately his adventurers never really got there, and I finished
the story with a keen sense of disappointment because, in the end, he
told so very little about the moon itself. As for the professional books
of the astronomers they are useless to me. Then, please tell me that
which, at this moment, with that wonderful orb actually in sight, I so
much desire to know.”
It was not possible to resist an appeal so earnestly urged, but I felt
compelled to say: “Since you remember so little about the fundamental
facts which generations of astronomers have accumulated concerning our
nearest neighbor in the sky, I must, for the sake of completeness, and
in order to put you _au courant_ with the more captivating things that
will come later, begin at the beginning, and the true beginning is not
among the mountains of the moon, but here on the earth. We must start
from our own globe—as the moon herself did.”
“What do you mean by that?” my friend asked with a tone of surprise.
“Have you not read, somewhere, in the last ten years, that the moon was
actually born from the earth?”
“Yes, now that you mention it, I dimly recall something of the kind, but
I took it for an extravagant speculation of some _savant_ who possessed
more imagination than solid knowledge.”
“The _savant_ who originally demonstrated the earthly origin of the
moon,” I replied, “is not one to be easily led into extravagance by his
imagination. It is Prof. George Darwin, the son of the famous author of
the ‘Origin of Species.’ I shall not mention his mathematics, which are
troublesome, but allow me to tell you, in a word, that his
investigations have satisfied astronomers that the earth and the moon
once composed a single body. How many million years ago that was we can
only guess. The causes of the separation which eventually occurred were
the plastic condition of the original body while it was yet hot and
molten, its swift axial rotation producing an immense centrifugal force
at its equator, and the attraction of the sun raising huge tides which
affected its entire mass instead of affecting only the waters of the
ocean as the tides do at present. At last there came a time when an
enormous portion of the swiftly rotating globe was torn loose. That
portion included about one-eightieth of the entire mass of the earth.
Some astronomers and geologists think that the ‘wound’ left in the side
of the earth by this stupendous excision is yet traceable in the basin
of the Pacific Ocean.
“The separation being once effected, the material that had escaped
gradually assumed a globular form under the influence of the gravitation
of its own particles; and, at the same time, by virtue of a curious
reaction of the tidal attractions of the two bodies upon each other, the
new-born globe was slowly forced away from its mother earth, becoming,
in fact, its satellite. Thus, by a process which certainly does seem
extravagantly imaginative, but which, nevertheless, is approved by
strict mathematical deductions from known physical facts, the moon is
believed to have had her birth.”
“Surely,” said my companion, “my imagination would never have dared to
form such a picture, even if it had been capable of so extraordinary a
flight.”
“No,” I replied, “nor the imagination of the most learned astronomer.
You perceive that in things celestial as in things terrestrial fact is
far more strange than fiction. We shall have occasion to refer to some
of the consequences of the earthly origin of the moon later on, but just
now in order that the knowledge you seek may not be too fragmentary, I
must tell you some other, more commonly known, facts about our
satellite.”
“Judging by myself I doubt if there are many such facts _commonly_
known.”
“Perhaps you are right, but do not judge too severely the authors of
astronomical books. Such books are written primarily for those who wish
to study, not for those who desire to be intellectually entertained. But
let me get through with my preliminaries, and then, under the guidance
of science and photography, we shall try to visit the moon. One of the
first questions that naturally arise concerning the objects that we see
in the heavens relates to their distance from us. The average, or mean,
distance of the moon from the earth is 238,840 miles. For the sake of a
round number we usually call it 240,000 miles. But the orbit, or path,
of the moon in her monthly journey around the earth, is so far from
being a true circle that the distance is variable to the extent of
31,000 miles. Even the form of the moon’s path in space is not constant.
Owing to the varying effects of the attraction of the earth and the sun,
her elliptical orbit becomes now a little more and now a little less
eccentric, the consequence being that the moon’s distance from the earth
is continually changing. When she is at her greatest possible distance
she is 253,000 miles away, but this distance at certain times, may be
reduced to only 221,600 miles. As a result of these changes of distance
the moon sometimes appears noticeably larger to our eyes than at other
times.
“This leads us next to inquire, ‘What is the actual size of the moon?’
When we know the distance of any body from the eye it is not difficult
to determine its size. The diameter of the moon is 2,163 miles. The face
of the full moon contains 7,300,000 square miles. It is a little larger
than the continent of South America. For a reason that we will speak of
presently, the moon always keeps the same side toward us no matter in
what part of its orbit it may be. Consequently we always see the same
features of her surface and, except through inference, we do not know
what exists on the other side of the lunar globe. Of the 7,300,000
square miles of surface which the moon presents to us, about 2,900,000
are occupied by those dark gray patches which you see so plainly
spotting her face, and which were once supposed to be seas. The
remaining 4,400,000 square miles consist of a very rough, broken
country, ridged with gigantic mountains and containing hundreds of
enormous craters, and mountain-ringed valleys, which are so vast that
one hesitates to call them, what many of them seem evidently to be,
extinct volcanoes. A single explosion of a volcano of the dimensions of
some of these lunar monsters would shake the whole earth to its center!”
“Please stop a moment,” my friend laughingly interrupted. “So many
merciless facts, chasing one at the heels of another, are as bad as the
books on your science that I have tried to read. Give my imagination
time to overtake you.”
“Very well,” I said, “then relieve your attention a little while by
regarding the face of the moon. Do you perceive the portrait of the Moon
Maiden there?”
“I believe I do, although I never noticed it before. It is in profile,
is it not?”
“Yes, and it occupies all the central portion of the western half of the
disk. Take the opera glass and you will see it more clearly.”
“Really, I find her quite charming,” said my companion, after gazing for
a minute through the glass. “But what a coquette! Look at the
magnificent jewel she wears at her throat, and the _parure_ of pearls
that binds her hair!”
“Yes,” I replied, “and no terrestrial coquette ever wore gems so
unpurchasable as those with which the Moon Maiden has decked herself.
That flaming jewel on her breast is a _volcano_, with a crater more than
fifty miles across! Tycho, astronomers call it. Observe with the glass
how broad rays shoot out from it in all directions. They are among the
greatest mysteries of lunar scenery. And the string of brilliants in her
hair consists of a _chain of mountains_ greater than the Alps—the lunar
Apennines. They extend more than 450 miles, and have peaks 20,000 feet
high, which gleam like polished facets.”
“Truly,” said my companion, smiling, “these gigantesque facts of yours
rather tend to dissipate the romantic impression that I had conceived of
the Moon Maiden.”
“No doubt,” I replied. “It is only distance that lends her enchantment.
But we must not disregard the facts. Her hair, you perceive, is formed
by some of the vast gray plains of which I spoke a few minutes ago. She
is like a face in the clouds—approach her, or change the point of view
and she disappears or dissolves into something else.
“Now, to return to my preliminaries, upon which I must insist. Knowing
the distance and the size of the moon, the next question relates to her
motions. You are aware that she travels around the earth about once
every month. There are two ways in which we measure the length of time
that the moon takes for each revolution. First, regarding the face of
the sky as a great dial, with the stars for marks upon it, we notice the
time that elapses between two successive conjunctions of the moon with
the same star. In the interval she has gone completely around the earth
and come back to the starting point. This is called the moon’s sidereal
revolution, and it occupies, on the average, twenty-seven days, seven
hours, forty-three minutes, twelve seconds. Every twenty-four hours the
moon advances among the stars, from west to east, about 13° 11´.
“But there is another, more usual way of measuring the orbital period of
the moon. This way is connected with her phases, or changes of shape,
from the sickle of the New Moon to the round disk of the Full Moon, and
back again to the reversed sickle of the waning moon. It is the time
that elapses from one New Moon to the next, or from one Full Moon to the
next which now concerns us, and it amounts, on the average, to
twenty-nine days, twelve hours, forty-four minutes. This is called the
moon’s synodic revolution, and it is equivalent to the ordinary lunar
month. It is variable to the amount of about thirteen hours. The reason
why the synodic revolution is more than two days longer than the
sidereal revolution is because the continual advance of the earth in its
orbit around the sun causes the latter to move eastward among the stars,
and before the moon’s monthly phases, which depend upon her position
with regard to the sun, can recommence, she must overtake the sun.”
“What a hopeless task to try to remember all that!”
“At any rate, if you cannot remember these things my conscience will be
clear, for I am simply doing my duty in telling you of them. If you
forget, go to the books on astronomy and refresh your memory. But do not
persuade yourself that the preliminaries are now finished. You are going
to think that my story of the moon resembles Walter Scott’s novels in
the length of its introduction; but if, in the end, I can interest you
half as much as he finally interests his readers I shall thank the stars
for my good fortune.
“The next thing that I must try to explain,” I continued, “is the cause
of the moon’s phases, or her continual changes of form. You know that
the New Moon is shaped like a thin crescent, and always appears in the
west immediately after sundown, with the convex side facing the setting
sun. The moon at First Quarter is a half circle and is visible in the
southern part of the sky just after sunset. The Full Moon, which we have
at present, is a complete round disk, and is always seen directly
opposite to the place of the sun, so that she rises when the sun sets.
The moon at last quarter is again a half circle, and appears on the
meridian in the south at sunrise. The waning moon is like the new moon,
crescent-shaped, but the convexity of the bow faces the rising sun, and
she is visible only in the morning sky just as dawn begins. To explain
the reasons for these changes of shape, which the moon regularly
undergoes every month, I must ask you to go indoors and examine a little
diagram which I have made.”
“Oh!” said my companion, “it is too bad to abandon this charming
spectacle, illuminated by rays so fascinating, for the sake of looking
at mathematical lines drawn on paper! But I suppose that this is one of
the sacrifices demanded by your inexorable science, and must be made.”
“Yes,” I said, “but if science sometimes demands sacrifices, at least
she always rewards them most generously.”
When we had returned to the house I placed upon the drawing-room table
this diagram.
[Illustration:
Phases and Rotation of the Moon.
]
As I spread it out, my companion, after a regretful glance through the
open door at the silvery lawn, on which the moon, having cleared the
obstructing branches of the bordering trees, was now pouring down the
full splendor of her rays, put her elbows on the table to follow my
explanation.
“The globe, half bright and half black, in the center,” I said,
“represents the earth. The large circle surrounding the earth we will
call the moon’s orbit, which she traverses once every month. The smaller
globe, also half white and half black, shown in four successive
positions in the orbit, is the moon. Suppose the sun to be away off here
on the left. It illuminates the earth and the moon only on the side
turned toward it. The opposite side of both is buried in night. Now, let
us begin with the moon at the point A. She is then between the earth and
the sun, the bright side being necessarily toward the sun and the dark
side toward the earth. In that position we do not see the moon at all
from the earth, unless she happens to come so exactly in a line with the
sun as to cover the latter, in which event we have an eclipse of the
sun. Now, suppose the moon to move in her orbit toward B. In a little
more than seven days she will arrive at B. In the meantime, while moving
away from the position of the sun, she begins to present a part of her
illuminated hemisphere toward the earth. This part appears in the form
of a sickle, or crescent, which grows gradually broader, until, at B, it
has grown to a half circle. In other words, when the moon is in the
position B we on the earth see one half of her illuminated surface. This
phase is called First Quarter. The narrow crescent, which appears as
soon as the moon begins to move from A toward B, is the New Moon. As the
moon continues on from B toward C, more and more of her illuminated half
is visible from the earth, and when she arrives at C, just opposite to
the position of the sun, she becomes a Full Moon. We then see, as occurs
to-night, the whole of that face of the moon which is presented sunward.
The upper half of the diagram shows how the moon moves from the position
of Full Moon back again to New Moon, or conjunction with the sun. During
this latter part of her course the moon rises later and later every
night, until, when she assumes the form of a waning crescent, she is
visible only in the morning sky just before sunrise.[1]
“Now, there is another interesting thing shown by this diagram,” I
continued—but my companion, who had followed my explanations thus far
with flattering attention, here suddenly ran to the door exclaiming:
“For mercy’s sake, what is happening to the moon?”
Footnote 1:
[Illustration:
The Moon’s Path with Respect to the Sun and the Earth.
]
It may be well to add to what is said in the text about the orbit of
the moon, that, while the moon does perform a revolution around the
earth once a month, yet her orbit is drawn out, by the common motion
of both earth and moon around the sun, into a long curve, whose radius
is continually changing, but which is always concave toward the sun.
This is illustrated in the accompanying diagram. Suppose we start with
the earth at A. The moon is then between the sun and the earth, or in
the phase of New Moon. The earth’s orbit at this point is more curved
than the moon’s, and the earth is moving relatively faster than the
moon. At B (First Quarter) the earth is directly ahead of the moon.
But now the moon’s orbit becomes more curved than the earth’s and it
begins to overtake the earth. At C (Full Moon) the moon has come up
even with the earth, but on the opposite side from the sun. From that
point to D (Last Quarter) the moon gains upon the earth until she is
directly ahead of it. Then, from D to E (New Moon, once more) the
earth gains until the two bodies are in the same relative positions
which they occupied at A. Throughout the entire lunation, however,
notwithstanding the changes which the shape of the moon’s orbit
undergoes, the latter is constantly concave toward the sun. This shows
that the sun’s attraction is really the governing force, and that the
attraction of the earth simply serves to vary the form of the moon’s
path, and cause it to move in a virtual ellipse with the earth for its
focus.
I glanced over her shoulder, and saw a smudgy scallop in the moon’s
edge.
“Really,” I said, “I am ashamed of myself. There is an eclipse of the
moon to-night, and I had positively forgotten it! What you see is the
shadow of the earth, which has the form of a long cone stretching away
more than eight hundred thousand miles into space, and whenever our
satellite at the time of Full Moon gets nearly in a direct line with the
earth and the sun, it passes through that shadow and undergoes an
eclipse. That is what is happening at the present moment.”
“And the shadow has a round form because the earth is round, I suppose.”
“Certainly; the shadow of a globe must have a circular outline. But the
shadow of the earth, although it finally diminishes to a point, is, at
the moon’s distance, still about 5,700 miles in diameter, or more than
two and a half times the diameter of the moon. In consequence of the
motion of the earth in its orbit around the sun, its shadow constantly
moves eastward, like a great pencil of darkness sweeping straight across
the heavens, but invisible to us except when the moon, traveling
eastward faster than the shadow, overtakes and passes through it. This
does not by any means happen at every full moon, because, for a reason
which I shall explain presently, the moon usually passes either above or
below the shadow of the earth, and thus escapes an eclipse. When an
eclipse does occur it lasts a long time because the shadow is moving in
the same direction as the moon. The moon must pass entirely through it
before the eclipse ends. On this occasion the moon will be in the shadow
more than three hours, and during an hour and a half she will be totally
immersed. We shall have plenty of time, then, to observe the phenomenon,
and after you have satisfied your curiosity a little by watching the
slow advance of the shadow movement across the moon, we can return to
our diagram and finish its explanation before the eclipse becomes
total.”
Accordingly, after having watched the progress of the eclipse for half
an hour, during which time the shadow began perceptibly to diminish the
moonlight in the park, we returned to the lamplight and the diagram on
the table.
“I was saying,” I resumed, “that another interesting thing in addition
to the cause of the moon’s changing phases is represented here. You
observe that a little cross stands on each of the four circles
representing the moon, and that, in every case, the cross is in the
center of that side of the moon which faces the earth. In fact the
position of the cross upon the moon is fixed and invariable, and it
always points toward the earth because the moon makes exactly one
rotation on her axis in the course of one revolution around her orbit,
or, as it is often called, one lunation. We know that this is so because
we always see the same features of the lunar surface, no matter where
the moon may be situated. This is true although, in consequence of the
phases, we cannot see the whole face of the moon except when she is
full. But whether it is the New Moon, or First Quarter, or Full Moon, or
Last Quarter, or Old Moon, that we look at, the mountains and plains
visible are identically the same. If the moon did not turn once on her
axis in going once around the earth we would see all of her sides in
succession, although only at Full Moon could we see an entire hemisphere
illuminated by the sun. At Old and New Moon the side presented to the
earth would be just the opposite to that presented at Full Moon. At Last
Quarter the side facing the earth would be the opposite to that facing
the earth at First Quarter.”
“But, tell me,” said my friend, “how did the moon ever come to so
humiliating a pass that she must be forever turning on her heel to face
the earth?”
“That,” I replied, “is a result of the same forces which originally
separated her from the earth and gradually pushed her off to her present
distance. In a word it is due to ‘tidal friction.’ Before the moon had
solidified, the attraction of the earth raised huge tides in her molten
mass. These tides acted on the rotating moon like brakes on a wheel, and
at length they slowed down her rotation until its period became
identical with that of her revolution around the earth. For the
mathematical calculations on which all this is based you must go to
Professor Darwin’s book on ‘The Tides,’ or some similar technical
treatise; but I imagine you will never do that.”
“Not just at present, I assure you. I do not know what unexpected
ambition for the acquirement of scientific knowledge may arise after I
have seen those wonders that you have promised to show me in the moon,
but, for the moment, I am content to accept your statement of the simple
fact.”
“Good!” I replied. “And now, perhaps, you will have the patience to
listen to an explanation of a very important relation which exists
between the moon and the earth. We are led to it by what I have just
said concerning tides. You know, of course that the tides in the oceans
are due principally to the attraction of the moon. The sun also raises
tides in the seas, but the moon, being so much nearer than the sun, is
the chief agent in producing them. Sometimes the moon and the sun act
together; at other times they pull in different directions. At Full Moon
and at New Moon they pull together, because then they are either on
opposite sides of the earth, or both on the same side. At such times we
have the highest tides in all our seaports. That occurs about once every
fortnight. But when the moon is at either First or Last Quarter, as you
will perceive by looking at the diagram, her position, as seen from the
earth, is at a right angle with a line drawn to the sun. Then the sun
raises tides in one direction and the moon in another direction. The
result is that at such periods the tides are lowest. An exact knowledge
of these things is very important for mariners because there are harbors
whose channels can be navigated by large ships only when the tides are
high. Tables predicting the times and heights of the tides have been
prepared for all the principal seaports of the world. In truth, the moon
renders important services to the inhabitants of the earth, not merely
in supplying them with a certain amount of light in the absence of the
sun, but also in enabling them to navigate waters which are too shallow
for ships except when deepened by the tide. The tides also, in many
cases, serve to scour out channels and keep them open.”
“Really, I am quite interested, and the more so because I find the moon,
like a dutiful daughter, trying to be of some use to her mother. But
have I not heard that the tides occur on both sides of the earth at
once, and not simply on the side where the moon happens to be at the
time? Please tell me how that can be so?”
“A complete reply to your question would carry us into the realm of
mathematical physics, but perhaps I can throw a little light upon the
matter with the aid of this second diagram.
[Illustration:
The Moon and the Tides.
]
“The eclipse is not yet total,” I continued, glancing out of the door,
“and we can finish our explanation before it becomes so. Have the
kindness, then, to look at the diagram. Suppose E to be the center of
the earth, and M the center of the moon. The protuberant portions of the
earth C A D and D B C represent the waters of the ocean pulled away from
the surface of the earth, if I may so describe it, by the moon’s
attraction. You are probably aware that the attraction of gravitation
varies with the distance of the attracting body. The distance from the
center of the earth to the center of the moon is about 239,000 miles.
But the earth being nearly 8,000 miles in diameter, the surface of the
ocean at A is about 4,000 miles nearer to the moon than is the center of
the earth E. It follows that the force of the moon’s attraction is
greater at A than at E. If the water of the ocean were a fixed, solid
part of the earth there would be no perceptible effect resulting from
this difference in the amount of the moon’s attraction. But since the
water is free to move, to a certain extent, it yields to the attraction,
and is drawn up a little toward the moon. At the same time it is, in
effect, drawn away from C and D. The consequence is the production of a
tide on the side facing the moon.
“Now, for the other tide, produced at the same time on that side of the
earth which is turned away from the moon. The point B is about 4,000
miles farther from the moon than E; consequently the moon’s attractive
force is less at B than at E. From this it results that the body of the
earth is more forcibly attracted by the moon than is the water at B. The
earth therefore tends to move away from the water at that point, and
another tidal protuberance is produced, with its highest part at B. I
should add that while the water of the ocean is, to a certain degree,
free to respond to these differences of attraction, the earth itself,
being solid, can only move as a single body, and, mathematically, we may
regard it as if its entire mass were concentrated at the center E.
Please remember, however, that this explanation is only elementary, only
intended as a graphic representation of the tides, and not as a
mathematical demonstration of the way they are produced. Such a
demonstration would only be suited to one of the technical books that
you have not found as interesting as—some other branches of literature.
“There is just one other thing to which I must ask your attention, and
then we shall return to the moon herself and the phenomena of the
eclipse now in progress. You will notice in the diagram, that two arrows
show the direction in which the earth is continually rotating on its
axis, and that a dotted curve, terminating with an arrow point,
indicates the course of the moon in her orbit surrounding the earth. The
rotation of the earth is so much more rapid than the motion of the moon
that the points A and B are carried out of the line drawn from the
center of the moon to the center of the earth, in the direction of the
arrows. From this it follows that the tides are never directly under the
moon, or exactly opposite to her, but sweep in great waves round the
globe. The tides produced by the attraction of the sun are only about
two fifths as high as those caused by the moon. As I have already
explained they are sometimes superposed upon the lunar tides—at New and
at Full Moon—and sometimes they are situated at right angles to the
lunar tides—at First and Last Quarters.”
“But the eclipse!” interrupted my friend, whose attention had evidently
begun to wander. “I think the totality of which you spoke must be at
hand, for notice how dark the park has become, and the fireflies are so
brilliant under the trees.”
The total phase of the eclipse was, indeed, beginning, and we stepped
out on the lawn before the door to watch it. The moon had now passed
entirely within the earth’s shadow, but although her light was almost
completely obscured as far as its power to illuminate the landscape was
concerned, still the face of the moon was dimly visible, as if concealed
behind a thick veil. Certain parts of it had a coppery color, producing
a very weird effect.
“Dear me!” exclaimed my companion, “I did not think it would look like
that! I naïvely supposed that one could not see the eclipsed moon at
all, but that she either disappeared or was turned into a kind of black
circle in the heavens. And what a strange color she has! Positively it
fills me with awe.”
“It is very rare,” I said, “for the moon to become invisible during an
eclipse. That can only occur when the earth is enveloped in clouds.”
“Indeed, and what have the clouds to do with it? If the solid body of
the earth cannot cast a shadow dense enough to hide the moon, I should
not expect things so evanescent as clouds to be more effective.”
“It is all owing to the earth’s atmosphere,” I replied. “If our globe
were not surrounded with a shell of air the moon would always be totally
invisible when eclipsed. But the atmosphere acts like a lens of glass
inclosing the earth; that is to say, it refracts, or bends the rays of
sunlight around the edge of the earth on all sides, and throws a portion
of them even into the middle of the shadow, at the moon’s distance. It
is these refracted rays which cause the singular illumination that you
perceive on the moon. But when, as occurs only occasionally, all that
part of the atmosphere which surrounds the earth along the edge visible
from the moon is filled with clouds, the air can no longer transmit the
refracted rays, and then, no light being sent into the shadow, a ‘dark
eclipse,’ as astronomers call it, results. An eclipse of the sun is a
very different thing. That is caused not by a shadow but by the opaque
globe of the moon passing between the earth and the solar orb. When this
occurs the sun is completely hidden behind the moon, and only its
corona, which projects beyond the moon on all sides, is visible.”
“Indeed! I supposed that all eclipses were very much the same thing.”
“By no means. An eclipse of the sun is an event of extreme importance to
astronomers, while an eclipse of the moon possesses comparatively little
scientific interest.”
“I do not see why that should be so.”
“It is so, for the reason that when the sun is eclipsed, as I have just
told you, the solar corona, which cannot be seen at any other time owing
to the overpowering brilliance of the solar orb, becomes plainly
visible, and by studying the form and other particulars of the corona
astronomers are able to draw most important conclusions concerning the
constitution of the sun, the mechanism of its radiation, and its effects
upon the earth. During an eclipse of the moon, on the other hand,
practically nothing new is revealed, and, accordingly, astronomers pay
comparatively little attention to such phenomena. Lunar eclipses,
however, possess a certain importance, because by predicting the times
of their occurrence and then comparing the predictions with the events,
something is learned about the motions of the moon. I should add that
recently eclipses of the moon have been carefully watched by a few
astronomers, notably by Prof. William H. Pickering, because of peculiar
effects which seem to be produced at certain points on the moon by the
chill which the shadow casts upon her surface. There are also
interesting observations to be made concerning the reflection of heat
from the moon during an eclipse. But, upon the whole, a lunar eclipse is
mainly interesting as a curious spectacle, and as a test of the
correctness of astronomical calculations of the motions of the heavenly
bodies.
“I may add, however, that eclipses of the moon have been of some use to
historians in fixing the dates of important occurrences thousands of
years ago. This is possible because astronomers can by calculation
ascertain the times of eclipses in the past as well as in the future.
Perhaps the most interesting of all instances of this kind is that which
relates to the date of the beginning of the Christian era. This has been
fixed by means of an eclipse of the moon mentioned by the ancients as
having happened the night before the death of Herod, king of the Jews.”
“It seems to me,” said my friend, “that the faint light on the moon’s
face is continually changing. It does not appear constantly to have the
same tint. While we have been standing here, I have noticed that some
parts have grown darker and others lighter, and that the red color on
the right has become a little more intense.”
“Yes, and that, too, is no doubt caused by the earth’s atmosphere. While
the eclipse lasts, the earth is rapidly rotating, and consequently new
parts of the atmosphere are continually brought to the edge where their
refractive effects come into play. If the atmosphere at the edge of the
earth is a little more or a little less dense its refraction varies
proportionally. Then, changes in the relative clearness or cloudiness of
the air are taking place all the time, and these are reflected in the
illumination on the moon.”
“It seems to me, then, that the earth would present a very remarkable
spectacle if we were now on the moon looking at it.”
“Surely it would. Seen from the moon the earth appears several times
larger than the sun. For the people of the moon, if we imagine them to
exist, an eclipse of the sun is now in progress. For them the earth now
occupies the same relative position which the moon occupies for us just
before it appears in the west as New Moon. They cannot see it except in
silhouette as it passes over the sun. More than an hour ago, if they
were watching (and if they exist, and are intelligent beings we may be
sure that they were on the alert), they suddenly perceived a black
round-edged notch in the side of the sun. Instead of being more or less
cloudlike and indefinite in outline, like the shadow of the earth on the
moon, this notch appeared to them perfectly black and smooth. At a
glance, they saw that the body producing it was much larger than the
sun. As the sun’s disk was gradually hidden behind the earth the shadow
of the latter fell over them, until the sun was wholly concealed.
Then—and this is true at the present moment—they perceived that the huge
disk of the earth was ringed with light, probably of a reddish tinge.
This light, as I have already indicated, is that which the atmosphere
refracts around the edge of the earth.”
“It must be truly a magnificent sight,” said my companion.
“Yes, and it is doubtless rendered far more magnificent by the other
phenomena which our people at the moon have before their eyes. In
consequence of the virtual absence of air there, an observer on the moon
would see all the stars, even in full daylight, blazing in a jet black
sky. The brilliance of the stars and of the Milky Way would hardly be
increased by the hiding of the sun, but probably the long silvery
streamers of the solar corona would glow perceptibly brighter when seen
projecting out on each side of the enormous disk of the earth.”
“But is it true that the moon has no air?”
“Very, very little, and what little she has is probably different in
composition from our atmosphere. Some observations seem to indicate that
there is a very rare atmosphere on the moon, but to us it would seem a
perfect vacuum. We could not breathe there at all.”
“How then do those intelligent inhabitants, whom you have pictured for
me watching the earth at this moment, manage to survive?”
“Ah, I did not say that there actually are inhabitants in the moon. I
only imagined them to exist for the sake of showing how this eclipse
would appear seen from the moon. Still, we cannot be absolutely sure
that there are no inhabitants on the moon. Even without air like ours it
is conceivable that beings of some kind, and intelligent beings, too,
_might_ exist there. However, astronomers have never yet been able to
discover evidence of their presence. Lately, indications have been found
of the probable existence of vegetation on the moon, but I shall speak
of that later, when with the aid of the series of lunar pictures made at
the Yerkes observatory we try to make a ‘photographic journey’ in the
moon.”
“But tell me, has the moon always been so airless?”
“That is another unsettled question. Some astronomers have thought that
formerly, ages ago, the moon possessed a much more dense atmosphere than
she has at present. Having separated from the earth, in the way I have
described, it is natural to suppose that at first she may have had an
atmosphere very like ours. The explanation of its disappearance which
was once generally accepted was that it had been absorbed into the lunar
rocks, as the globe of the moon cooled off. But recent progress in our
knowledge of the nature of the gases composing the atmosphere has led to
a different explanation. This assumes that nearly all of the moon’s
atmosphere has _flown away from her_ because the lunar globe does not
possess sufficient gravitating force or attraction to retain it. If the
mass of the earth were no greater than that of the moon, our atmosphere
also would probably have escaped by flying off into space.”
“But how, and why, do these gases fly away?”
“They do it by virtue of what physicists call their molecular velocity.
A gas, of whatever kind, is a mass of molecules which are in continual
vibration, moving in all directions among one another with very great
velocities. These velocities have been measured, and it has been found
that the molecules of nitrogen, one of the components of the air, move
at the rate of two miles in a second. The velocity of the molecules of
oxygen is a little less; that of the molecules of hydrogen is very
great, nearly seven and a half miles in a second! Now, it is also known
that the attraction of the earth is sufficient to retain permanently
upon its surface all moving particles or molecules which have a velocity
less than seven miles in a second, while the attraction of the moon only
suffices to retain those whose velocities fall under a mile and a half
in a second. So you perceive that all of the gases I have named would
soon escape from the moon, even if they were present upon it at the
beginning of its history.
“I must also remind you that there is no water upon the moon, at least
not in the form of rivers, oceans, lakes, ponds, or even of clouds. But
Professor Pickering has recently noted certain appearances which may be
due to the formation of a kind of hoar frost. If there were once oceans
upon the moon, as the great plains, called _maria_, or seas, in the
lunar charts, seem to indicate, they, too, have escaped by evaporation.
The velocity of the molecules of water vapor is two and a half miles per
second, a mile greater than the ‘critical velocity’ which the attraction
of the moon would be able to control.”
“But,” interrupted my companion, “I am puzzled to understand how you
know so much about the power of the moon to hold things.”
“It is really quite simple,” I replied. “The attraction of gravitation,
which is a property belonging to all known bodies, is measured by the
mass, or amount of matter, in a body. It also varies with the distance
between the attracting and attracted bodies. We know, by means which I
shall not attempt to describe here, the mass both of the earth and of
the moon. We also know the size of both of these bodies. They attract
objects as if their entire masses were concentrated at their centers. A
body of a certain kind and size at the surface of the earth weighs just
one pound. If the earth were reduced to half its actual diameter, while
retaining the same mass or amount of matter, more closely packed
together, the body which now weighs one pound would then weigh four
pounds, because it would be twice as near to the center of the earth as
before, and the attraction of gravitation varies according to the square
of the distance from the center. As the distance diminishes the force
increases. The square of two is four, therefore the body would be
attracted with four times the force which it experiences at present.
Now, the moon is not only much smaller than the earth, but its average
density, or the closeness with which the molecules of its rocks are
packed together, is less. It results from these facts that the ratio of
the entire mass of the moon is to that of the earth as one to
eighty-one. Hence the inherent power of the moon to attract bodies is
less than one-eightieth as great as the earth’s. If the diameter of the
moon were the same as that of the earth, a body weighing one pound on
the earth would weigh only one eighty-oneth part of a pound on the moon.
But the diameter of the moon is less than one quarter as great as that
of the earth. It follows that bodies on the moon are almost four times
(more accurately about 3.66 times) nearer to the center of attraction.
This fact must be taken into account in calculating the force of gravity
on the moon’s surface. As far as the mass of the moon is concerned,
bodies on her surface experience less than one-eightieth of the
attractive force which the earth exercises upon bodies on its surface,
but this is so far counterbalanced by their greater nearness to the
center, that the actual attraction upon them is about one sixth of that
which they would experience on the earth.”
“Thank you,” said my companion dryly, “your explanation appears to me to
be very scientific.”
“Not by any means as scientific as it might be, or as it ought to be,” I
replied, laughing. “But, really, if you wish to understand these things
you should not be too much afraid of the bugbear ‘science.’ Science
makes the world go nowadays, and everybody ought to know a little about
it, just as everybody with any pretensions to education a hundred years
ago had to learn more or less Greek and Latin. But let me continue a
little farther. Since the force of attraction on the moon is only one
sixth as great as it is on the earth, the weight of all bodies is in the
same proportion. Pardon me if I guess at your weight; it is, perhaps,
120 pounds. Very well, translated to the moon you would weigh only 20
pounds.”
“Dear me, then skipping the rope may be the favorite pastime of
middle-aged ladies on the moon.”
“And throwing somersaults that of gray-haired lunar gentlemen. Let me
tell you of one very interesting consequence of the small force of the
moon’s gravity, which affects not merely the weight of bodies but the
flight of projectiles, and, indeed, all motions of every kind. You will
see, when we come to the photographs, that some of the lunar volcanoes
are of a magnitude almost incredible. This is doubtless due to the fact
that the ejections from volcanic craters there were able, with no
greater expenditure of explosive force, to attain an elevation six times
that which they would attain if thrown from a volcano on the earth.
During the eruption of Vesuvius in April, 1906, the column of smoke,
steam, and cinders from its crater reached, according to the measures of
Professor Matteucci, a maximum height of about eight miles. On the moon
the same force would have blown these things almost fifty miles high! It
is not difficult, in view of such facts, to see how the giant volcanic
craters and mountain rings of the moon were formed.”
In the meantime the eclipse continued, and, having tired of watching it,
we returned to the drawing-room.
“When shall we see these famous photographs and begin our imaginary
journey in the moon?” my companion asked.
“To-morrow,” I replied. “But I shall have to demand one more brief
exercise of your patience this evening, while I finish with this subject
of eclipses.”
“Then we are not through yet?”
“Not quite, for I have not yet told you why the moon is not eclipsed
every time she approaches the earth’s shadow, and why she does not
eclipse the sun once every month at the time of New Moon.”
“Well, tell me then, and I promise to be as interested as possible; only
please don’t talk any more mathematics than is absolutely necessary.”
“Very well, I’ll spare your attention as much as possible. To begin with
the eclipses of the moon: The reason why they are not of regular monthly
occurrence is simply because the orbit of the moon is a little inclined,
about 5¼°, to the orbit of the earth. Even then there would be an
eclipse once every month if the orbit of the moon were fixed in space,
and if the point where that orbit crosses the plane of the earth’s orbit
were always directly opposite to the sun. But instead of being fixed in
position the orbit of the moon has a curious motion of revolution of its
own. This causes the two opposite points, where it crosses the plane of
the earth’s orbit, and which are called the moon’s ‘nodes,’ to move
continually onward in a direction opposite to that in which the moon
revolves, but much more slowly. A period of about nineteen years is
required for the moon’s nodes to complete a revolution. The consequence
is that the nodes are not always in line with the earth and the sun, and
except when they _are_ nearly in line no eclipse can occur. To enter
into a complete explanation of this would require more ‘mathematics’
than you would like, but what I have said may at least serve to give you
an idea of the reason why eclipses are comparatively of rare
occurrence.”
“I think I understand the reason sufficiently. But what a complicated
affair you astronomers make of what, it seems to me, should really be a
very simple thing.”
“It is like a sewing machine,” I replied, “which seems very simple when
you see it running smoothly, and do not trouble yourself about all the
various parts of its mechanism. But if you undertake to explain to
yourself, or to make clear to another person, exactly how the machine
works, you find that your attention is rather severely taxed, and that
the apparent simplicity is based upon no little complexity of
construction and interaction of parts. You will have understood from
what I have said, that the reason why the moon does not eclipse the sun
once every month is based upon the same fact, namely, the inclination of
the moon’s orbit to the plane of the orbit of the earth; and that when
she does eclipse the sun her nodes must be somewhere near a line drawn
from the earth to the sun. There is one broad difference between an
eclipse of the moon and an eclipse of the sun which I have not yet
mentioned. This arises from the fact that the moon being so much smaller
than the earth, her shadow, when she hides the sun, does not cover the
entire earth, as the earth’s shadow covers the whole moon, but comes
almost to a point before reaching the earth. The average length of the
moon’s shadow is only 232,150 miles, 6,690 miles less than the average
distance between the moon and the earth. But since, in consequence of
the eccentricity of her orbit, the moon’s distance is continually
varying, the length of her shadow also varies to the extent of about
4,000 miles each way. Thus it may be as short as 228,300 miles, or as
long as 236,050 miles. When the greatest length of the moon’s shadow
coincides with her least distance from the earth (221,600 miles), her
shadow extends more than 18,000 miles beyond the earth. Under such
circumstances its diameter at the surface of the earth is about 167
miles. That is the greatest diameter that the shadow of the moon can
have at its intersection with the earth. Ordinarily, when it reaches the
earth at all, its diameter is less than 100 miles, and often very much
less. If the earth and the moon were motionless during an eclipse, her
shadow would form a round, dark spot on the earth, and all observers
within the circumference of that spot would behold the sun totally
eclipsed. But, in consequence both of the motion of the moon in her
orbit, and the rotation of the earth on its axis, the shadow spot moves
swiftly in an easterly direction over the earth’s surface, forming what
is called the path of the eclipse. The astronomer calculates beforehand
across what parts of the earth the path will lie, and selects his points
of observation accordingly.
“When the length of the shadow is too small to reach the earth, the moon
appears projected against the sun as a round black disk, hiding the
center of the solar orb, but leaving a brilliant ring all around. Such
phenomena are called annular eclipses. There are about three annular
eclipses for every two total ones. When the moon, as often occurs, does
not traverse the center of the sun’s disk, as seen from any part of the
earth, a partial eclipse is the result. This means that only a portion
of the sun is hidden by the moon. Even a total eclipse appears as a
partial one to observers who are not placed within the limits of the
shadow path.”
“But it seems to me,” said my friend, “you have hedged round your
eclipses with so many difficulties, what with the tip of the moon’s
orbit, and what with the shortness of her shadow, that they must be very
few in number. Yet I often hear of an eclipse, although I have never
seen one before to-night.”
“They are not so rare as you might suppose,” I replied. “It is not
necessary, in order that an eclipse, either partial, or total, or
annular, may occur, that the moon’s nodes be in a _direct_ line with the
centers of the sun and the earth. The moon may be a few degrees out of
line, and yet either pass into the earth’s shadow or be seen crossing
the sun from one point or another on the earth. There are about 70
eclipses in every eighteen years, 41 of the sun and 29 of the moon, but
the number varies a little. Generally there can be no more than two
eclipses of the moon in any one year, but it is possible for three to
occur. The greatest number of solar eclipses in a year is five, but this
is very rare, the usual number being two. In fact, there must be at
least two solar eclipses in a year, but there are many years which have
no eclipses of the moon at all. And now, I think I have said all that is
necessary about eclipses, and we arrive very opportunely at the end of
the discourse, for behold the moon is passing out of the shadow, and her
light begins once more to glow in the park.”
This was indeed the case. Going to the door, we saw the earth’s shadow
slowly withdrawing from the face of the moon, while the landscape was
brightening under her returning rays. For a few minutes we watched, in
silence, the brilliant spectacle. Then my companion turned to me.
“Would you know my whole thought?” she asked. “I fear that I cannot
recall many of the scientific facts you have just been telling me, but
for them I can go back, at need, to the books. Yet one thing I feel that
I have certainly gained. It is a sense of friendly, companionable
interest in the moon. Henceforth she will be more to me than she ever
was before. I shall always be conscious, when looking at her face, that
she is the offspring of the earth, and that there exists between these
two bodies an intimacy that I had never imagined possible. For me your
tides and your eclipses seem an inarticulate language, a caressing
exchange of communications between these two celestial beings of one
blood. To my mind they are, in a certain sense, personalities, and, as a
creature of the earth, I feel now my relationship to the moon.”
“Very good,” I replied. “All science and all forms of knowledge are
rooted in the imagination. To-morrow we shall begin with the
photographs, and many most interesting things that I have not yet
mentioned will then naturally present themselves before us.”
“Good night then,” said my companion, “and to-morrow I shall count upon
the delights of a photographic journey in the moon.”
------------------------------------------------------------------------
I
NEW MOON TO FIRST QUARTER
------------------------------------------------------------------------
I
NEW MOON TO FIRST QUARTER
AT breakfast the next morning I asked my friend if she still had
sufficient curiosity concerning the moon to induce her to undertake the
contemplated journey amid lunar scenes.
“Yes, surely,” she replied. “My dreams last night were filled with
wonderful spectacles; great cones of shadow flitted continually through
the heavens, eclipsing, in turn, moon, sun, and stars; and I stared, as
it seemed, for hours at strange faces veiled behind a maze of
mathematical diagrams covering the moon. I am not sure that your
discourses have made me scientifically much wiser, but I feel that my
imagination is sufficiently aroused to enable me to enjoy the
photographic excursion that you have proposed, and I am quite ready to
start at once.”
“Excellent!” I said, producing my portfolio. “Here then are the
photographs which I trust will enable us, in imagination, to spend an
interesting month upon the moon. These photographs were made at the
Yerkes observatory and they represent the moon, as you will perceive, in
all of her principal phases, beginning with the narrow crescent of the
New Moon, and ending with the similar, but reversed, sickle of the Old
Moon.”
“Let us take them out into the park under the trees,” my friend
suggested.
The shafts of morning sunshine, falling through the branches and
illuminating the broad lawns and brilliant flower-beds, offered the
greatest possible contrast with the strange scenes of the preceding
night. We chose the shadow of a huge elm, and had a table placed there
for our accommodation. On this I spread the photographs, and my
companion began to examine them with many expressions of interest.
“It is not often,” I said, “that science finds so flattering an
audience.”
“And I hope, surely, never so small a one,” she responded, laughing.
“But you must admit that science very seldom presents herself in so
attractive a form as that of these pictures.”
“They are indeed of the highest excellence,” I replied. “It is the very
moon herself that you see there.”
“But are you certain that they have not been embellished? Has not the
hand of an artist retouched and improved them—particularly these large
ones that seem to contain a thousand curious things which I can hardly
believe really exist on the moon?”
“No,” I said, “there is nothing fictitious or imaginary in what you see.
The only art displayed here is that of the astronomer-photographer,
whose greatest ambition is to make his pictures absolutely true to
nature. A defect in one of his plates, producing the appearance of a
speck of light or shadow which does not actually exist, causes him as
much distress of mind as you would experience upon hearing a false note
from your piano. Indeed, the astronomer is so desirous of having nothing
but the truth represented in his pictures that he often prefers, for his
own study, the original negatives alone, because every time that they
are reversed to make a ‘positive’ copy something is sure to be lost, and
some slight defect is certain to be introduced. Let us begin, if you
please, with the series of smaller pictures showing the various phases,
and the gradual advance of daylight across the moon’s surface. Take
first the photograph which I have labeled No. 1. It shows the New Moon
when it is between three and four days old. You must often have seen it
in that form in the western sky soon after sunset. Photographs of the
New Moon have been made when the crescent is still narrower than that
here shown, but there is no such photograph in this series, and it would
possess little interest for you because almost no details of mountains,
craters, and plains would be visible. It is hardly possible to make a
good photograph of the moon when it is only one or two days from the sun
in its monthly journey, on account both of the glare of the solar light
in our atmosphere and of the nearness of the moon to the horizon, where
the air lacks transparency and steadiness. In the photograph before us
you will observe a great number of strange forms and shadings. I shall
tell you what these are presently, but first let me call your attention
to the fact that the picture does not exhibit a phenomenon which you
would behold if you were actually looking at the moon in the phase here
represented. You see here the New Moon very clearly, but not the Old
Moon in her arms.”
“Indeed! It is a pity that the photograph does not show so interesting a
sight.”
“Yes, it is a pity. The cause lies in the defect of light from what I
have called the ‘Old Moon.’ The part that we see in the photograph is
illuminated with sunshine, while the remainder of the moon reflects only
the earthshine, which is too faint to be photographed (at least with the
amount of exposure required to make a good picture of the brightly
lighted crescent); although, as I have said, you would see it clearly if
you were looking at the New Moon herself.”
[Illustration:
NO. 1. FEBRUARY 19, 1904; MOON’S AGE 3.85 DAYS.
]
“But,” interrupted my companion, “do you mean to tell me that the earth
illuminates the moon?”
“Surely it does. Why not?” I replied, smiling. “You must remember that
the earth is simply a huge moon to our imagined inhabitants of the lunar
world. Our globe sends to the moon about fourteen times as much
reflected sunlight as the moon sends to the earth. The consequence is
than an earthlit night on the moon is far more brilliant than a moonlit
night on the earth.”
“Then why do we not always see the moon shining with light from the
earth?”
“It is a question of contrast. You cannot see a faint light in the
immediate presence of an overpoweringly brighter light. The part of the
moon that the sun illuminates is in the full glare of day, and this is
so much more brilliant than the reflected earthlight that that portion
of the moon which enjoys only the latter is not visible to us, except
for a few days after New Moon, when the amount of light from the
crescent is not yet great enough to dazzle our eyes and hide the rest
from sight. I should advise you when the next New Moon occurs—you can
find the date in any almanac—to look at it in the western sky. You will
see in addition to the bright crescent the full round orb of the moon,
shining faintly, with a dull, rather copperish, tint, and you will find
it interesting, then, to remember that that light is reflected from our
earth.
“And now,” I continued, “let us examine our photograph more closely.
There is one remark that I had expected which you have not made; it
concerns the position of the crescent. You observe that it is bowed
toward the left. If you saw it with the naked eye in the sky it would be
bowed toward the right, or toward the place of sunset. The reason is
that the photograph presents the moon as seen with a telescope, which
reverses objects, turning them top for bottom. In this picture, and in
all the others that we shall examine, the southern part of the moon is
at the top and the northern part at the bottom, the western part at the
left and the eastern part at the right. The first thing that you
probably notice in the photograph is a conspicuous oval plain, somewhat
below the center of the crescent.”
“Yes, and I see clearly why you call it a plain, for it is perfectly
flat and smooth.”
“Not quite so flat and smooth as you suppose. This object is one of the
most celebrated on the moon. It is the so-called _Mare Crisium_, or Sea
of Crises, as we may translate the name given to it by the astronomers
of a couple of centuries ago, many of whom knew more Latin than science.
Owing to its apparent smoothness of surface, as well as to its form and
general aspect, they took it for a great lake or sea.”
“To tell you the truth,” said my friend, “if I were an astronomer and
had discovered this curious place on the moon, I should certainly
believe just what your Latin-loving predecessors believed, but I doubt
if I should have been capable of inventing so singular a name for it.”
“In the singularity of the names they chose for objects on the moon,” I
replied, “their invention is unrivaled. We shall see some remarkable
examples. Of course they are not at all to be blamed for thinking that
this oval spot, and other similar ones of much greater magnitude, were
seas and oceans. They simply judged by appearance and by analogy.
Finding mountains on the moon, they saw no improbability in supposing
that there were bodies of water also. They had not the means of knowing,
as we know to-day, that there is no water on the moon. Yet, perhaps,
they were not so far wrong after all. The _Mare Crisium_ certainly has
the look of an empty sea bed, and I should not be willing to assert that
ages ago it was not filled with water.”
“Like the Great Salt Lake, dried up,” suggested my companion.
“Not exactly, for the Great Salt Lake dried up would probably present a
surface as white as snow, whereas the _Mare Crisium_ is very dark. It
must be admitted, however, that gradually the white deposit would grow
darker, and there may be much significance in the fact, which some
observers have noticed, that, at times, parts of the dark plains on the
moon seem to glitter with minute points of light. Your imagination is at
liberty to see deposits of salt there.”
“In that case,” said my companion, laughing, “I should prefer to regard
the _Mare Crisium_ as resembling that wonderful valley discovered by
Sindbad the Sailor, whose floor was sprinkled with diamonds.”
“Well,” I replied, “science certainly cannot deny the possibility of
diamonds on the moon, for she is _par excellence_ the world of
volcanoes, and one of the most striking discoveries of recent years is
that of the intimate association existing between ancient volcanic vents
and deposits of diamonds. The diamonds of South Africa are found in lava
rocks that cooled off ages ago.”
“Then I hope that no future Columbus will find a way to the moon, for we
should have too many diamonds, and they would lose all their charm.”
“That is true, but suppose that not only diamonds but even more
beautiful gems should be discovered in the lunar world? You surely would
not object to a transethereal traffic bringing them to our doors.
However, there is not the slightest prospect that we shall ever be able
to go from the earth to the moon. Let us resume our examination of the
photograph, and concentrate our attention on the known facts.”
I then proceeded to tell my friend, whose interest I was delighted to
find had not yet begun to flag even in the face of comparatively
matter-of-fact statements, that the _Mare Crisium_ is a profound
depression, about 350 miles in length by 280 in breadth. Exactly how far
it lies below the general level of the lunar surface we do not know;
but, at any rate, if it was ever filled with water it formed a deep,
navigable sea. Its encircling mountains, which appear generally bright
in the photograph, especially along the eastern border, where the
sunlight strikes directly against their slopes, are in many places steep
and abrupt. At one place, on the southwestern side, there is a
mountainous promontory 11,000 feet in height. There are a number of
small craters on the floor of the _Mare Crisium_, but the scale of this
photograph is not large enough to show them clearly.
“You will notice,” I continued, “that there is a kind of bay on the
eastern side, which runs back into the mountains, and is bordered with
high, steep cliffs. Near this point, on that part of the moon over which
the sun has not yet risen, there is a very remarkable mountain which we
shall see in a later photograph. But let us finish with this one. Look
at the comparatively small oval adjoining the _Mare Crisium_ below
(toward the north). It is one of the great crater rings of the moon, and
is named Cleomedes. It is much larger than it looks, being nearly 80
miles in its greatest diameter, and there is a peak on its surrounding
wall 10,000 feet in height. Still farther toward the north you will
observe two or three other smaller craters or rings, which are very
interesting when studied with the telescope.
“Now, please turn your attention to the photograph bearing the number 2.
You see again the _Mare Crisium_, and nearly in the center of the
crescent, and just on the border line between day and night, a perfect
oval ring with a central peak. It is called Langrenus. It is even larger
than Cleomedes, being about 90 miles across. It has the form of an oval,
as we see it, but that is an effect of perspective, since it is so far
round the side of the lunar globe. In reality it is a nearly circular
circumvallation, or rather an almost perfect hexagon, composed of
gigantic mountains including a valley, in the center of which rises a
cluster of peaks 3,000 feet in height.”
“This second photograph,” interrupted my friend, “was taken later than
the first, I suppose, since it shows more of the moon’s surface.”
[Illustration:
NO. 2. SEPTEMBER 24, 1903; MOON’S AGE 3.87 DAYS.
]
“I should have told you that,” I replied. “Yes, it does represent the
moon at a time when more of its surface, visible to us, is illuminated
by the sun. In fact, we may regard it as a picture of the moon made
about a day later than the other. But I must now tell you that these
photographs were not all taken in regular succession, a day apart, or
even two days apart. That was impracticable for reasons that I need not
explain. Some of them were made at one season of the year and some at
another. Yet taken together they form a sufficiently continuous series
to enable us, with their aid, to follow the changing aspects of the moon
during more than three weeks, or all that part of a lunation in which
the moon is a conspicuous object in the sky.”[2]
Footnote 2:
In addition to what is said in the text concerning the photographs the
reader should be informed that, in consequence of her “librations,”
the moon does not, all the time, present _exactly_ the same surface
toward the earth. If she did we should never see more than one half of
her surface. In fact, however, at one time or another, we see, in all
(but never at the same time), about fifty-nine per cent of her
surface, leaving forty-one per cent which is forever invisible because
never turned in our direction. The librations, or “balancings,” of the
moon, which bring now one and now another portion of the usually
invisible hemisphere into view, are of three kinds: First, the
libration in latitude, arising from the combined effects of the
inclination of the moon’s orbit to the plane of the earth’s orbit, and
the inclination of her axis of rotation to the plane of her own orbit.
When added together these two inclinations make the axis of the moon
lean one way or the other with respect to the earth about 6½°. But,
since the inclination of the moon’s orbit to that of the earth is
continually varying to a small extent, the amount of this libration is
also variable. Its effect is to cause now the North and now the South
Pole of the moon to incline slightly toward the observer on the earth,
so that he can see alternately a little way round the northern and the
southern edges of the moon’s disk.
Second, the libration in longitude, which arises from the eccentricity
of the moon’s orbit, causing her to move a little faster when she is
nearer the earth, or in perigee, and a little slower when she is
farther from the earth, or in apogee. In consequence of this, she gets
alternately about 6° ahead of, or behind, the position which she would
have if her orbit were a perfect circle and her motion perfectly
uniform. But, inasmuch as her rotation on her axis is never either
faster or slower, she shows a little of her usually invisible
hemisphere on the western side when she is between perigee and apogee,
and a little on the eastern side when she is between apogee and
perigee. The accompanying diagram is designed to aid the reader in
understanding these effects.
[Illustration:
Effect of Moon, Varying Velocity in Orbit Producing Libration in
Longitude.
]
Third, the diurnal libration, which arises from the fact that the
diameter of the earth bears a considerable proportion to the distance
of the moon. If the observer were at the center of the earth there
would be no effect of this kind, but being situated about 4,000 miles
from the center, there is a parallactic effect in consequence of which
we see a little around the western side of the moon when she is rising
and a little around the eastern side when she is setting. The maximum
diurnal libration is a little more than one degree. The maximum
libration in latitude is 6° 44´, and that in longitude 7° 45´. An
illustration of the results of libration will be found by comparing
photographs Nos. 1 and 2. They were both taken at nearly the same “age
of the moon,” about three days, twenty hours, but under different
librations, so that in No. 2 more of the western edge of the moon is
visible, and the crescent appears broader. Even more remarkable
examples of the results of libration are seen in Nos. 6 and 7, and 8
and 9. In No. 6, the moon is actually “older” by about half a day than
in No. 7, yet, owing to libration, the “terminator,” or line between
day and night on the moon, is considerably farther toward the east in
the latter than in the former. A similar effect is seen in comparing
Nos. 8 and 9. The exact dates and ages of the moon corresponding to
these photographs are given in the Appendix.
“If you will follow the curve of the terminator toward the south (upward
in the photograph), you will perceive that there is a long line of
ovals, more or less resembling Langrenus. The first of these, darker in
appearance than Langrenus, is named Vendelinus.”
“What extraordinary names!” exclaimed my companion, “and how
unpicturesque!”
“Yes, it is true that the invention of the old astronomers who supplied
these names seems to have failed a little at times. They did exceedingly
well in naming the ‘seas’ and similar objects, but for the mountains,
craters, and ring plains they could think of no better plan than that of
attaching to them their own names, and the names of other _savants_, or
supposed _savants_ of their time, or of preceding centuries. And in
Latinizing these names they gave them a kind of uniformity, which is
hardly pleasing to our taste to-day. But let me continue. Vendelinus is
an extremely beautiful sight when the sunlight strikes its broken walls
in such a manner as to bring into prominence, by contrast with the deep
shadows, the rugged peaks, precipices, and ridges of which its very
irregular ring is composed. You should see it with a powerful telescope,
especially under the rays of the setting sun. Then the bottom of the
valley within has been described by Mr. Eiger, an English student of
lunar phenomena, as appearing punctured like a sieve with holes.”
“And what are they?”
“Volcanic craters, probably, long since extinct.”
“So many volcanoes in one place?”
“Oh, yes. You have been at Naples and have seen Vesuvius. But probably
you have not visited the Phlægrean Fields which lie northwest of Naples.
If you had had a passion for geology when you were in Italy you would
have explored that region, and there you would have found something not
altogether unlike the valley of Vendelinus in the moon. There is a great
number of extinct volcanic craters near Naples, and they show how
similar in many ways the moon is, or has been, to the earth.”
“But, dear me,” my friend exclaimed, “are we going to see nothing but
burned-out craters and wild, ragged mountains on the moon? I am sure
that I should never have thought of visiting Naples for the sake of
looking at its Phlægrean Fields.”
“Still,” I replied, “you must certainly know that Pompeii and
Herculaneum and the memories of their tragic fate are the most vivid
attraction of Naples to-day, although the Pompeiians have all been dead
for almost 2,000 years. So in looking at these spectacles in the moon we
cannot but be interested by the reflection that they are reminders and
relics of a wonderful history, whatever its precise character may have
been. The moon seems to me to stand for the most affecting of all
tragedies—the passing of a world. When I survey its extraordinary
landscapes, it is like looking upon a long-abandoned stage, whose actors
are in their graves, whose scenery is moldering under a gaping roof,
whose machinery is broken, whose very traditions are forgotten, but
which yet retains a semblance of its former brilliance. I do not have to
imagine inhabitants in the moon at the present day in order to find it
interesting. The possibility that it may once have had inhabitants is
enough, remembering its nearness to the earth and the manner of its
origin, to make it the most fascinating thing that the heavens contain.”
“Indeed, I had never thought of the moon quite in that way,” was the
reply. “If you can read a history for me in these craters and ring
plains I believe I shall find them more interesting than I expected.”
“I cannot promise you a history as full of romantic details as that of
Herodotus,” I said, “but it may contain nearly as many actual facts.
However, we shall see about that as we go along. Let us now return to
the inspection of the photograph. Be kind enough to look a little above
Vendelinus. You observe there another still larger ring plain, or walled
valley, with a conspicuous mountain in the center. This is Petavius. It
belongs to the chain of similar formations which includes Langrenus and
Vendelinus, but it is more wonderful than either of them. It is nearly a
hundred miles long from north to south. For some reason, as with
Vendelinus, its ruggedness and complexity of structure are more
conspicuous in the lunar afternoon than in the lunar morning. It is a
question of the direction in which the light falls across it. A curious
thing about Petavius is the convexity of its vast floor. The center is
about 800 feet higher than the edges along the feet of the surrounding
mountains.”
“How do you know that?”
“The shadows tell the story. The height of objects on the moon is
measured by observing the length of their shadows under a known
inclination of the sun’s rays. When I stand this book upright on the
table, allowing the sunlight to strike it on one side, it casts a shadow
on the table. If I did not know the height of the book, and could not
measure it directly, I could find it out by measuring the length of its
shadow, other simple trigonometrical data, easily ascertained, being
known. There is an enormous cleft not clearly visible in the photograph,
extending from the central mountains of Petavius to the southwestern
wall of the valley. Still farther south, above Petavius, you will notice
another conspicuous oval plain and several smaller ones near it. The
largest of these is named Furnerius. They all lay in the morning
sunshine, not far from the terminator, when this photograph was taken.”
“Tell me, please, about the ‘terminator’ of which you have spoken
several times. As I understand you it is the line between day and night
on the moon.”
“Yes, and a very wonderful line it is, too. There is nothing just like
it on the earth. Owing to the effects of our atmosphere in dispersing
the light, day and night do not stand face to face with one another on
the earth in the same way that they do on the moon. Here we have
twilight in the evening and dawn in the morning, and night neither comes
nor goes for us with the startling suddenness that characterizes it on
the moon. For an hour or two after sunset and before sunrise, we receive
rays of reflected and refracted light from the atmosphere above us,
which spread a soft, pleasing illumination over the landscape, and
render all objects more or less distinctly visible. But if you were on
the moon in certain situations, the passage from day to night or from
night to-day would be as rapid as the falling or rising of a curtain.
Imagine yourself standing on the western wall of Vendelinus or Petavius
at the time when this photograph was taken. You would be in a blaze of
pitiless, untempered sunshine, but glancing down the precipice at your
feet you would seem to be looking into a gulf of blackness. But for the
light reflected back from the eastern cliff, and that coming from the
earth, there would be scarcely a ray of illumination on the rocks below
you. You would look down into inky darkness, and would scarcely dare to
make a step from fear of falling over the edge of a bottomless pit. At
the same time, as I told you last night, you would see the stars all
about you in the sky, even close to the sun.
“This is the reason,” I continued, “why the march of day across the
moon, always keeping sharp on the heels of night, is a spectacle so
imposing and unparalleled. It is this wonderful march that we are going
to follow with the aid of the photographs. I shall now ask you to give
your attention to photograph No. 3. It was made more than a day and a
half later than the others, measured by the age of the moon, which, in
this case, was about five days and a half. You notice how in the
interval the sunlight has swept eastward over the moon’s surface. The
_Mare Crisium_ is recognizable in the lowest or most northerly, of three
large, dark plains. The small white oval a considerable distance above
it is our old acquaintance Langrenus, whose floor and walls are now very
brilliant in the full sunshine, which falls upon them at a high angle.
Vendelinus and Petavius are less conspicuous. The broad, dark plain
which has come into view eastward from Langrenus is the _Mare
Fœcunditatis_, which we may translate ‘Sea of Fecundity’! You certainly
cannot aver that on this occasion the invention of the old astronomers
failed in the matter of romantic suggestiveness. The name calls up
pictures of a great body of tranquil water, fanned by gentle,
stimulating breezes, filled with fish of every variety, dotted with
vine- and flower-garlanded islets, and bordered by well-watered shores,
rich with vegetation, and supporting a numerous and happy population.
Some such idea of the _Mare Fœcunditatis_ may have been in the minds of
its sponsors a couple of centuries ago. But telescopes have become too
powerful in our day to permit us to be any longer deceived as to the
actual nature of this singular lunar region. Like the _Mare Crisium_, it
may have been the bed of a sea many years ago, but at the present time
it contains no water, and its shores present an endless succession of
fire-scarred cliffs, peaks, and volcanoes. The only ‘islands’ in it are
extinct craters.”
“But,” said my companion, smiling, “where then is its history?”
“Ah!” I replied, “is not this old sea itself history enough? When it has
receded sufficiently into the past, all history loses its details, and
presents only its setting and its grand primary elements. Suppose that,
some ages in the future, you should be an inhabitant of a distant
planet, surveying with a telescope the dried-up basin of the Atlantic
Ocean. Provided only that you were convinced, in your own mind, that it
had once been an ocean, with fertile, inhabited shores, and with ships
sailing upon it, you would be singularly lacking in imagination if you
could not reconstruct its history for yourself. The details could safely
be left to your invention and you could change them from time to time to
suit your varying moods. Terrestrial historians have sometimes done
that.”
[Illustration:
NO. 3. JULY 29, 1903; MOON’S AGE 5.54 DAYS.
]
“But do _you_ believe that the _Mare Fœcunditatis_ was ever such a sea,
and the scene of such events?”
“That is certainly a very pointed question. Questions of that kind are
always in order when one is treating of ascertained verifiable facts,
but just now, you know, we have wandered a little aside from the
straight path of scientific exactitude. Still, I will be frank with you
and say that I really possess no settled opinion concerning the former
condition of the moon, except so far as what we may call its
‘geological’ history is revealed by its present state. I am sure that
the moon was once the seat of tremendous volcanic action, and I think it
not improbable that its great depressed plains were once occupied by
water, but as to inhabitants, I know no more about them than you do.
Still, I am disposed to think that, as we go on, _you_, at least, will
reach the conclusion that all life has not yet disappeared from the
moon. We are going to learn some very suggestive and significant things
before we are through.
“Farther toward the south and closer toward the terminator you will see
in the photograph a third dark plain with five sides, the northern one
convex and ill-defined. At its upper corner is an incomplete ring plain.
This region bears a still more curious name than the _Mare
Fœcunditatis_. It is the _Mare Nectaris_ or ‘Sea of Nectar.’”
“Apparently your astronomers of old took the moon for an abode of the
gods.”
“Yes, or for their wine cellar. But we shall get a better look at the
surroundings of this Sea of Nectar in a later photograph, and then I
shall have more to tell you about it. In the meantime let us return to
the _Mare Crisium_. To the east (right-hand side) of the _Mare Crisium_
you will observe a diamond-shaped district, not very dark, with a bright
point at the corner which faces the _Mare_. You could never guess its
name. It is called the _Palus Somnii_, which may be translated ‘Marsh of
a Dream.’ It is a very singular place, and, seen with the telescope,
possesses a color which is unique upon the moon, a kind of light brown,
quite unlike the hue of any of the other plains or mountain regions. It
is covered all over with short, low ridges, as if its surface had been
broken up in a most irregular manner with a giant plow. What the person
who named it saw there to lead him to connect it in his mind with dreams
I have never been able to imagine. The bright point on its western edge
is a remarkable crater mountain, named Proclus. What that mountain is
made of nobody knows, but it gleams with extraordinary brilliance when
the sun strikes it.”
“Why may it not be snow-covered?”
“That is a suggestion which has often been made, but one great objection
to it is that we have reason for believing that snow, at least in such a
situation, cannot exist on the moon. Another objection is that only a
few of the lunar mountains are comparable in brightness with Proclus,
and they are not the loftiest ones. Upon the whole it is much more
probable that the reflecting power of Proclus is due to the composition
of its rocks, perhaps to broad crystalline surfaces exposed in the
sunshine.”
“It is a surprise to me, then, that that ‘earthly godfather’ of lunar
wonders, who had a sufficiently vivid fancy to invent the ‘Marsh of a
Dream’ close by, did not name this mountain for some jewel, real or
imaginary.”
“It would have been more poetic, indeed, but as I have already told you,
the mountains and volcanoes of the moon nearly all bear very prosaic
designations, while a wealth of fancy has been lavished in naming the
‘seas’ and plains. The astronomer Riccioli is responsible for most of
the commonplace nomenclature that we find in lunar charts. If you will
now glance at the northern (lower) ‘horn’ of the moon in the photograph
you will notice, near the terminator, about two thirds of the way from
the _Mare Crisium_ to the end of the horn, a pair of ring plains, or
crater rings, apparently almost touching one another. They are Atlas and
Hercules, the latter being the smaller one on the right. A darker oval
below them near the bright edge of the moon is Endymion.”
“That, at least,” exclaimed my companion, delighted, “is a romantic and
appropriate name! I am enchanted to think that Endymion has not been
separated by your cold-hearted science from her who loved him so well.”
“But if you should look at Endymion with a telescope you would wonder
what the moon could find in him to admire. He has been turned into a
huge, broken-walled ring plain. You will observe that the other, the
southern or upper horn of the moon in the photograph, appears
extraordinarily roughened. It is completely pitted with craters and
rings. There are so many of them, and they are so entangled, that I
shall not undertake to indicate them by their individual names,
especially as there is none among them of the very first importance. If,
however, you will bring your attention back to the _Mare Nectaris_ I
shall be able to point out to you a very extraordinary object, which
lies just on the border between day and night here, but will be seen in
the next photograph that we examine, in full morning light. The object
that I mean is a ring on the right-hand edge of the _Mare Nectaris_. Its
eastern wall and the top of its central peak are brightly illuminated by
the rays of the rising sun; while beyond it, to the eastward,
everything, with the exception of the tips of one or two high peaks, is
steeped in night. This is one of the mightiest volcanic formations that
the moon contains. Its name is Theophilus. To see it and certain
gigantic neighbors that it has, fully displayed, we shall turn, after
this glance at its first appearance, to photograph No. 4.
“In this photograph the sunrise line on the moon has advanced so much
farther eastward that the _Mare Nectaris_ lies well within the
illuminated part of the disk, and Theophilus has become the most
conspicuous object of the kind in view. You now observe that it does not
stand alone, but is linked, so to speak, with another similar ring on
its southeastern side, while still farther southward is a third less
regular ring which seems to belong to the same group.”
“Oh, yes,” cried my companion, “they certainly do seem to be connected.
They look like three links of an enormous broken chain dropped upon the
moon.”
“The ring nearest to Theophilus,” I continued, “and whose northwestern
side has been destroyed to give room for the full circle of the wall of
Theophilus, is named Cyrillus. The other more distant one is Catharina.
If you wish to become a little learned in the geography of the moon it
is necessary that you should remember these names. As to the objects
that the names designate, they are far too wonderful ever to be
forgotten, and it is impossible to confuse them with any other features
of the lunar world. There is a great deal of ‘history’ connected with
these three enormous volcanic formations, but I am going to reserve that
for a while, because by and by we shall examine a larger photograph of
these same objects in which you will see their marvelous details
displayed. Now let me direct your attention to the first chain of
mountains that we have found upon the moon. Above Catharina you will
notice a thin, crinkled line of light passing through a comparatively
level district and ending at another ring. It is a range of peaks and
cliffs named the Altai Mountains. They are of no great height, and
cannot be compared in magnificence with the lunar Alps and the lunar
Apennines which we shall see in the photographs taken a few days later,
but they are nevertheless very interesting. The ring mountain at which
the Altai range terminates is named Piccolomini. It is another marvelous
object for telescopic study. The incomplete ring, with a dark interior,
which forms the southern corner of the _Mare Nectaris_, resembling a
semicircular bay, is Fracastorius. It is a very curious object because
close inspection reveals that the missing part of its ring has been
submerged, but is still faintly visible through the surface of the
_Mare_.”
[Illustration:
NO. 4. NOVEMBER 24, 1903; MOON’S AGE 5.74 DAYS.
]
“I suppose it cannot be water that has covered it, since you have so
often assured me that there is no water on the moon.”
“No, it is not water, but rock or sand or solidified lava, or some kind
of solid matter. It looks as though the whole bed of the _Mare Nectaris_
had welled up in one mighty convulsive outpouring of liquid lava, which
broke down the wall of Fracastorius, inundated the interior, and then
hardened like a floor of cement. The probability that a catastrophe of
the kind I have described has occurred here is heightened by the fact
that the bed of the _Mare Nectaris_ is concave, sunken in the center, as
if it had broken and settled down ‘like ice upon a pond.’ Scattered more
or less all over its surface and particularly near its shores, there are
indications of this breaking down, and of something that has been
covered up.”
“To me it seems very mysterious,” said my friend, “and very terrible
also.”
“It is more or less mysterious to the astronomer likewise. Still,
geology shows that there have been somewhat similar occurrences on the
earth. If you will now direct your eyes to the lower (northern) part of
the photograph you will notice some additional things that have come
into view with the advance of the sunlight. You observe that a vast
somber region occupies the inner portion of the crescent below the
center. This consists of two immense plains, one of which sends a large
‘bay’ as far south as the ring of Theophilus, where it is connected by a
narrow ‘strait’ with the _Mare Nectaris_.
“Turning to photograph No. 5 we see the two plains to which I have
referred more fully displayed. The sun has now risen over their entire
surface. The upper one is the _Mare Tranquillitatis_, ‘Sea of
Tranquillity’; and the lower one the _Mare Serenitatis_, ‘Sea of
Serenity.’”
“I have always thought that astronomers must be happy persons,” said my
companion, with a smile, “and these names are convincing.”
[Illustration:
NO. 5. JULY 1, 1903; MOON’S AGE 6.24 DAYS.
]
“Yes, perhaps, but then in bestowing the names they may have been
transferring to the moon ideals of tranquillity and serenity which they
did not find realized upon the earth. I am not going to talk about these
two ‘seas’ at present because they are better represented upon one of
the large photographs which we shall examine later. I prefer to direct
your attention just now to some other things. In the first place look
once more at Theophilus and its companion rings, and observe how they
maintain their preëminence. The entire surface of the moon to the
eastward and southward is broken and heaped up with mountains, craters,
and rings, but nowhere do we see anything comparable with Theophilus
except, perhaps, far toward the south, where near the inner border
appear two still larger, but less regular, rings lying in line at a
right angle to the terminator. The one on the left is Maurolycus, and
the other, still half obscured by night, is Stöfler.”
“The names of old astronomers, I suppose.”
“Yes, astronomers sufficiently famous in their day, but who would be
virtually forgotten at the present time if their friend Riccioli had not
thus immortalized them. You see it is a great piece of good fortune to
have your name in the moon. It is a kind of revenge for the neglect of
future generations at home.”
“And it seems to me an equal good fortune to have had an admirer willing
to set your name up in the moon.”
“Surely. But Riccioli’s own name is there also. Afterwards I shall show
you his lunar monument, a truly magnificent one. Permit me now to tell
you that Maurolycus is much greater in extent than any of the rings that
we have yet seen. Not by any means so perfect in form as Theophilus, it
covers a vast extent of surface, as much as 150 miles across, with an
amazing mass of broken rings, walls, ramparts, ridges and chasms. Some
of its peaks are 14,000 or 15,000 feet in height. It has a very lofty
central mountain, visible in the photograph, and whose peak comes into
view when the sun is rising long before the surroundings have been
illuminated, so that it resembles a star glowing amid the blackest
night. The neighbor of Maurolycus, Stöfler, is equally extensive and
almost equally wild and magnificent when the sunlight is leaping across
it from pinnacle to pinnacle and ridge to ridge. In this photograph,
however, it is too near the terminator to be well seen. We shall
presently pass to photograph No. 6, where Stöfler appears in full light,
but before doing so let us glance at the northern part of the moon as
here pictured. Close to the terminator, below the grand oval form of the
_Mare Serenitatis_, you will perceive two rings, one above the other.
They seem to be the complement of the other pair, Atlas and Hercules,
which we looked at when the sun had recently risen upon them in another
photograph, and which now appear far off toward the west. You observe
that Atlas and Hercules lie upon an east and west line, and the others
upon a north and south line. The northernmost one is named Aristoteles,
and the other Eudoxus. They are situated near the edge of a plain called
the _Mare Frigoris_, ‘Sea of Cold,’ thus named, I suppose, because it
lies so far north. Aristoteles is about 60 miles in diameter, and its
immense wall is very high and splendidly terraced. Eudoxus, equally
deep, is only 40 miles in diameter.
“Turning to photograph No. 6, taken when the moon was more than a day
older than it was when No. 5 was made, we have a striking example of the
effect of libration in presenting the moon at perceptibly different
angles to our line of sight at corresponding phases. We have now arrived
at First Quarter, and behold all the western half of the moon
illuminated by the sun. You will perceive that we now have in view,
simultaneously, six of the great plains called ‘seas,’ namely, the _Mare
Crisium_, the _Mare Fœcunditatis_, the _Mare Nectaris_, the _Mare
Tranquillitatis_, the _Mare Serenitatis_, and the _Mare Frigoris_, while
others are beginning to emerge out of night on the east. Maurolycus and
Stöfler, the pair of giant rings in the south, are better seen than
before because daylight has advanced farther across them. In fact
Stöfler now appears more imposing than its great neighbor, and a smaller
ring breaking the continuity of its wall on the western side is visible.
Above these, in the direction of the south pole of the moon, and around
the pole itself, the surface is marvelously rough and broken. It looks
as if it would be impossible to find a level acre of ground in all that
region. The rings and craters are veritably innumerable. It is the
existence of these irregularities which causes the terminator to appear
so crooked and broken. At some places you perceive small bright points
within the edge of the night half of the moon. These, of course, are the
summits of peaks, which have just been touched by the sunlight while the
surface all around them is still covered with darkness.
“Below Stöfler, all along the terminator, as far as the middle of the
moon, an irregular row of rings appears. Three of these bear some
resemblance to the great group of which Theophilus is the chief member.
They are, counting from south toward north, Aliacensis, Werner, and
Blanchinus. Below them two other much larger ones are conspicuous,
Albategnius, the more southerly, and Hipparchus. These two are full of
moon history. Albategnius, the smaller, is very deep and comparatively
perfect in condition, while Hipparchus, more than 90 miles across, has
been vividly described as a ‘wreck and ruin,’ its walls, once possibly
of great height, being now low and broken, and traversed with gaps and
valleys, while a great cleft exists crossing a part of the broad,
irregular floor. It is probable that Hipparchus is an older formation
than Albategnius.”
[Illustration:
NO. 6. NOVEMBER 26, 1903; MOON’S AGE 7.75 DAYS.
]
“Pardon me,” interrupted my companion, “but I must cry for mercy.
Really, these strange names escape from my mind as fast as you mention
them. Is there not something a little more romantic in the
moon—something to relieve the strain of all this nomenclature of words
terminating in ‘us,’ and this frightful lunar geology?”
“Yes,” I said, “I believe that on the other half of the moon, which has
not yet seen the sun rise, we shall find something better to your taste.
But do not be too impatient. Reflect that these names represent very
wonderful things visible to us in another world than ours, things the
knowledge of which has cost the lifelong labors of many gifted men, and
that will be remembered, studied, talked, and written about centuries
after we are dead. Fortunately for your powers of attention the eastern
half of the moon, upon which day will be seen gradually dawning in the
next set of photographs, has a general character quite different from
that of the western half. It contains the greatest ranges of lunar
mountains, yet upon the whole it is more level, being covered to a great
extent with broad plains, in the midst and along the borders of which
stand the most remarkable and interesting of all the lunar formations.
In and around some of them we shall search for the evidences which some
astronomers think that they have found of life upon the moon.”
“Oh, that indeed will be interesting!” exclaimed my friend with reviving
animation.
“But,” I added, “do not place your expectations too high. Keep your
imagination under control, try always to be just a little ‘scientific’
in your way of looking at things, and then I believe you will not be
disappointed.”
“Oh, please do not think that I have been disappointed,” she said
deprecatingly. “But positively you must admit that ‘Albategnius,’
‘Aliacensis,’ ‘Blanchinus,’ and ‘Maurolycus,’ are not precisely
captivating. Remember that I have read little except poetry and romance,
and those histories that are full of stories.”
“You will find a deep vein of poetry and romance in the moon,” I
replied, “before we have finished, and after you have reflected upon
what we have seen and what we have been saying.”
Leaving the remaining photographs to be examined after lunch, we now
entered the house.
------------------------------------------------------------------------
II
FIRST QUARTER TO FULL MOON
------------------------------------------------------------------------
II
FIRST QUARTER TO FULL MOON
NOTWITHSTANDING the signs of impatience which my friend had manifested
when we were passing, in our review of the photographs, from one lunar
ring mountain to another, all more or less similar in appearance and
characteristics, I was gratified to see that her mind was still
attracted to the subject of the moon, and during the lunch she, of her
own accord, began to talk of it.
“You have said so much about volcanic occurrences on the moon,” she
remarked, “that I wonder why you do not call those immense mountains
‘volcanoes.’ I observe that you always speak of them as ‘rings,’ or
‘mountain rings,’ or ‘ring plains’; while to me, although to be sure I
am no geologist and have perhaps no right to an opinion, they seem
plainly to be just huge volcanoes and nothing else.”
“Your observation is quite correct,” I replied, “as far as superficial
appearance goes, and I may add that these great rings are often called
volcanoes. If we apply the proper adjective and name them ‘lunar
volcanoes,’ perhaps there can be no objection to the term. But they are
certainly widely different from our terrestrial volcanoes. The
difference is not in size alone, although in that regard it is enormous.
There is a far more significant difference, which you could hardly be
expected to notice in a simple inspection of the photographs, although
it is evident when once pointed out. I refer to the fact that what seem
to be the craters of lunar volcanoes are not situated on the tops of
mountains. They are immense plains, more or less irregular in surface,
and often having a peak or a group of peaks in the center, while around
these plains always extends a mountain ring, steep on the inner side,
and having a gradual slope without. But most significant fact of all,
the plains, or floors inside the ring, are almost invariably situated
thousands of feet below the general level of the moon. If the
terrestrial volcanoes were formed on the plan of the lunar ones, when we
visit Vesuvius, instead of climbing up a mountain rising out of the
midst of a plain and capped with a cone, having a funnel-shaped crater
in the center, we should find before us a relatively low, circular
elevation, on surmounting which there would appear on the inside of the
circle a great basinlike hollow, far below the level of the surrounding
country. In the center of this, distant from the lofty encircling walls,
would be seen a conical hill with smoke and vapor issuing from a vent at
its summit. The top of this crater hill would be lower than the rim of
the basin-shaped hollow, so that the whole volcano with its immediate
surroundings would be inclosed and shut off from the environing upper
world by the sides of the basin. While you finish your coffee I will
make a sketch which may render this difference between lunar and
terrestrial volcanoes evident at a glance.”
[Illustration:
Lunar Volcano, in Section.
]
[Illustration:
Terrestrial Volcano, in Section.
]
Accordingly, after a few minutes, I presented to her these two diagrams,
remarking that it should be borne in mind that the two sketches were not
made on the same relative scale. “I was compelled,” I said, “to change
the true proportions in the section of the lunar volcano, for if I had
drawn them as they are in fact, the width of the basin would have been
enormous in proportion to its depth. You will recall that I told you
that such rings as Albategnius and Maurolycus are a hundred miles and
even more in diameter, while their depth does not exceed two or three
miles. It results from this necessary falsification of proportions in
the sketch that the terrestrial volcano, although so widely different in
form, appears comparable in magnitude with the lunar one. But the fact
is that you could take a dozen of the largest volcanic mountains on the
earth and throw them into one of the great lunar rings without filling
it.”
“I am the more astonished by what you say,” remarked my friend, “because
you have already told me that the moon is so much smaller than the
earth. How does it happen, then, that her volcanoes are so much larger?
I should think that in a little world all things would be small in
proportion.”
“It is quite natural to think so,” I replied, “until you reflect upon
the consequences of the smaller force of gravitation on a small world. I
told you last evening that gravitation on the moon, is only one sixth as
powerful as it is on the earth, and you will recall that one consequence
which I pointed out was that you would weigh only twenty pounds if you
were on the moon. Since the same reasoning applies to all objects in the
lunar world, it is clear that a similar force exerted there would be
able to produce enormously greater effects, as for instance in the
formation of vast hollows or depressions, by violent explosions, the
products of which would be thrown to immense distances. Some
selenographers, which is a term applied to those who study the features
of the lunar world, have suggested that in this cause alone is to be
found the explanation of the giant lunar ring mountains. At some remote
period of the past, according to them, the volcanic forces of the moon
reached a maximum of activity and energy. The lava, cinders, ashes, and
other products of ejection, were hurled to a height of scores of miles,
and when this fell back at a great distance from the centers of eruption
these were piled up in huge rings, fifty, eighty, or a hundred miles in
diameter, while the surface of the moon within the rings sank in
consequence of the withdrawal of the material thus ejected. To account
for the existence of the central mountains so often found in the middle
of the rings, it has been suggested that at a much later period, when
the volcanic energy had become comparatively insignificant, as a result
of the cooling of the interior of the moon, less violent explosions, not
greater than many that have occurred on the earth, took place, and by
these the central peaks were formed.”
“You are going to think me too romantic, or too imaginative, again,”
said my friend, with a smile, “but I cannot prevent myself from
wondering what the inhabitants of the moon did and thought while all
those marvelous things were happening.”
“I have not said that there were inhabitants of the moon.”
“No, but you have confessed that there might have been inhabitants, some
time, and I should like to know whether they were there when those
terrible volcanoes were formed.”
“If they were,” I replied, “they could not have survived such a
universal upheaval as the surface of the moon has undergone. You have
seen in the photographs that the great rings and smaller craters are
scattered thickly over the moon. It is true that comparatively few are
found in the level expanses called ‘seas,’ but if those regions were
covered with water they could only have been inhabited by beings
provided with gills and fins.”
“How long ago did these explosions occur?”
“I cannot tell you, except that it must have been many ages in the past;
so long ago, indeed, that the whole course of human history seems but a
day in comparison.”
“Then,” said my friend with animation, “there has been time enough
_since_ that dreadful period for inhabitants to develop upon the moon,
has there not?”
“Yes, time enough, perhaps, provided that sufficient water and air and
other vital requisites remained after the exhaustion of the volcanic
energies.”
“Oh, let us say that they did remain. I am eager to believe that the
moon has not always been so desolate as she appears at present.”
“Very well, you are at liberty to believe that if you like. No
astronomer is likely positively to contradict you, although he may smile
a little incredulously. Besides, as I have already told you, there are
certain rather inconclusive indications of some kind of life, and of
some kind of activity, still on the moon.”
“Please show them to me, then, or tell me about them. Perhaps I shall
find them less inconclusive than you do.”
“Everything in its turn,” I replied. “We shall come to the indications
that I have spoken of after we resume the inspection of the
photographs.”
“Then I am ready to resume at once.”
Accordingly we returned to the table and the photographs under the
pleasant shade of the elm. Taking up the photograph numbered 7, I
remarked that it exhibited the moon as it appears a little after First
Quarter; that is to say, a trifle more than half the face turned toward
the earth is in the sunlight. I called attention once more to the six
“seas,” which we had already remarked, and to the continued
conspicuousness of Theophilus and its companions, a little above the
middle of the visible hemisphere.
“You observe now,” I continued, “how the rotundity of the lunar globe
begins to manifest itself as the sunlight sweeps farther eastward. The
crescent shape is gone and the line between day and night begins to be
bowed outward, convexly. The _Mare Crisium_ is particularly well
defined, and also the diamond-shaped region called the _Palus Somnii_.
With the sun so nearly vertical above it, the remarkable peak of
Proclus, between the _Palus Somnii_ and the _Mare Crisium_, has become
very brilliant. In a telescope you would see it glowing almost like a
star. You observe also that several long, straight, bright rays proceed
from it in several directions.”
“All the more reason, it seems to me,” said my friend, “why your
unimaginative astronomer, Riccioli, should have named it for some
brilliant gem instead of attaching to so dazzling an object the prosaic
designation of ‘Proclus.’”
“After all,” I replied, “what’s in a name?” Now that you are familiar
with the appearance of Proclus, its name will henceforth call up to your
mind an image as brilliant as if it had been named ‘Mount Diamond’ or
‘Mount Amethyst.’”
[Illustration:
NO. 7. JULY 2, 1903; MOON’S AGE 7.24 DAYS.
]
“Pardon me,” said my friend, “but it was not of names like those that I
was thinking. Observe how he who named the neighboring _Palus Somnii_,
‘Marsh of a Dream,’ exhibited an exquisite delicacy of fancy. It
suggests something indefinitely strange, romantic, imaginative. That
unknown astronomer, unknown at least to me, put a little of himself, a
little of his inmost mind, into the name, and I thank him for it. I
shall never forget the ‘Marsh of a Dream’ in the moon. It will haunt my
own dreams. I shall be all my life seeking and never finding its
meaning.”
“Since you are in so poetic a mood,” I responded, “I rejoice that
besides its bald facts, its fireless volcanoes, and its dried-up plains,
the moon possesses many things that can stir the imagination of the most
sentimental observer. But, in order that we may not wander too far from
the paths of science, let me recall your attention to the photograph. We
have been going over ground already trodden by returning to the
neighborhood of the _Mare Crisium_. I shall now lead you back to the
terminator, where we shall find a little that is new. Still nearly
hidden in night we perceive many great rings on which the sun is
beginning to rise, and four of the most important ranges of mountains
are coming into view. One of these, on the southern border of the _Mare
Serenitatis_, is visible throughout its entire extent. It forms a
portion of the coquettish ornaments with which the Moon Maiden has
decorated her hair, as we shall see clearly in the next photograph. This
range is named the Hæmus mountains. Near its center, quite at the edge
of the ‘sea,’ is a bright crater ring, one of the most conspicuous on
the moon. It is called Menelaus.”
“Menelaus?” exclaimed my friend. “Ah, then Riccioli did not confine his
favoritism to the astronomers and philosophers in putting their names in
the moon. Menelaus, if I remember my classical reading correctly, was
the husband of Helen of Troy.”
“Yes, the brother of Agamemnon himself. You must admit that Riccioli
occasionally felt his imagination a little awakened. He was not
altogether destitute of the spirit of poetry.”
“But did he also put Helen in the moon?”
“I am sorry to say that he did not. It would have been a very suitable
abode for her. However, if you like, you may recognize Helen in the Moon
Maiden herself.”
“Thank you, that will be, indeed, an unexpected pleasure.”
“Meanwhile allow me to point out to you that there is a curious light
streak, very faintly shown in the photograph, which crosses the _Mare
Serenitatis_ from Menelaus to the opposite shore, and reappears more
distinctly, on the lighter-colored plain toward the north. This streak
comes all the way from a great ring mountain named Tycho in the southern
part of the moon. It is more than 2,000 miles long, and is one of the
greatest mysteries of the lunar world. Tycho, which lies just on the
sunrise line, is not well seen in this photograph. It has a great number
of these strange streaks or rays proceeding from it in all directions.
We shall study them in one of the photographs which are to come. One
word in regard to the plain north of the _Mare Serenitatis_ of which I
have just spoken. It, too, has a name that is calculated to appeal to
your lively imagination. It is called the _Lacus Somniorum_, which if my
knowledge of Latin is correct, means ‘Lake of the Sleepers.’”
“Then your old friend Riccioli certainly did not bestow the
appellation.”
“No, it was one of his more fanciful, or, if you prefer, more poetical
predecessors, perhaps the same who imagined the ‘Marsh of a Dream.’”
“Oh, that gives me another reason to think of him with admiration and
gratitude. He, at least, had a soul that rose above mere prosaic facts.”
“Perhaps. But do not think too lightly of the facts of the moon. After
all the human mind must base itself upon the solid ground of fact.
Without that we should become mere dreamers, and be suited only to
inhabit your favorite ‘Marsh.’”
“The other mountain ranges of which I have spoken,” I continued, “are
faintly distinguishable eastward from the _Mare Serenitatis_. They are
the Apennines, the Caucasus, and the Alps. But perhaps we had better
turn at once to photograph No. 8 where they are much more clearly seen,
because the sunrise there has advanced a couple of hundred miles farther
east.”
“But, dear me, how slowly the sun rises on the moon! Was this photograph
taken a day later than the other?”
[Illustration:
NO. 8. AUGUST 31, 1903; MOON’S AGE 9.22 DAYS.
]
“Almost exactly two days later. When it was made the moon was nearly
nine and a quarter days old, and its age at the time No. 7 was made was
only seven and a quarter days. But, owing to the effects of libration,
an explanation of which I have put into a note for your private reading
when you feel like it, [see p. 57, footnote], the difference of phase
amounts to less than two days. You are right, however, in remarking that
sunrise is a very slow process on the moon. It requires about two weeks
to pass from the western side of the moon to the eastern side, and both
day and night at any point on the moon last about a fortnight. This
results from the fact that, as I have told you, the moon does not turn
rapidly on its axis like our own globe, but keeps always the same side
directed toward the earth. Accordingly, a lunar day and night are
together about a month long.”
“And was it so when, as I must persist in believing, there were
inhabitants on the moon?”
“Probably, although it may have been shorter then. The consequences of
these excessively long days and nights would be very serious to beings
fashioned upon the terrestrial plan. In the practical absence of an
atmosphere the heat of the sun’s rays, pouring down without interruption
and without the intervention of any clouds or vapors for fourteen days
at a time, must be simply overpowering. And then, during the equally
long night that ensues, the radiation into open space must quickly leave
the surface of the moon exposed to the most frightful degree of cold,
comparable with the absolute zero of empty space!”
“But think, what a merciless environment you are picturing for my
inhabitants of the moon. Please do not forget that I insist that their
comfort shall be considered.”
“Oh, as for that, you know you were content a little while ago to
relegate your inhabitants to a remote period in the past, after the
volcanic fury of the lunar world had ceased, and before its present
airless and waterless condition had supervened. Possibly at that time
things were not so uncomfortable for them. They may have had clouds to
temper the sunshine, rains to cool the days and dews the nights, and
shady parks like yours for philosophic and scientific contemplation.”
“Do not forget the poets.”
“Certainly not. But is not the moon herself the very spirit of poetry?
What in nature is more poetical in its suggestions than the moon wading
through fleecy clouds on a serene summer’s night? But pardon me, we are
forgetting my mountains, upon which I insist as strongly as you do upon
your inhabitants. The mountains have this advantage that they are very
real, and no exercise of the imagination is required to bring them
clearly before us. In photograph No. 8 they are all visible. The
Apennines, the greatest of them, start from the eastern end of the _Mare
Serenitatis_, and run in a slightly curved line southeastward, a
distance of about 450 miles. They form the singular ornament which the
Moon Maiden (or shall we now call her Helen of Troy?) wears upon her
forehead. Turn the photograph upside down so that the moon is presented
as the naked eye sees it in the sky, and you will find that, although he
aimed only to be scientifically exact and to exclude everything but the
real facts, Mr. Wallace has produced an excellent picture of this
wonderful face in the moon.”
“But what is that face?”
“It is humanity projected upon the moon. It is a lesson on the powers of
the imagination. We perceive a certain collocation of mountains, peaks,
and plains on the disk of the moon, and our fancy sees in them a human
likeness. We should congratulate ourselves that we are able to do this.
It is a kind of proof of superiority. Many brute animals do not
recognize even their own likenesses in a mirror, much less in a picture.
But the Moon Maiden is perhaps as real as your inhabitants.”
“I am not prepared to confess that yet.”
“Very well, let us go on. The lunar Caucasus is the broader, but
shorter, range of mountains at the northeastern corner of the _Mare
Serenitatis_, and the Alps extend eastward from the Caucasus to a
conspicuous dark oval close to the terminator, which is one of the most
remarkable formations on the moon, and which, when we come to study it
in one of the larger photographs, will probably interest you deeply
because it is one of the places where recent studies have discovered
indications of what may possibly be some form of lunar life. I wish now
to direct your attention to the central and upper parts of the
photograph. Running downward from the south, a little west of the
terminator, you will perceive a double row of immense rings and ring
plains. They are not only remarkable individually, but quite as
remarkable for their juxtaposition in two long ranges. Among them, in
the westernmost row, are three or four whose names you may
remember—Maurolycus, Stöfler, Aliacensis and Werner. Still larger ones
are included in the eastern row, the largest of all being at the bottom.
It is rather a hexagon than a circle. It is 115 miles in diameter, and
the flat plain inside the bordering mountains contains about 9,000
square miles. By close inspection you will perceive a small crater
mountain near the northwestern side. This immense walled plain is named
Ptolemæus after a great astronomer of antiquity, the author of the
Ptolemæic system, which treated the earth as the center of the universe.
“Still more interesting are the things visible farther south. You cannot
fail to remark a very beautiful ring, a perfect circle, brightly
illuminated on the eastern side, and having a bright point symmetrically
placed in the exact center. It is named Tycho, after another great
astronomer, and is generally regarded as the most perfect crater ring on
the moon. It is 54 miles in diameter, and its walls are about 17,000
feet high on the inner side, more than a thousand feet higher than Mt.
Blanc, the giant of the terrestrial Alps. Its central mountain is 5,000
feet high. The most remarkable thing about Tycho is the vast system of
‘rays’ or bands which seem to shoot out from it in all directions,
traversing the surface of the moon, north, south, east, and west for
hundreds of miles, and never turning aside on account of any obstacle.
They lie straight across mountains, valleys, and plains. We have already
seen one of them, the largest of all perhaps, crossing the _Mare
Serenitatis_ and the _Lacus Somniorum_, in the northern hemisphere of
the moon. Nobody knows exactly what these rays mean or what they consist
of. We shall from this time on see them in all the photographs that we
examine, and later I shall have more to say about them, and the
speculations to which they have given rise.
“About half way between Tycho and the south pole of the moon, you will
see an enormous irregular plain, with lofty broken walls, interrupted by
a number of crater rings. Several similar rings also appear in the
interior of the plain. If Tycho is the most perfect in form of the lunar
crater rings, this great inclosure, which is named Clavius, is the
finest example of the walled valleys. It is more than 140 miles across,
and covers an area of not less than 16,000 square miles. Two of the
rings within it, which seem so small in comparison, are 25 miles across.
A smaller walled plain, yet one of really immense size, is seen half way
between Tycho and Clavius, and farther from the terminator than either
of them. This is Maginus, and it possesses the peculiarity that at full
moon it practically disappears!”
“But how can that be possible? I see nothing behind which it can be
hidden.”
“It is the sunlight that hides it. You must have noticed already that
the rings and mountains are best seen when at no great distance from the
terminator, because there the sunlight strikes across them at a low
angle, and their shadows are thrown sharply upon the adjoining slopes
and levels. Look at the western part of the moon in the photograph
before us. Many of the huge rings and walled plains that were so
striking in appearance when the sun was rising upon them are now barely
visible. Langrenus and Petavius, for instance, have become no more than
whitish blotches, and even Theophilus is no longer conspicuous. The
reason is because when the sunlight falls vertically upon any part of
the moon there are no shadows there, and without shadows there can be no
appearance of relief. Then the mightiest mountains are almost lost from
sight in the universal glare. The same thing would be apparent if you
were suspended above the earth at a great height in a balloon and
looking down upon the tops of the snowclad Rockies. Without shadows
serving to reveal their true character and to throw their outlines in
silhouette upon the adjacent plains, they would resemble only white
spots and lines on the generally darker expanse of the continent. But
Maginus is an extreme case. Owing to the relatively small elevation of
its walls, and their broken-up state, and owing also, probably, to a
similarity of color between the mountain ring and the inclosed plain,
when the light is vertical upon them, as at the time of Full Moon, they
blend together and become barely distinguishable from one another, and
from the surrounding surface of the moon.
“Take now photograph No. 9. The age of the moon here is actually less
than it was in the photograph that we last examined, yet, in consequence
of libration, which has caused the moon, in effect, to roll a little to
one side, the sunlight is farther advanced toward the east, and we see
many features of the lunar world that before had not yet emerged from
night. Clavius you will notice is much more fully illuminated. See how
distinctly the shadow of its vast western wall is cast upon the floor of
the valley within, while the opposite eastern wall with its immense
cliffs and precipices glows in full sunshine, its shadow, thrown toward
the east, blending with the darkness of night still covering that side
of the moon. Southeast of Tycho, which is beautifully shown here, two
other great walled plains have come into view. The uppermost of these is
Longomontanus and the other Wilhelm I. For a considerable distance below
these (toward the north) the surface continues broken with rings and
craters, but at length these give place to a dark, level expanse. This
is a part of the _Mare Nubium_, or ‘Sea of Clouds.’”
“Not quite so romantic a name as some of the others,” remarked my
friend, “but still I think I can be sure that Riccioli had nothing to do
with the selection. There is certainly something poetic in the idea of a
sea of clouds.”
“It is a very beautiful region when examined with a telescope,” I
continued, “and its mountainous shores contain many interesting
formations. Farther north, you will observe, near the terminator, and
apparently lying in the midst of the _Mare Nubium_, a large ring, as
perfect in form as Tycho itself. This is a very famous object, and it
bears the name of the great astronomer Copernicus, who overthrew the
Ptolemæic system and established in its place the true idea of the solar
system, namely, that the sun is its center, while the earth and the
other planets revolve as satellites around him.”
“Surely,” said my friend, “Copernicus deserved to have his name placed
in the moon, and very conspicuously, too.”
[Illustration:
NO. 9. AUGUST 2, 1903; MOON’S AGE 8.97 DAYS.
]
“It could not have been made more conspicuous,” I replied, “for the
situation of the great ring mountain called Copernicus, in the midst of
an immense level expanse, makes it one of the most marked features of
the lunar world. Copernicus is the subject of one of the larger
photographs that we are going to examine later, and I reserve a
description of its peculiarities. North of Copernicus you will observe
apparently a continuation of the _Mare Nubium_. But it is really another
‘sea’ that we are looking upon there, the _Mare Imbrium_, ‘Sea of
Rains.’ The baylike projection that runs out into the bright highlands
west of Copernicus bears the name of the _Sinus Medii_, ‘Central Gulf,’
and the one just below it is the _Sinus Æstuum_, ‘Gulf of Heats,’ which
is certainly suggestive of dog days on the moon. Observe that the _Sinus
Æstuum_ merges on the west with a dark, oval area, which is called the
_Mare Vaporum_, ‘Sea of Mists.’ It is one of the darkest districts on
the moon. If you will now turn the photograph upside down you will find
that the _Sinus Medii_ constitutes the dark eye of the Moon Maiden,
while the _Sinus Æstuum_ and the _Mare Vaporum_ form that portion of her
hair which droops upon her forehead.”
“Why not frankly call it frizzed?”
“Because I feared that you would not consider that a sufficiently poetic
term.”
“But I find poetry enough in the names ‘Gulf of Heats’ and ‘Sea of
Mists.’ My admiration for the man who could think of such appellations
continually increases.”
“Then please reverse the photograph, for we must not lose ourselves in
dreams. You will notice that the range of the lunar Apennines runs
between the _Mare Vaporum_ and the _Sinus Æstuum_ on one side, and the
_Mare Imbrium_ on the other. The entire chain of the Apennines is
beautifully shown here. They are exceedingly steep on the side facing
the _Mare Imbrium_, and gigantic peaks standing upon their long wall
cast immense shadows over the ‘sea.’ Their southwestern slopes are
comparatively gentle, rising gradually from the level of the _Mare
Vaporum_. At their upper or southern end, in the direction of
Copernicus, they suddenly terminate with a beautiful ring, which is
called Eratosthenes. This is a fine example of the disk or cup shape of
the lunar volcano. The bottom of Eratosthenes lies 8,000 feet below the
level of the surrounding _Mare_, while peaks on its wall are as much as
15,000 or 16,000 feet in height. Between the lower end of the Apennines
and the upper end of the Caucasus Mountains a strait opens a broad,
level way between the _Mare Imbrium_ and the _Mare Serenitatis_. On one
of the large photographs these two ‘seas’ and the strait connecting them
are represented in all their picturesque details, as you will see when
we come to study them. I promise you at that time a free rein to your
imagination and plenty of room for its flights. On the northern border
of the _Mare Imbrium_ and close to the terminator we see once more the
remarkable oval valley to which I referred when pointing out the lunar
Alps, and which bears the name of Plato. I call your attention to it and
also, again, to Copernicus, in order that you may compare their
appearance here with that which they present in the next photograph,
taken when the moon’s age was eleven and three-quarter days.”
[Illustration:
NO. 10. NOVEMBER 30, 1903; MOON’S AGE 11.78 DAYS.
]
We hereupon turned to photograph No. 10.
“Now,” I continued, “observe the difference that some two days’ advance
of the sunlight has produced. Plato is far within the illuminated part
of the disk, and it looks darker than before. Copernicus, on the other
hand, which appeared as a sharp ring with one border dark when it was
near the sunrise line, has now become a round, white spot, somewhat
darker in the center, with a great grayish splatter surrounding it upon
the surface of the _Mare_. In the meantime, over nearly the whole extent
of the _Mare Imbrium_ the sun has risen and two other _mares_ have made
their appearance, one of which, extending across half the width of the
eastern hemisphere, might be called the Pacific Ocean of the moon, if it
had any water. It is named the _Oceanus Procellarum_, the ‘Ocean of
Tempests,’ while at its southern extremity a very dark nearly circular
expanse, inclosed with mountains, bears the name of the _Mare Humorum_,
‘Sea of Humors.’”
“Evidently the astronomer who bestowed that name was not in a joking
mood else he would surely have called it the ‘Sea of Humor.’”
“No, apparently he was in deep earnest. But what kind of humors he was
thinking of I cannot tell. Perhaps the name occurred to him because the
_Mare Humorum_ is the darkest of all the great levels on the moon. It is
very conspicuous to the naked eye at Full Moon. You will perceive that
Tycho has now become the most prominent of all the rings on the moon. It
will maintain this distinction and continue to gain in conspicuousness
up to the time of Full Moon. Seen as we now see it, Tycho manifestly
merits the appellation sometimes bestowed upon it of the ‘metropolitan
crater of the moon.’ Notice how bright the mysterious bands radiating
from it have become. The higher the sun rises upon them the more
brilliantly they glow, almost as if they were streaks of new-fallen
snow. They spread over the whole of the southwestern quarter of the
moon, hiding rings and mountains with their brightness. One very notable
ray runs down into the _Mare Nubium_, and a fainter one parallel with it
produces the semblance of a long, walled way.
“The South Pole of the moon lies in the midst of a marvelously upheaved
and tumbled region, where one huge ring is seen breaking into another on
every hand. One of these rings, named Newton—it lies just on the upper
edge of the disk, south of Clavius—surrounds the deepest known
depression on the moon. Its bottom sinks to a depth of 24,000 feet below
the highest point on the wall. This gigantic hole is so profound that,
situated where it is, close to the pole, where the sun can never rise
very high, its depths remain forever buried in night. It is the very
ideal of a dungeon, for if you were imprisoned at the bottom you would
never see either the sun or the earth.”
“You make me shudder! Truly, after all, the moon appears to be a world
filled with dreadful things. Who would ever imagine it, seeing how
serene and beautiful she is in a calm night?”
“Yet is there not a kind of beauty even in those things, like the abyss
of Newton, which appall you only when you know the real facts about
them? There is a certain grace in their shapes and outlines, and a great
attraction for the eye in their contrasts of light and shadow. It is the
same sort of attraction which we find in such terrestrial scenes as the
Yosemite Valley viewed from Inspiration Point, or the awful depths and
chasms of the Grand Cañon of the Colorado. The presence of man and his
works is not always essential in order to fix our attention upon the
wonders of nature. Their very grandeur exalts us until we forget our
little race and its ephemeral achievements.”
“Still, I hope that you will show me something on the moon less
awe-inspiring and suited to awaken more quiet thoughts, and especially
to reassure me concerning my lunarians, as I suppose you would call
them.”
“You shall not be kept long in expectation. Turn your eyes once more to
the _Mare Imbrium_. You will observe that its northern shore consists of
a series of curves, each terminating with a promontory projecting into
the sea. When looking at it I am often reminded of an entrancing view
which I once enjoyed from the summit of Mt. Etna over the island of
Sicily. From that great elevation nearly the whole eastern and
southeastern coast of the island was visible as upon a map. The indented
shore stretched away in long, graceful curves, where the blue
Mediterranean contrasted sharply with the yellow sands, and the eye,
wandering from Catania to Syracuse, was enchanted with the beauty of
those geometric lines. But the winding coast of the _Mare Imbrium_ is
far longer than the shores of Sicily, and the mountains and cliffs
bordering it are more wonderful than any corresponding scenes on the
earth. I wish, particularly, to have you look at the easternmost of the
indentations on the northern side of the _mare_. It bears a designation
that must surely please your imagination. It is the _Sinus Iridum_,
‘Gulf or Bay of Rainbows.’”
“I recognize the work of my old friend the unknown astronomer. Verily he
had a poetic soul! And he has written his poem on the chart of the moon,
for those to read who can.”
“It is a charming landscape that the telescope reveals there,” I said,
“even though no rainbows are visible.”
“But you will not deny that they may once have spanned that bay and its
shores with their exquisite arches?”
“No, I shall not deny so pleasing a possibility. I will only say that it
lies beyond the ken, and even outside the field, of science.”
“Then I regard it as fortunate that _he_ was not too exclusive in his
devotion to science, for then he could never have seen the rainbows with
the eye of fancy, and your charts would not have been adorned with so
delightful a name.”
“Let me tell you about this bay or gulf,” I said, tapping the photograph
to recall her from her reverie. “You observe that it terminates at each
end with a promontory. That at the western end is named Laplace, and the
other Heraclides. The latter is the more picturesque. If ever you have
an opportunity to see the moon with a good telescope do not fail to look
at the promontory of Heraclides, for if you are fortunate in the choice
of the time of observation when the setting sun is throwing its shadow
over the adjoining ‘bay,’ you will find that the serrated outlines of
the promontory represent, in a very striking manner, the profile of a
woman, more sharply defined than the face of our familiar Moon Maiden,
but a mere miniature in relative size. The shores of the _Sinus Iridum_
are bordered with high cliffs, behind which rise the peaks of a mighty
mountain mass. Just back of the center of the great bowed shore of the
‘bay’ appears, in the photograph, a small, bright crater ring. This
bears the name of Bianchini. It is a lunar volcano, 18 miles in
diameter, rising out of the midst of many ranges of nearly parallel
hills and mountains, the general direction of which corresponds with
that of the shore of the ‘bay.’ If there is any place on the moon where
one is tempted to think that the scenes of a living world might once
have been witnessed it is the _Sinus Iridum_ and its neighborhood. Its
latitude is between 40° and 50° north, corresponding with the most
thickly populated zone of our own globe. The surface of the ‘bay’—once
its bottom, if we admit that it was ever filled with water—is gently
undulating, with winding ridges that suggest the action of tides and
currents in sweeping to and fro deposits of sand and gravel, and piling
them in long rows of bars and shallows. One can hardly help picturing in
the mind’s eye waves breaking on the curving beach and dashing against
the projecting rocks of the promontories; a white city seated just at
the center of the shore of the ‘bay,’ near Bianchini, like Naples at the
feet of Vesuvius; a rich vegetation covering the slopes of the mountain
valleys, and romantic sails dotting the ‘bay’ and the neighboring
‘sea.’”
“I am very glad to observe,” interrupted my friend, “that you are not
hopelessly prejudiced against my opinion that the moon has not always
been ‘dead,’ as you call it.”
“I am so far from it,” I replied, “that I am half disposed to admit that
she is not altogether dead even yet. But it is my duty to keep you as
close as possible to the known facts. We shall see the _Mare Imbrium_
and the neighborhood of the ‘Bay of Rainbows’ again. Meanwhile, suppose
we turn to the next photograph of the series, No. 11. The age of the
moon here is about thirteen days. She is fast approaching the phase of
Full Moon. The first thing to which I would direct your attention now is
the exceedingly brilliant point of light which has come into view near
the terminator, a little north of east where the _Mare Imbrium_ merges
into the _Oceanus Procellarum_. In several ways this is the most
noteworthy object on the moon. It led the famous English astronomer, Sir
William Herschel, to believe that he had seen an active volcano on our
satellite. He naïvely wrote in his notebook on a certain occasion: ‘The
volcano glows more brightly to-night!’ Yet it is no more active than the
other craters and crater rings in the lunar world. It is only
extraordinarily, almost incredibly brilliant—by far the most dazzling
point on the moon. It is a ring mountain, and is named Aristarchus. It
has a near neighbor, barely visible in this photograph, close by toward
the east named Herodotus. Herodotus is by no means remarkable for
brilliancy. The central peak and a part of the floor and the east wall
of Aristarchus consist of some material—nobody can tell what it is—which
gleams in the sunlight, I had almost said like diamonds, although that
would be an exaggeration. There are three or four other crater rings on
the moon, including Proclus, which are also very brilliant, but not one
of them can be regarded as a rival of Aristarchus. Its power of
reflection is so great that it is even visible with a telescope in the
lunar night, when the only light of any consequence that reaches it is
that sent from the earth. It was, indeed, this fact which misled
Herschel. He saw Aristarchus shining on the night side of the moon, and
naturally thought that only the fires of an active volcano could have
rendered it thus visible.”
[Illustration:
NO. 11. DECEMBER 1, 1903; MOON’S AGE 12.98 DAYS.
]
“And are you sure that he was mistaken?”
“Positively. There is no fire in Aristarchus, and has been none for
ages.”
“But why do not astronomers undertake to find out what it is that makes
Aristarchus so brilliant, then?”
“They have almost no data to go upon. You should be informed that even
the greatest telescopes, with their highest powers, are unable to bring
the moon within less than an apparent distance of say forty miles. At
such a distance it is manifestly impossible to tell of what a lunar
formation consists. We cannot analyze the moon with the spectroscope as
we can the sun and the stars, because she does not shine with her own
inherent light. We can only infer that a large part of the substance of
Aristarchus consists of something which reflects a very great proportion
of the light that falls upon it. If a mountain on the earth were
composed of a vast mass of crystals, or of bare polished metal, we might
expect it to present, when seen from the moon, some such appearance as
we notice when we look at Aristarchus.
“In this photograph the _Sinus Iridum_, having the sun higher above it,
is more brilliantly illuminated than in No. 10. Particularly you will
notice the brightness of the line of cliffs along its eastern curve,
terminating at the promontory of Heraclides.”
“That is the promontory which presents the profile of a woman’s face, if
I recall correctly what you told me.”
“Yes. Please observe also that the oval of Plato is as dark as ever,
while Copernicus has, if possible, increased in brightness, and the
great splatter of broken rays around it seems to have extended farther
over the surrounding maria. Almost directly east of Copernicus, in the
_Oceanus Procellarum_, appears a much smaller crater ring, Kepler, which
resembles a miniature of Copernicus because it, too, is encircled with a
kind of corona of short, bright rays. Copernicus, Kepler, and
Aristarchus mark the corners of a large triangle. Speaking of rays
recalls us to Tycho. You will see that, as I told you, this wonderful
formation grows in relative prominence when the period of Full Moon
approaches. Its ringed wall and central mountain are obscured by their
own brilliance, while the gigantic system of bright bands, or rays,
which have their center of origin at Tycho, is gradually becoming the
master feature of the bright part of the moon.
“I have told you that the _Mare Humorum_, which is very sharply defined
in the picture before us, is the darkest of all the level areas that go
under the name of ‘seas.’ It is not, however, the darkest _spot_ on the
moon. There are several places where the surface appears, at times, much
duskier than in any part of the _Mare Humorum_. Three or four of these
are clearly discernible in this photograph. They lie westward from
Copernicus in the _Sinus Medii_, the _Sinus Æstuum_, and the _Mare
Vaporum_. Their dusky hue strikes the eye at once. They give the
impression of sink holes. No special name is attached to them, but they
must have been evident to the first observers, with the smallest
telescopes, and it is rather surprising they should have escaped special
designation on the lunar charts. A fact which will especially interest
you is that some observers look upon these and other dusky areas on the
moon as being, possibly, indications of the existence of some kind of
vegetation there.”
“But if there is vegetation there may be other kinds of life also, may
there not?”
“Ah, I have not said positively that there _is_ vegetation, but _if_
there is then your conclusion as to other life may be correct. Glance
next at the upper part of the disk along the terminator. Two or three
broad oval rings have come into view there. The largest of these with
its long eastern wall lying exactly on the line between day and night is
an extremely interesting formation, bearing the name of Schickard. The
plain within the ring is almost large enough to have been called a ‘sea’
or at least a ‘lake.’ It is about 134 miles in diameter, and is in
reality much more nearly circular than it appears to be. Like all
similar formations situated near the ‘limb’ of the moon, by which we
mean the edge as viewed from the earth, it is greatly foreshortened by
perspective. The scale of the photograph is, unfortunately, not large
enough to reveal an unique thing in the immediate neighborhood of
Schickard, toward the southeast. I refer to what, as far as its
telescopic appearance goes, might be described as an enormous bubble—a
bubble 54 miles in diameter. Unlike the other formations the surface of
this singular ring is elevated above the general level of the moon. When
we come to examine it in detail it hardly answers, perhaps, to my
designation of a bubble, since the edges are a little higher than the
center, giving it the form of a shallow dish. If we could visit it we
should find on approaching that we were climbing the slopes of what
would seem to be a chain of low mountains, and on reaching the summit we
should see before us an elevated circular plain, sinking gradually
toward the middle. Filled with water it would form a shallow lake lying
on the top of a broad, flat mountain. There is nothing else quite like
it on the moon and certainly nothing on the earth.”
“It must have been a great curiosity in the days when the moon was
inhabited, and I suppose that scientific ‘lunarians’ organized
expeditions to explore it.”
“Perhaps, if you choose to regard it in that way. Now look again at the
_Mare Humorum_. You perceive that its eastern side is lined with
mountains and crater rings, while near the center of the northern border
there is a conspicuous ring with a bright line running from the southern
edge to the center. This is one of the most beautiful of lunar
formations, and is named Gassendi. It is a favorite object for those who
study the moon with telescopes on account of the great variety and
singularity of the details visible within the ring. When you become a
selenographer and possess your own telescope you will find few things
more interesting to study than Gassendi.
“Next let us take up photograph No. 12. Here the moon is once more a
little ‘older’ than before, and the sunrise line has again advanced a
little eastward. This advance does not appear so rapid when the
terminator is near the moon’s limb, because, on account of the rounding
away of the lunar globe, the illuminated surface is foreshortened from
our point of view on the earth. In this photograph you perceive that the
wonderful shining mountain Aristarchus has become even brighter than it
was before, or at least it is more conspicuous on account of the
appearance of what seems to be a short ray shooting out from it in a
southeasterly direction. There is also a light spot just below it which
is caused by a little mountain group called the Harbinger Mountains. The
bright ray connects Aristarchus with its neighbor Herodotus, of which I
spoke a little while ago. There is a very remarkable feature of the moon
here, not shown in the photograph, but to which I must briefly refer. It
is an enormous cleft, or crack, or, if you please, cañon, which starts
from Herodotus, whose northern wall seems to have been broken through to
give passage to it, and goes winding across the surface of the _Oceanus
Procellarum_ with several sharp turns and angles for a total distance of
nearly a hundred miles. What produced this remarkable chasm on the moon
it is difficult to say. Some have suggested that it may once have been
the bed of a river, but there are many serious objections to that view.
Nevertheless, there seems to be little doubt that if we were to visit
the moon we should find, in many ways, a striking resemblance between
this prodigious cañon and that of the Colorado River.”
“And are not all these things so ancient, as far as you can tell, that,
like the terrible volcanic rings, they might have been formed before the
appearance of inhabitants upon the moon?”
[Illustration:
NO. 12. SEPTEMBER 4, 1903; MOON’S AGE 13.27 DAYS.
]
“They certainly seem to be very ancient, and I cannot deny the
_possibility_ of what you say.”
“Very well, then, I, for my part, am convinced that curious eyes, filled
with the light of intelligence, have peered down from the verge of that
chasm into its fearful depths. If you will not permit me this flight of
imagination I shall refuse to take any further interest in the moon.”
“Oh, I should not think of refusing. Imagine what you will, and draw
your own inferences, only remembering that they are not supported by
_ascertained_ facts, and probably never will be. Yet for all that they
may have an element of truth.”
“Pardon me for saying that your astronomical science, as far as it
concerns the moon at least, does not seem to me quite satisfying. You
are not bold enough in drawing conclusions.”
“On the contrary many astronomers think that some of their brethren are
altogether too bold in that respect. However, it must be freely
confessed that astronomical science, except perhaps in its mathematics,
is not satisfying even to those who have created it. Nobody would
rejoice more sincerely than the astronomer at the discovery of evidence
of the former, or even the present, habitability of the moon. It is
surely a great disappointment that we have not been able to settle so
apparently simple a question in regard to our nearest neighbor in the
sky.”
“Then if I were a multimillionaire I should certainly devote several of
my millions to the construction of a telescope great and powerful enough
to reveal so interesting a secret.”
“With your great telescope you could probably render possible many
discoveries at present beyond our reach. But the mightiest telescope
that you could make would enable no one to _see_ inhabitants on the
moon, even if they existed.”
“Not if it magnified the moon a million times?”
“No, for optical imperfections and the disturbances to clear vision
produced by our atmosphere would absolutely prohibit the use of any such
magnification. And even supposing that one could use a magnifying power
of 1,000,000 diameters in viewing the moon, how near do you think that
would place us to the lunar surface? It would still appear to be more
than a quarter of a mile away.”
“That is not much. I am sure I can see people at that distance.”
“Oh, yes, but the distinctness of view would be nothing like so great as
if you were looking at the same objects on the earth. Still, if we could
obviate the atmospheric and other difficulties, a magnifying power of
one million would certainly enable us to discover the works of the
moon’s inhabitants—their houses, their fields, their plantations, their
great establishments of art and industry. But I assure you that a
telescope of such power is a mere dream. It could never be constructed
without some fundamentally new and unheard-of discovery in optics. We
shall do better to turn once more to our photographs which, at least,
have no deceptions. Dropping No. 12, we shall take up No. 13, which
brings us practically to the Full Moon phase. The moon’s age at the time
this photograph was made was nearly fourteen and one-half days. You see
that its whole eastward face is now lying in the sunlight. The march of
day across its surface has been completed, and on the western edge of
the moon the sun is about to set, while on the eastern edge it is just
rising. Among the new things that have come into view is a conspicuous
dark oval, shaped like Plato, but very much larger, near the eastern
edge. This is a walled plain named Grimaldi, and it enjoys the
distinction of being the darkest on the moon. Near it on the northeast
and consequently closer to the limb is another walled plain, which I
promised some time ago to point out to you because it bears the name of
the astronomer Riccioli, the great bestower of names on the moon, and
upon whose lack of imagination you have so severely commented. But, as
you have already learned, the time of Full Moon is not the best for
studying the mountains and rings, because then the light strikes too
nearly vertical upon them and they cast no shadows. But it is the best
time for seeing the broad general features of the lunar surface. Turn
the picture upside down again, thus bringing the disk into its natural
position as seen with the naked eye, and this photograph shows the moon
very much as it appears with a small pocket telescope, or with a
powerful binocular. The new prism binoculars that have come into use
within the past few years are excellent for general views of the moon.
Their defining powers are superb, and one who has never seen the moon
with such a glass is always greatly surprised and delighted with the
view which it affords. You see now that Tycho forms a blazing brooch,
resting on the Maiden’s neck, while its rays extend across her profile,
and the long one lying over the _Mare Serenitatis_ bears some
resemblance to a pin displayed in her hair, with the crater ring,
Menelaus, glittering at its lower end. The other bright point, to the
left of Menelaus (we will henceforth keep the picture reversed), is a
ring mountain named Manilius. After the detailed study which we have
given to the various ‘seas’ and formations you should be able to
recognize them with the picture in this position, and I wish that you
should do so because, as I have just remarked, this is the position of
the Full Moon as it is always seen with the naked eye or with a simple
binocular, for the latter does not reverse it, as does a telescope. The
western edge is now at the right hand, and the north at the top. All the
_mares_ are clearly visible. On the right the _Mare Crisium_, the _Mare
Fœcunditatis_, the _Mare Nectaris_ and the _Mare Tranquillitatis_; in
the center, above, the _Mare Serenitatis_; on the left the _Mare
Imbrium_, the _Mare Vaporum_, the _Mare Nubium_, the _Mare Humorum_, and
the _Oceanus Procellarum_. The two bright spots on the right, lower than
the _Mare Fœcunditatis_, are Petavius and a neighboring ring. Vendelinus
forms a less brilliant spot at the western edge of an extension of the
_Mare Fœcunditatis_, and Langrenus is distinctly seen on the western
shore of the main body of that _mare_. Proclus and the remarkable
diamond of the ‘Marsh of a Dream’ are very plain just under the large
oval of the _Mare Crisium_. The mountains and cliffs encircling the
_Mare Imbrium_ on the west, north, and east you will recognize at a
glance. The dark Plato is conspicuous in the lighter mountainous area
north of this ‘sea,’ and the semicircle of the ‘Bay of Rainbows’ is
sharply defined. Farther north is the long, dark _Mare Frigoris_, whose
eastern end merges into the broad _Oceanus Procellarum_. Aristarchus
appears as a very bright point in this ‘ocean,’ and far to the right of
Aristarchus, toward the center of the disk, Copernicus, with its
splatter of irregular rays, is conspicuous. Following the eastern limb
round toward the south we see again the dark oval of Grimaldi, beyond
which the bright mountainous region broadens as we approach the South
Pole.
[Illustration:
NO. 13. SEPTEMBER 5, 1903; MOON’S AGE 14.40 DAYS.
]
“There is just one other thing on which I should like to dwell a little
while we have the Full Moon before us. I have already referred to it
once or twice—I mean the system of bright rays or bands radiating from
Tycho. These rays, as I have told you, are among the greatest mysteries
of the moon. Their appearance is so singular and, if I may so describe
it, unnatural, that when the first photographs of the Full Moon were
published, some persons actually thought that they were being imposed
upon. They imagined that the photographer had indulged in a practical
joke, by photographing a peeled orange and dubbing it ‘the moon.’ The
mysterious rays do not start from the central mountain of Tycho, nor
even from the ring itself, but from a considerable distance outside the
ring. Nevertheless, Tycho is manifestly the center from which they
arise. It looks as though some irresistible force had been focused at
that point—a force that split the moon along a hundred radiating lines.
This is, in substance, the theory of the English selenographer Nasmyth.
He supposed that, the lunar globe being burst by internal stress, molten
lava welled up and filled the cracks. After solidifying this lava
possessed a lighter color and greater reflecting power than its
surroundings and thus gave rise to the appearance of long bands.”
“Really, your moon history seems to me to be made up of extremely
tragical chapters. But I am content as long as you put all these
terrific events sufficiently far in the past to leave time for the moon
to have enjoyed a different kind of history since they occurred.”
“But,” I said, “even if I grant what you wish, you must admit that the
greatest tragedy of all succeeded.”
“What do you mean?”
“I mean simply that your imagined lunar age of gold, when the moon was
full of animated existences and beautiful scenes, has also become a
thing of the past; and what geological cataclysm can be compared in
tragic intensity with the disappearance of a world of life?”
“But that disappearance was gradual, was it not?”
“Very likely it was, if it depended upon the slow withdrawal of the
atmosphere and water.”
“Good! Then again I am fairly well content, for all things must have an
end. The most beautiful life finally merges into old age and death. I
think I have read that some of your _savants_ predict that the earth
will not always be a living world. All that I ask is that you leave room
somewhere in your lunar history for an age of life on the moon.”
“Very well then. As I have told you several times, Science does not
positively forbid you to picture such an age if you will. She only says
that she cannot find the evidences of its existence. Still, as we are
going to see later, there are those who think that they can perceive
indications of some simple forms of life on the moon even now. I will
grant you that in the past these may have been more numerous and more
highly organized.”
By this time the afternoon had waned and the trees were lengthening
their shadows upon the lawns of the park.
“Perhaps,” I said, “we had better postpone an examination of the
remaining photographs of the series exhibiting the moon’s various phases
until after dinner. They will show very well in the light of the
electric chandelier. I have but a few words to add concerning the rays
of Tycho. The opinion of Nasmyth concerning their mode of origin has not
been universally accepted. Prof. William H. Pickering, for instance, has
suggested that the rays are formed by some whitish deposit from the
emanations blown out of comparatively minute craters lying in rows. He
supposes large quantities of gas and steam given forth from craters
surrounding the rim of Tycho, and, in consequence of these gases and
vapors being absorbed and condensed in more distant regions, a wind
constantly blowing away from Tycho and distributing the white deposit in
windrows. A similar explanation has been applied to the shorter and more
irregular systems of rays surrounding Copernicus, and a few other ring
mountains.”
“I prefer the Nasmyth hypothesis,” said my friend, as we rose and took
the path to the house. “It is, to be sure, more gigantically tragic, but
then it is simpler and more easily comprehended.”
------------------------------------------------------------------------
III
FULL MOON TO OLD MOON
------------------------------------------------------------------------
III
FULL MOON TO OLD MOON
AFTER dinner, in the brilliantly lighted drawing-room, we once more
spread out the photographs on a table.
“This time,” I said, taking up No. 14, “we are going to watch the
advance of night over the moon. Before, it was the march of sunrise that
we followed. Both begin at the same place, the western edge or limb of
the moon. Comparing this photograph, which was taken when the moon was
about fifteen and two-third days old, with No. 13, taken when the moon’s
age was more than a day less, you perceive, at a glance, wherein the
chief difference lies. In No. 13 sunrise is just reaching the eastern
limb; in No. 14 sunset has begun at the western limb. Having watched day
sweep across the lunar world, we shall now see night following on its
track. West of the _Mare Crisium_ and the _Mare Fœcunditatis_, which I
expect you to recognize on sight by this time, darkness has already
fallen, and the edge of the moon in that direction is invisible. The
long, cold night of a fortnight’s duration has begun its reign there.
The setting sun illuminates the western wall of the ring mountain
Langrenus, which you will remember was one of the first notable
formations of the kind that we saw emerging in the lunar morning. But
then it was its eastern wall that was most conspicuous in the increasing
sunlight. For the selenographer the difference of aspect presented by
the various objects of the lunar world when seen first under morning and
then under evening illumination is extremely interesting and important.
Many details not readily seen, or not visible at all, in the one case
become conspicuous in the other. But it is only close along the line
where night is advancing that notable changes are to be seen. Over the
general surface of the moon there is not yet any perceptible change,
because the sunshine still falls nearly vertical upon it. Tycho’s rays
are as conspicuous as ever. Aristarchus, away over on the eastern side,
is, if possible, brighter than before, and the three small dark ovals,
Endymion a little west of the north (or lower) point, Plato at the edge
of the _Mare Imbrium_, and Grimaldi near the bright eastern limb, are
all conspicuous.”
[Illustration:
NO. 14. AUGUST 26, 1904; MOON’S AGE 15.65 DAYS.
]
“But look!” exclaimed my friend, putting her finger upon the photograph.
“Here is something that you have not mentioned at all. I believe that I
have made a discovery, although you probably will not accept it as a
scientific one. I see here a dark woman in the moon.”
“I confess,” I replied, “that I am not acquainted with her, and do not
even see her. Please point her out to me.”
“She appears in profile, like the brilliant Moon Maiden, but is not so
much of a beauty. In fact I begin to suspect that she is the ‘Old Woman
in the Moon,’ that I have often heard of.”
“Positively I do not see her.”
“Then I will try to recall some of the names that you have been telling
me in order to indicate where you should. She faces west and occupies
most of the eastern half of the disk. Her head is under Tycho, toward
the northeast, I suppose you would say. The bright double ray that you
pointed out in one of the preceding pictures lies across the top of her
head and over her ear. Her face seems to be formed by a part of the
_Mare Nubium_—you observe how well I have learned your selenographical
terms—and her hooked nose is composed of a kind of bay, projecting into
the bright part below Tycho. Her front hair is banged, and the _Mare
Humorum_ constitutes her chignon. She has a short neck, and a humped
back, consisting of the _Oceanus Procellarum_. Copernicus resembles a
starry badge that she wears on her breast, and Aristarchus glitters on
the inner side of the elbow of her long arm. The _Mare Imbrium_ seems to
be a sort of round, bulky object that she carries on her knee, and she
appears to be gazing with intentness in the direction of the _Mare
Tranquillitatis_.”
“Ah, yes,” I said, laughing, “I see her plainly enough now. I really
cannot say that your discovery is likely to be recorded in astronomical
annals, but nevertheless I congratulate you upon having made it, if only
for the reason that henceforth you can never forget the names and
locations of the lunar ‘seas’ and other objects that you have been
compelled to remember in pointing out your ‘dark woman.’ In truth, her
features are almost as well marked as those of the Moon Maiden, but you
will hardly be able to find her again, except in a photograph, or with
the aid of a telescope, because you must recollect that this picture
shows the moon reversed top for bottom as compared with her appearance
to the naked eye, or with an opera glass. But please look again at the
objects along the western edge, for we are about to turn our attention
to photograph No. 15 in which this will be no longer visible. You must
say ‘good-by,’ or rather ‘good night,’ to the _Mare Crisium_ and the
_Mare Fœcunditatis_; for you will see them no more, until another lunar
day has dawned.”
We next picked up photograph No. 15.
[Illustration:
NO. 15. AUGUST 28, 1904; MOON’S AGE 17.41 DAYS.
]
“Here the age of the moon has increased to nearly seventeen and a half
days. The sunset line has advanced to the borders of the _Mare Nectaris_
and the _Mare Tranquillitatis_. Toward the south a vast region which was
very brilliant in the morning and midday light with the reflections from
mountain slopes and the rays of Tycho, has passed under the curtain of
night. The great crater rings on the eastern border of the _Mare
Nectaris_, and thence upward to the South Pole, are beginning to
reappear, but with the shadows of their walls thrown in a direction
opposite to that which they assumed before. By a little close inspection
you will recognize Theophilus and its neighbors which were so
conspicuous for many days while the sunrise was advancing, but which
have been almost concealed in the universal glare of the perpendicular
sunshine since the Full Moon phase was approached. On the _Mare
Tranquillitatis_ and the _Mare Serenitatis_ it is late afternoon, and
your favorite ‘Marsh of a Dream’ has become a true dreamland.”
“This oncoming of night,” said my friend, “seems to me more imposing,
and more suggestive of mystery than was the advance of day.”
“Surely it is. Do we not experience similar sensations when night
silently creeps over the earth? But it imparts a feeling of loneliness
and desolation when we watch it swallowing up the barren mountains and
plains of the lunar world that we do not experience in terrestrial life.
There are no cheerful interiors on the moon to which one can retreat
when darkness hides the landscapes. There is another thing about the
lunar night to which I have made but scant reference thus far. I mean
it’s more than Arctic chill. Imagine yourself standing there in the
midst of the broad plain of the _Mare Tranquillitatis_. Toward the east
you would see the sun close to the horizon, yet blazing bright and hot,
without clouds or mists to temper its rays. The rocks or soil beneath
your feet would perhaps be cold to the touch, because the surface of the
moon radiates away the heat very quickly, but your face and hands would
be almost scorched by the intense solar beams. Looking toward the west
you would see the shining tips of mountains suddenly extinguished, one
after another, and when the sharply defined edge of the advancing night
passed over you it would be as if you had plunged into a cold bath. In a
little while, if you remained motionless, you would be frozen. No
clothing would suffice to keep you warm. Nothing that polar explorers
have ever experienced can be likened to the cold of the lunar night.
Only the apparatus of the laboratories for producing temperatures,
capable, when combined with pressure, of liquifying and solidifying the
air itself, can bring about upon the earth a lowering of temperature
comparable with that which occurs during the lunar night.”
“But I do not exactly see why night should be so much colder on the moon
than on the earth. She is not farther from the sun.”
“No, her average distance from the sun is the same as that of the earth.
The reason why her nights are so cold is to be found in the absence of
an atmosphere like ours. The air is the earth’s blanket, which serves a
double purpose, tempering the heat by day with its vapors and winds, and
keeping the earth warm at night by preventing the rapid radiation into
space of the heat accumulated during the daylight hours. If there is any
atmosphere at all upon the moon—and I shall tell you by and by what has
been learned on that subject—it is so rare as compared with ours that it
can exercise very little effect upon the temperature of the lunar
surface.
“Now, look at the great range of the lunar Apennines. You will see that
the eastern faces of these mountains are in the sunlight, and they cast
no shadows, as they did in the lunar morning, over the _Mare Imbrium_.
The same is true of the lunar Caucasus, and the lunar Alps. All of these
mountains are very steep on the side facing the plains, and that is the
side presented sunward in the lunar afternoon. By turning to photograph
No. 16, we shall see this phenomenon more clearly displayed. This
photograph, measured by the age of the moon when it was taken, is more
than a day older than the other, but once again the effect of libration
has, in part, counteracted for us the advance of the line of sunset.
Still it has distinctly advanced. You will observe that it has now
passed completely across the _Mare Nectaris_, and more than half across
the _Mare Tranquillitatis_, while only the mountain tops along the
western edge of the _Mare Serenitatis_ remain to indicate its outlines
in that direction. Theophilus, Cyrillus, and Catharina, on the eastern
border of the _Mare Nectaris_, have again become very conspicuous, but
this time in evening instead of morning light. See how sharply the
western wall of Theophilus stands out against the darkness of night
behind it, and how its central peak glows in the setting sun while all
the vast hollow beneath it is black. The floors of Cyrillus and
Catharina, being less profoundly sunken, are still illuminated. Below
the _Mare Serenitatis_, the twin rings, Aristoteles and Eudoxus, are
very conspicuous, and they show the same change of illumination as
Theophilus, their western sides being strongly illuminated on their
inner faces, while the eastern walls cast shadows into the interior. The
mountainous character of the surface in the neighborhood of the North
Pole of the moon seems to be more clearly brought out in evening than in
morning light. In this picture the North Polar Region seems to be almost
as much broken up with gigantic rings as is that surrounding the South
Pole. In both cases, you observe, many of the rings are poised just on
the edge of the lunar disk, and their libration alternately swings them
in or out of view.”
[Illustration:
NO. 16. AUGUST 29, 1904; MOON’S AGE 18.62 DAYS.
]
“Then the other side of the moon may not be very different from the side
that is turned toward us.”
“In its general features I doubt if it is at all different. There was
once a theory, which had considerable vogue, that the side of the moon
turned away from the earth presented a great contrast with its earthward
side. A German mathematician, Hansen, drew conclusions, which are no
longer accepted, as to the form of the moon. He thought that the moon
was elongated in the direction of the earth, somewhat like an egg, her
center of figure being about thirty miles nearer to us than her center
of gravity. This, if true, would make the part of the lunar surface that
we see lie at a great elevation as compared with the other part, and the
center of gravity being toward the other side would cause the atmosphere
and water to gravitate in that direction.”
“What a pity that so interesting a theory should have been abandoned!”
“If interest were the only test of the value of a scientific theory
knowledge would not advance very fast. Notice how this very photograph
before us vindicates the true scientific attitude toward nature. It
records all the facts within its range, and leaves the theories to us.
The features of your ‘dark woman’ are, in their way, as clearly marked
in the photograph as is the range of the lunar Apennines. It is for us
to recognize the essential difference between the interpretations which
we choose to put upon these two phenomena. Giving play to fancy, we see
the figure of an old woman in the one case, and employing our reason we
find a chain of unmistakable mountains in the other.”
“But surely you do not mean to aver that science has no other business
than that of recording facts.”
“By no means. It is also the business of science to find hypotheses and
to build up theories that will explain its facts and connect them
together systematically, according to some underlying law. But as I have
just intimated it is the mark of true science that it never retains a
theory merely because it is interesting. The truth is the only
touchstone. Still, even the most conscientious scientific investigator
may be misled by his imagination. His greatest virtue is that he never
lets his fancies deceive him after he has recognized their false
character. Point out your ‘dark woman’ to the child, or the savage, and
it will be in vain afterward to explain that her profile is made up of
plains and mountains. The child and the savage are not scientific but
imaginative, and only after a long education will they abandon the
apparent for the real.
“I will ask you now to take up photograph No. 17. The age of the moon
here is twenty days. Comparing it with the last photograph we see that
Theophilus has disappeared, although Cyrillus and Catharina, being a
little farther east, are yet visible. Half of the _Mare Serenitatis_ is
buried in night, and only a little of the eastern edge of the _Mare
Tranquillitatis_ remains visible. Aristoteles and Eudoxus are now very
close to the terminator, and the shadows of their eastern walls are
spreading farther over their floors. Aristarchus is very brilliant, as
it is still early afternoon on that part of the moon, and the sunshine
is intense. Observe that Kepler, the crater ring directly east of
Copernicus, has become more conspicuous than we have seen it in any
preceding photograph. This is especially true of the system of bright
rays surrounding it, and it is due to the change of illumination. In the
southern part of the moon, west of Tycho, you will now recognize many
gigantic formations which we first saw when the sun was rising over
them. Some of them are even more prominent in the sunset light. Among
these is our old acquaintance Maurolycus, whose western wall is so
brilliant that it resembles a tiny crescent moon. The double row of
broad, dish-shaped walled plains along the central meridian has also
become visible once more. In fact the amount of delicate detail and the
sharpness of the definition in these photographs are very remarkable.
Observe the curious mottling of the ‘seas.’ It is in some of the
differences of tint, which correspond in telescopic views of the moon
more or less closely with the varying shades in the photographs, that
some selenographers have thought they could detect evidences of the
presence of vegetation on the moon. We shall talk about that more in
detail another time. It is sufficient just now to notice that the beds
of the _mares_ are by no means uniform either in tint or in level. All
of them are more or less ‘rolling,’ like many of our prairies, and often
winding chains of hills and huge cracklike ravines are visible in them.
In this photograph the amount of detail shown in the _Mare Imbrium_ is
particularly striking. Notice how some of the crinkled rays from
Copernicus extend almost to the center of the ‘sea,’ and how in front of
the precipitous base of the Apennine range the lighter-colored ground,
with three prominent ring plains in it, presents the appearance of
shallows. Lying off the shore south of Plato and the Alps a number of
isolated mountain peaks are seen, mere white specks on the gray
background. The undulating character of the ‘bottom’ of the ‘Bay of
Rainbows’ is also distinctly indicated. By the way, I should perhaps
mention the names of the three rings lying off the front of the
Apennines, for although they are among the most interesting on the moon
they have hitherto escaped our special attention. The largest of the
three is Archimedes, the second in size is Aristillus, and the smallest
is Autolycus. You will hear of them again when we come to the large
photograph of the _Mare Imbrium_ and the _Mare Serenitatis_.
[Illustration:
NO. 17. OCTOBER 10, 1903; MOON’S AGE 20.06 DAYS.
]
“Let me now prepare you for an almost dramatic change in the appearance
of some of the most conspicuous lunar features which will take place
when we pass from this photograph to No. 18. Direct your attention
particularly to the chain of the Apennines. In No. 17 it lies very
brilliant in the sunlight, with its western slopes distinctly visible,
rising gradually from the shores of the _Mare Serenitatis_ and the _Mare
Vaporum_, while the ‘sea’ along its eastern front is bright with day. In
No. 18 the Apennines have become simply a chain of illuminated mountain
tips with comparative darkness all around them. Their western slopes are
practically invisible, the _Mare Imbrium_ on the east has turned dark,
as if twilight had fallen over it—although as I have told you there is
no twilight on the moon—and at its northern end the great range, with
only its summits illuminated, projects like a row of electric lights far
into the black night that has covered the plains beneath.
“Yet, although the _Mare Imbrium_ has turned so dark as to be barely
visible over its western half, the sun has by no means set upon it, and
the darkness is perhaps greater than it should, theoretically, be under
the circumstances. This phenomenon of the rapid darkening of the great
lunar levels as the sun declines is one of the arguments that have been
found to favor the hypothesis of the existence of vegetation. If, for
the sake of discussion, we admit the possibility of vegetation growing
on the lunar plains, it will be interesting once more to compare
photographs Nos. 17 and 18.”
“Don’t say that it is merely for the sake of a discussion,” interrupted
my friend. “I shall be far more deeply interested if you will simply say
that it may be true.”
[Illustration:
NO. 18. SEPTEMBER 29, 1904; MOON’S AGE 20.50 DAYS.
]
“Very well, let us put it that way, then. As I was remarking, if we
again compare the two photographs, keeping the vegetation hypothesis in
view, we may ascribe at least a part of the rapid darkening of the plain
of the _Mare Imbrium_ to a change in the color of the—what shall I say,
grass?—covering it.”
“Good! good!” exclaimed my friend, clapping her hands. “Just listen to
him! After gravely rebuking me so many times for my unscientific faith
in the lunar inhabitants of a long past age, now you are talking of
‘grass’ on the moon.”
“You are hardly fair,” I protested. “It is you who have just led me to
make an admission which many astronomers would laugh at, and you ought
to support me with all the brilliance of your imagination when I try to
picture a state of things so consistent with your predilections about
the moon.”
“Oh, I do support you with all my heart!” she replied. “Pray go on, and
tell me about the lunar grass.”
“Not just at present,” I said. “We are going to take that subject up
again, and I may then succeed in convincing you that there is far more
evidence for believing that vegetation exists on the moon in the present
day than for believing that intellectual beings inhabited it at some
unknown former period. I should warn you, too, that I have been using
the contrasts of light and darkness between these two successive
photographs simply as an illustration of what occurs in visual
telescopic views; but that, for some reason, the lunar plains nearly
always appear darker in photographs when contrasted with the mountainous
regions than they do when viewed with the eye. Owing, also, to a variety
of influences two successive photographs of the moon may differ in tone
when the eye would detect no corresponding difference. All this,
however, does not invalidate what I have said about the lunar ‘seas,’ or
plains, darkening near sunset more rapidly than we should expect them to
do, as a simple result of the low angle at which the sunlight strikes
them.
“You will notice that the waning of day between photographs Nos. 17 and
18 has produced a remarkable change in the appearance of Tycho. Since
the Full Moon phase Tycho has resembled a button rather than a volcanic
crater, but now it has once more assumed the form of a very beautiful
ring with its central peak clearly shown, its western wall, bright and
its eastern wall casting a broad, black shadow. Most of the rays have
now disappeared, only two or three, running over the eastern hemisphere,
remaining visible. The immense walled plains near Tycho have again
become prominent, Maginus toward the southwest, Clavius toward the
south, and Longomontanus toward the southeast being the most
conspicuous. Clavius is always a wonderful object for the telescope, but
it is rather more interesting in the lunar morning than in the evening.
Away over near the eastern limb, where the sun is still high, Grimaldi
shows its dark oval, with a couple of mountain peaks on its western
rampart shining brilliantly. The small, dark spot below it, toward the
east, is in the walled plain, Riccioli. The bright spot with starlike
rays, a long way south of Grimaldi, and east of the _Mare Humorum_, is
Byrgius, a walled plain near which exists a small system of bright
streaks resembling those surrounding Copernicus and Kepler, but much
less extensive.”
“Do you recall my expression of impatience this morning when you were
giving me the names of a long string of crater rings?” said my friend,
smiling. “Well, I am now going to make a confession. Perhaps it is
slightly of a penitential nature. I find now that these names, although
they certainly are far from picturesque in most cases, begin to interest
me, because, I suppose, I understand better the character and meaning of
the things that they represent. The ceaseless Latin terminations no
longer annoy me, for I do not think of them, but of the things
themselves.”
“It is always so,” I replied, “whenever one takes up a new study. I know
that you have dipped a little into botany, and I am sure that the Latin
names which abound in that science must have repelled you at first. But
after a time, when you had begun to recognize the beautiful flowers and
the remarkable plants for which they stood, you found that even these
names assumed a new character and became interesting and memorable. You
will find it the same if you continue to study the moon. The most stupid
designations will derive interest from their applications.”
“Yes, that is no doubt true. Still, I wish that Riccioli had possessed a
little more imagination.”
“Be thankful, then, that he did not name the lunar ‘seas’ and ‘bays.’
You must now bid good night to your ‘dark woman.’ You observe that the
_Mare Nubium_ is beginning to fall under the shadow, and that her
features are growing indistinct. If you will turn the photograph upside
down you will find that the Moon Maiden has retired. She belongs
exclusively to the western hemisphere, and it is only the eastern
hemisphere of the moon that now remains visible to us, for we are close
to the phase of Last Quarter. This is an aspect of the moon with which
you may not be very familiar. To see the moon at Last Quarter, and
particularly after she has passed that phase, we must rise near midnight
and devote the early morning hours to observation. During these later
phases, however, one may see the moon in the heavens during the daytime
all through the forenoon and a part of the afternoon. She is a very
beautiful object then, although few persons, I fear, ever take the
trouble to look at her. The lighter parts of her surface assume a
silvery tint in the daylight, and the dark plains seem suffused with a
delicate blue from the surrounding sky. Exquisite views of the moon may
then be obtained with a telescope. The glare of reflected light from the
mountains and crater rings, which dazzles the eye at night, is so
reduced that the telescopic image becomes beautiful, soft, and pleasing.
The same principle has been very successfully applied in recent years to
the study of the planet Venus. Her atmosphere is so abundant, in
contrast to what we find on the moon, that she is as blinding in a
telescope as a ball of snow glittering in full sunshine; but when seen
in the daytime, her features, indistinct at the best, may be more
clearly discerned.”
“Oh, you interest me deeply! If Venus is supplied with such an abundance
of air, I suppose she is inhabited?”
“It is not exactly orthodox among those calling themselves astronomers
to talk of inhabitants on the planets, but I do not mind telling you
privately that I think that Venus is most likely a world filled with all
kinds of animate existences. Our present business, however, is with the
moon, and I must recall your attention to the photographs. We shall next
take up No. 19. Here the crescent shape becomes again evident, but
reversed in position as compared with the crescent of the new and waxing
moon. Only two of the ‘seas’ now remain completely in view—the _Mare
Humorum_ and the _Oceanus Procellarum_.”
“That term I think you have translated as the ‘Ocean of Tempests.’ Pray,
do you know any reason why it should have been thus named?”
[Illustration:
NO. 19. AUGUST 16, 1903; MOON’S AGE 23.81 DAYS.
]
“There is not the slightest reason that I know of. You must ascribe it
to the vivid imagination of that old astronomer whom you so greatly
admire. I regret, sometimes, that he cannot be here to explain to you
the thoughts that occupied his mind. They must surely have been very
captivating, even though not very scientific. Remark that there are many
of the features of the eastern part of the moon which we can now discern
more clearly than in any of the preceding pictures. Beginning at the top
we see the vast inclosure of Longomontanus with the top of its
encircling walls illuminated, while the interior is all in deep shadow.
Its western rampart projects into the night and seems detached from the
main body of the moon. Along the terminator below Longomontanus, what
appears to be another immense walled plain presents a similar aspect.
This, however, consists of several smaller formations grouped near
together, only their loftiest points being illuminated. The steep
borders of the _Mare Humorum_ are finely shown. Notice how the floor of
that little ‘sea,’ which is about the size of England, as Mr. Elger has
remarked, is mottled with whitish spots, and how distinct the ring of
Gassendi appears at the northern end of the _mare_. You can even see the
comparatively small crater that crowns the northern wall of the ring.
Southeast of the _Mare Humorum_ are visible the great flat plains of
Schiller and Schickard. Notice also how all the surface of the moon in
that direction is freckled with crater pits, which resemble the
impressions made by raindrops in soft sand. But the smallest of these
pits is larger than the greatest volcanic crater on the earth.
“The _Oceanus Procellarum_ is beautifully illuminated in this picture.
In several places, particularly north of the _Mare Humorum_, parts of
_submerged_ rings are visible. These are great curiosities, and we shall
see more of them elsewhere. Some selenographers believe that they are
the remains of an earlier world in the moon, which was buried by a
tremendous upheaval and outrush of molten material from the interior.
You will remember, perhaps, that I spoke of a catastrophe of that kind
when pointing out the half-buried ring of Fracastorius at the southern
end of the _Mare Nectaris_.”
“Did that catastrophe occur after the formation of the huge lunar
volcanoes?”
“It is difficult to say just when it occurred, but the appearances
generally favor the view that it was subsequent to the great volcanic
age. It is the opinion of Mr. Elger, whom I have once or twice mentioned
as an English observer who has devoted special attention to the study of
the moon’s surface, that the _mares_, as we now see them, do not
represent the original beds of the lunar oceans. These beds, which,
according to this view, were at first deeper, have been covered up, at
least over a great part of their areas, by the outrush of molten lava.
If they were ever filled with water it was very likely prior to that
occurrence. But you must remember that all this is speculation, very
interesting, it is true, but based upon insufficient data to enable us
to be sure of our conclusions. I shall show you later that some recent
students of lunar phenomena have formed the opinion that there is a
strong argument to be drawn from geological analogies in favor of the
view that the lunar _mares_, practically in the state in which we see
them, have been true sea beds.
“Let us continue our inspection of photograph No. 19, which is one of
the most interesting of the series. Look at the crater ring Kepler, in
the midst of the _Oceanus Procellarum_. We have not before seen it in
the aspect which it now presents. Hitherto it has appeared only as a
bright point surrounded by a light patch covered with radiating streaks.
But now, with the late afternoon sunlight striking across it, its walls
are illuminated in such a manner that its very perfect ring shows very
clearly, about half of the interior lying in shadow, which serves to
give it a striking relief. If we suppose a time when the _Oceanus
Procellarum_ was a real ocean, and when Kepler was an active volcano
rising above its waters, its situation, far from all shores, would have
been not unlike that of the great volcano of Kilauea in the Hawaiian
Islands. In that case we might assume that the streaks around it
represent ancient lava flows, which spread far about over the bed of the
ocean. The same explanation would apply to the streaks and rays around
Copernicus, and half a dozen other similar ring mountains.
“You will also observe that the afternoon slant of the solar rays has
considerably changed the appearance of Aristarchus. Now for the first
time the crateriform shape of that most remarkable mountain has become
evident on account of the shadow in the interior. This shadow has almost
reached the central peak which is the brightest part of the entire
formation. You may be interested in the fact that the brilliance of the
central peak of Aristarchus is so great that it stands in an order by
itself, in what may be called the photometry of the moon’s surface. Ten
orders of relative brightness have been adopted to represent the various
reflective powers of different parts and spots of the moon. I copy them
from Mr. Elger’s list. They are as follows:
“0° = Black (example, the shadows of mountains).
“1° = Gray black (example, darkest places in the walled plains of
Grimaldi and Riccioli).
“2° = Dark gray (example, the floor of Endymion).
“3° = Medium gray (example, interior of Theophilus).
“4° = Yellowish gray (example, interior of Manilius).
“5° = Pure light gray (example, surface around Kepler).
“6° = Light whitish gray (example, walls of Macrobius).
“7° = Grayish white (example, Kepler).
“8° = Pure white (example, walls of Copernicus).
“9° = Glittering white (example, Proclus).
“10° = Dazzling white (sole example, the central peak of Aristarchus).”
“Really, I am greatly surprised by what you tell me,” said my friend. “I
would never have imagined that there were so many different neutral
tints on the moon.”
“You would be still more surprised,” I replied, “if I could present to
you a similar table of the different tints of color that have been
discovered there. But I am not aware that any scale of lunar colors has
been prepared. There are, however, various shades of brown, yellow, and
green. Most of them are found in the _mares_ and walled inclosures. Some
of them appear to be variable, and some are only to be detected under
particular illuminations.”
“Are not such colors an indication of something living there?”
“It may be so—an indication, for instance, of the existence of ‘lunar
grass,’ the mention of which so amused you a little while ago.”
“Oh, it was not the ‘grass’ that amused me, but your unexpected way of
introducing it. I _want_ to be convinced that there is grass there, and
a great many other things besides grass. But I am not yet satisfied
concerning that unique peak in Aristarchus. ‘Dazzling white’ you say is
its description in the scale of tints. That excites my curiosity
immensely. I think you have told me already that it cannot be snow, but
you have spoken of the possibility of crystals and of metal. Do you
know, I like the idea of ascribing the phenomenon to metal. It recalls
something that I read in childhood about the first discoverer of a
silver mine in Mexico. As I remember the story, an Aztec hunter, chasing
his game across a mountain, seized upon a bush to aid him, and the roots
giving way disclosed a glittering mass of silver. Why not let me imagine
that the peak of Aristarchus is composed of pure silver?”
“There is no harm in imagining that if you wish to do so. But then your
imagination, or rather your knowledge, should go a little farther and
recall the fact that silver does not remain dazzling bright when
exposed.”
“Ah, but you say there is no air, no water, no rains, no moisture on the
moon. Under such circumstances might not a metal remain bright?”
“It is possible, but I hardly think that it would. It is likely that
other corroding influences exist. A better explanation, I think, is
afforded by supposing that the reflecting surface is simply composed of
a rocky mineral, resembling in its power of reflection a mass of quartz
crystals or imbedded planes of mica. There is no absolute impossibility
involved in thinking that it may be simply white rock.”
“Why not say marble—a gigantic Carrara mountain on the moon?”
“I fear that that would involve a geological history for the lunar world
for which we have not sufficient warrant in observed facts. I prefer to
assume a volcanic origin for the phenomenon. Since you are so interested
in the mystery of Aristarchus I may add that a part of the floor and the
inner side of the ring are also extremely bright, but not quite so
bright as the central peak. That alone stands at the top of the scale.
Putting the peak at 10°, Mr. Elger finds that the other brilliant parts
of Aristarchus possess only 9½° of brightness. Yet the whole interior is
so glistening that when the sunlight falls vertically it almost
resembles the inside of a crystal cup, and details are hidden in the
glare.
“Now please look at the ‘Bay of Rainbows’ in the photograph before us.
Cape Laplace at its western end lies close to the terminator and appears
as a minute speck of light. The great bow-shaped shore is clearly
defined, the level surface within being very dark and the highlands
around it comparatively bright. The crater mountain Bianchini you will
recognize near the center of the bow. Several other similar crateriform
mountains are visible toward the north and east. In this light the
surface of the moon eastward from the North Pole appears as rough and
broken with craters and crater plains as we saw in the earlier pictures
that it is toward the west.
“Before directing our attention to photograph No. 20, let us return for
a moment to Aristarchus. When speaking of that formation a few minutes
ago I interrupted myself in order to give you the scale of tints on the
moon, which demonstrated the unique brilliance of the peak inclosed by
the ring. I intended to point out to you then the fact that in
photograph No. 19 we see, for the first time, not only the ring of
Aristarchus but its curious neighbor Herodotus. A light streak, which we
observed in an earlier picture, seems to connect the two. It is better,
however, to notice this now because in turning from No. 19 to No. 20 you
will perceive once more a change in the appearance of Aristarchus and
its neighborhood. In No. 20 Aristarchus is distinctly more conspicuous.
The night has advanced during almost exactly twenty-four hours, having
in the meantime swept across the entire length of the ‘Bay of Rainbows,’
which we now no longer see. If we had been using a telescope during that
interval we should have beheld a very interesting spectacle, for sunset
on the ‘Bay of Rainbows’ is quite as remarkable, although in a very
different way, as sunset on the Bay of Naples. The astronomer, seated
amid the lonely gloom of his observatory dome, and watching the change
of light and illumination on the surface of the moon, has many an hour
of solitary enjoyment of aspects of nature that are quite impossible on
the earth, and that frequently lure him into poetic meditations which
find no place in his notebook.”
“I am very glad to hear you say that. It enhances my opinion of the
astronomers, and convinces me that after all they are not so severely
scientific as they describe themselves.”
[Illustration:
NO. 20. AUGUST 17, 1903; MOON’S AGE 24.84 DAYS.
]
“If they were,” I replied, “or if all of them were, it would be a bad
augury for the future of their science. Do not think that in
occasionally seeking to restrain your imagination I wish to express
condemnation of what, after all, is the noblest of human faculties. But
again we are forgetting our principal business, which is with the facts.
Aristarchus, as I have said, has undergone another distinct change of
appearance from that which it showed before. The central peak is now
covered by the shadow of the eastern wall, but still the reflection from
the western wall alone is sufficient to make it the brightest spot on
the moon. Herodotus, on the other hand, has become indistinct and the
Harbinger Mountains are practically invisible, but we can detect the
existence of the enormous chasm or cañon, which I told you once issues
from the interior of Herodotus and goes winding nearly a hundred miles
over the floor of the _Oceanus Procellarum_.
“Notice, also, how clearly visible three or four relatively small
craters east of the ‘Bay of Rainbows’ have become, and how conspicuous
are several large walled plains on the northern ‘horn.’ The dark level
south of these formations and between them and the small craters has
also a name which I have not before mentioned. It is the _Sinus Roris_,
‘Gulf of Dew.’ It connects the _Mare Frigoris_ with the _Oceanus
Procellarum_. It is another legacy from your friend the imaginative
astronomer.”
“Then once more he receives my thanks for having done his best to make
the moon an ideal world. It is always painful to have one’s ideals
destroyed.”
“I hope that I have not been destroying any of yours.”
“No, but at least you have caused a change in my impressions about the
character of the moon. Henceforth there will be an element of terror as
well as of unexpected grandeur mingled with my thoughts of the ‘Queen of
Night.’”
“That element will not be diminished by what I am about to point out.
Look far over near the eastern border of the _Oceanus Procellarum_,
directly east of Aristarchus. There you will distinguish the outlines of
two or three vast submerged ring plains, which we may regard as relics
of that earlier lunar world, which preceded the outgush of lava that Mr.
Elger thinks covered the sea bottoms. Observe also the singular light
streak that runs from Kepler, now barely visible at the edge of night,
to a dark little crater, beyond which lies a bright point off the coast
of the ‘ocean.’ South of this there are other submerged ring plains, one
of which, named Letronne, has a high western wall, which forms in the
picture a sort of promontory projecting from the southern border of the
_Oceanus Procellarum_, almost directly north of Gassendi. The latter is
very clearly shown at the lower end of the _Mare Humorum_, the western
side of which is in shadow, while its whole surface has turned very
dark. On the southern horn of the crescent the ring plains, Schickard
and Schiller, are still prominent, and the northern and eastern edges of
the _Mare Humorum_ appear more ragged with mountains and crater rings
than before.”
“And have all these mountains and craters names?”
“Not all of them, but many more, perhaps, than you suppose. On the whole
visible surface of the moon about 500 objects, not including the ‘seas,’
have received names. It may surprise you to learn that the position of
the most important of these objects has been ascertained with an
accuracy which is still lacking in our determination of positions on the
earth. In other words our charts of the moon are more exact than those
of our own planet.”
“That does indeed surprise me. I should have thought that, living on the
earth, we could make very correct maps of it, while, as for the moon,
two or three hundred thousand miles away, it seems to me not so easy to
do that.”
“It is mainly because we are on the earth that we find such great
difficulty in making accurate maps of it. We cannot look at the earth as
a whole, but we have to crawl over its surface, making measurements as
we go, and afterwards translating those measurements into lines and
angles on paper. Thus we are still uncertain about the precise distance
between many important points on our globe, while for points on the moon
no corresponding uncertainty exists. The moon hangs before us in the
sky, with no clouds except those in our own atmosphere to obscure it,
and it is only necessary carefully to observe the position of particular
points, and with the proper instruments to measure their distance and
directions from one another. But even this is not a thing that can be
accomplished without much pains and much knowledge. The astronomer, no
matter what field he chooses, is necessarily a hard worker, and his
motto, above everything else, is accuracy. No one is more tempted than
he by the sublimity and the extraordinary character of the objects of
his study, to give rein to the imagination, and yet imagination is the
thing of all others from whose vagaries he must most carefully guard
himself. So you must not blame him too severely if he has not dotted the
shores of the moon with cities, and populated its plains with
industrious farmers.”
“If you will permit me to wander a little aside from our photographic
studies for a few minutes,” said my friend, “I should like to ask you
about two or three things concerning the moon which have long puzzled
me. From my earliest days, living the greater part of the time in the
country, I have heard that the moon exercises a decided influence over
the weather, and over the growth of vegetation. I have neighbors who
would never think of planting certain things except ‘in the New of the
moon’! Some will not cut timber except ‘in the Old of the Moon,’ as they
say that the sap is drawn up by the moon’s influence when she is
growing. Is there really any truth in all this?”
“Not the least. At any rate there is no scientific evidence whatever for
such statements, and no probability that they are based on facts. They
are the result of faulty observation, misled by coincidences. It is
_imaginable_ that the light of the moon might have some influence upon
vegetable growth if it were an original kind of light coming from the
moon herself. But moonlight is only reflected sunlight, and when we
examine it with the spectroscope we do not find that the rays of light
in visiting the moon and returning thence to the earth have had either
anything added to or anything taken away from them, except intensity.
The total amount of light reflected from the moon upon the earth is
estimated to be about 1/618000 of the total amount that comes to us from
the sun. Curiously enough the moon appears to reflect proportionally
more heat than light, the amount of lunar heat received by the earth
being reckoned at 1/185000 of the amount coming from the sun. The
popular idea that the moon affects the movement of sap in plants is
equally illusory.”
“But about the weather? I know people who believe that a change of the
moon from one phase to another brings about a change of weather. Is that
true?”
“Certainly it is not true. The moon is changing its apparent form all
the time. There is no sudden alteration at any phase. The popular
belief, however, has always been so firmly fixed that many
investigations have been made to ascertain whether there is, in reality,
any foundation for it. These investigations have shown that no
measurable effect of the kind exists.”
“And the Full Moon does not drive away clouds, as some assert?”
“Surely she does not. I will now tell you something that the persons who
plant and sow and cut timber according to the phases of the moon, and
who believe that she exercises a kind of magic control over the clouds,
probably have never heard of, although if they knew it they might use it
as an argument in favor of lunar influences. It is this: The alternate
approach and retreat of the moon with respect to the earth, as she
travels round her elliptical orbit, produce measurable, although slight,
disturbances of the magnetism of our planet. The distance of the moon
varies to the extent of about 30,000 miles. Now, if it could be shown
that these magnetic disturbances were reflected in the character of the
weather, then the supposed influence of the moon would be established.
But that has not been shown, and if it were shown it would still be
found that the phases of the moon had no relation to the fact, for the
moon may be at its greatest or its least distance from the earth, or at
any intermediate distance during any possible phase.”
“You will, perhaps, think me very persistent in asking foolish
questions, but there is one other on my mind that I should like to put,
now that we have gone so far. It is this: I have read, since the great
earthquakes at San Francisco and Valparaiso, and the great eruption of
Vesuvius in the same year, 1906, that the moon has an influence over
such things. Is this another unfounded popular superstition?”
“It is not a notion of _popular_ origin at all,” I replied. “It
originated rather from scientific considerations, and there may possibly
be a germ of truth in it, although it yet remains to be demonstrated,
and the evidence concerning it is confusingly contradictory. You will
recall, I trust, what has been said about the sun and the moon producing
tides in the oceans. We have also seen that before our globe had assumed
its present condition, while it was yet more or less plastic throughout
its whole mass, and before the birth of the moon, great tides were
produced in the body of the earth. The _tendency_ to the production of
such bodily tides still exists, and now that the moon has become a
near-by attendant of the earth, she acts more effectively in this regard
than does the sun. If the earth were still plastic the moon would
produce bodily tides in it. In other words the earth would be deformed
by the attraction of the moon. The question has arisen whether or not
the tendency to the production of such tides, now that the earth has
become rigid, may not disturb its crust sufficiently to induce
earthquakes and volcanic eruptions. Some students of the subject have
thought that they could detect evidence that this is the case. It has
frequently happened that such phenomena have occurred on a large scale,
at or very close to, the periods of New and of Full Moon. Those are the
times, as we saw when we were talking of the oceanic tides, when the sun
and the moon pull together. If all great eruptions and earthquakes
occurred at these conjunctions there would be little doubt of the
correctness of the theory. But, unfortunately for the clearness of our
conceptions, this is by no means the case. There have been many
earthquakes and volcanic outbursts when the sun and the moon were not
thus combining their tidal attractions. Thus the evidence is found to be
contradictory or inconsistent, and the question remains unsettled. It
is, however, a very interesting one, and the time will come, it is to be
hoped, when it will be answered decisively one way or the other.”
After this digression we returned to the study of the photographs.
“Photograph No. 20, which we have just been examining,” I said,
“represents the moon at the age of about twenty-four days and twenty
hours. The next, and the last of the series showing the moon in
progressive phases, is No. 21. Here the age of the moon is about
twenty-six days and twenty hours. It is the fast waning sickle of the
Old Moon which we behold. You perceive that it is relatively
uninteresting when compared with No. 20, because very little except the
eastern limb is illuminated. Nearly all the great circular and oval
formations and craters, and all the ‘seas,’ have passed into the lunar
night. Only the eastern verge of the _Oceanus Procellarum_ remains in
sight, dulling the brilliance of the inner curve of the sickle. The dark
walled plain above the center is Riccioli, and just below it appears
Hevel, a smaller, but yet large formation, with a low central mountain.
It is hardly worth our while to attempt to identify the other features
shown in the photograph. They include none that we have previously
studied. Yet this picture has an interest all its own because it is an
excellent representation of the moon at a time when she is so near to
the sun. Do not forget that, as I warned you when we began with the
crescent of the New Moon, in these photographs the moon appears reversed
top for bottom. Seen in the sky in the early morning this sickle would
have its rounded edge toward the left hand and directed more or less
downward, according to the position of the sun. A great deal of
confusion exists in the minds of well-educated people concerning the
position of the sickle of the New and the Old Moon. You have, of course,
heard of the classic instances in which artists have drawn the New Moon
with the concave side toward the sun! It is only necessary to remember
that a line drawn straight from the center of the convex side of the
sickle, whether it be the New Moon or the Old Moon, always extends
directly toward the place occupied by the sun.”
[Illustration:
NO. 21. AUGUST 19, 1903; MOON’S AGE 26.89 DAYS.
]
“There is,” said my friend, “an interesting old superstition
which I have often heard—I suppose it must of course be a
superstition—concerning ‘wet moons’ and ‘dry moons.’ As I recall it they
say that when the sickle of the New Moon appears nearly upright in the
sky that is a sign of dry weather, because the moon is then like an
overturned cup, but when the sickle has its ends turned upward that is a
sign of wet weather, because then the cup can hold water. I suppose that
these various positions of the moon actually occur, but I do not know
how they are brought about.”
“The supposed influence of the position of the New Moon on the weather,”
I replied, “is too gross a superstition to be worthy of any notice, but
the different attitudes of the sickle are interesting. They arise from
the changes in the position of the moon as seen from the earth with
respect to the direction of the sun, and these changes depend in turn on
the inclination of the moon’s path in the sky to the plane of the
earth’s equator as well as to the plane of the ecliptic or the earth’s
orbit. The ecliptic has an inclination of about 23½° to the plane of the
equator, and the moon’s orbit is inclined a little over 5° to the
ecliptic. The moon may, in consequence, appear more than 28° above or
below the equator. But since, as I told you in the beginning, the orbit
of the moon itself turns slowly about in space, the distance of the moon
above or below the equator is not constant. It may be only a little more
than 18°. In consequence of these changes of relative position the
situation of the horns of the crescent moon varies. But you need never
be in doubt as to what position they will occupy at any time if you will
simply remember that a straight line drawn from the point of one horn to
that of the other must always form a right angle with the direction of
the sun.
[Illustration:
Diagram Showing Why the Winter Moon Runs High.
]
“There is another very interesting fact about the position of the moon
in the sky which we should not neglect to notice. Did you ever observe
the superior brilliancy of the light of the Full Moon in winter? It is
one of the compensations that nature offers us. Since the Full Moon is
necessarily situated opposite to the point occupied by the sun, and
since the sun is far south of the equator in midwinter, it follows that
at the same season the Full Moon appears high above the equator in the
northern hemisphere. You will, perhaps, permit me to show you a diagram
intended to explain this phenomenon.
You observe that the sun being south of the equator, in the direction
indicated by the dotted line, the Full Moon is correspondingly situated
north of the equator, and must necessarily appear high in the sky at
midnight, when the sun is at its lowest declination. This is the reason
why the winter Full Moons are so brilliant, making the snow-clad hills
gleam with a splendor that sometimes dazzles the eyes of the beholders.
In the Arctic regions the long winter night, when the sun does not rise
for months, is periodically brightened by the presence of the Full Moon.
Just the opposite condition of affairs exists in summer. Then the sun
being north of the equator the Full Moon is south of it, and ‘runs low,’
appearing in high latitudes to skim along the southern horizon.”
“Thank you, and now I will ask you one more question,” said my friend.
“I have often heard of the ‘Harvest Moon’ and the ‘Hunter’s Moon.’ Will
you not kindly explain what is meant by these terms and when the
‘Harvest Moon’ can be seen? There is a poetic suggestiveness in the name
that appeals to me.”
“I will try with pleasure,” I said, “but I fear that I shall have to
trouble you with another diagram, or perhaps with two.”
“Oh, I shall not mind that at all. I have grown used to diagrams as well
as to the nomenclature of the moon.”
“Well, if my diagrams conduct your thoughts to things as interesting as
many that lie concealed behind the prosaic names on the moon I shall be
content. To begin, then, I must remind you that in her monthly journey
around the earth the moon moves from west toward east in her orbit, and
thus she gets a little over 12° farther east every twenty-four hours, as
reckoned from the position of the sun. The earth turning on its axis in
the same direction causes the moon to appear to rise in the east and set
in the west once every twenty-four hours. But in consequence of the
constant eastward motion of the moon she rises at a later hour every
night. Here is a graphic representation of what I mean:
“The earth is turning on its axis in the direction represented by the
arrows, and simultaneously the moon is moving in its orbit in the same
direction, as is shown by the large arrow. Suppose that some night the
moon is seen rising at a particular hour from the point A on the earth.
Then, the following night, when the observer has again arrived at A,
with the rotation of the earth, the moon will have advanced from M^1 to
M^2, and will not be seen rising until the point occupied by the
observer has arrived at B. This retardation of the hour of moonrise is
variable on account of changes in the position of the moon, arising from
the inclination of her orbit to the plane of the equator, and from the
inequalities of her motion, to which I have before referred. On the
average it amounts to fifty-one minutes daily. It varies also with the
distance of the observer from the equator, the variation being greater
in high latitudes. In the latitude of New York the retardation of
moonrise may be as great as an hour and a quarter, or as little as
twenty-three minutes.
[Illustration:
Diagram Showing Why Moon Rises Later Every Night.
]
“Now it is upon this variation that the phenomenon of the ‘Harvest’ and
the ‘Hunter’s Moon’ depends. If I had a celestial globe here I could
show you that at the time of the Autumn Equinox, September 22d, when the
sun crosses the equator moving southward, the apparent path of the moon
in the sky intersects the eastern horizon at a comparatively small
inclination. In other words the moon at that time instead of rising
steeply from the horizon rises on a long slope almost parallel with the
horizon. The consequence is that for several evenings in succession the
Full Moon near the time of the Autumn Equinox may be seen rising just
after sunset at almost the same hour. Look at this second diagram and
you will see why this is so.
[Illustration:
Diagram Illustrating the Harvest Moon.
]
“The little circles M show the moon at several successive positions in
her orbit, just twenty-four hours apart. You perceive that in
consequence of the slight inclination to the eastern horizon the sinking
of the latter caused by the earth’s rotation will bring the moon into
view night after night at almost the same hour. In fact, in high
northern latitudes like those of Norway and Sweden the moon’s path at
this time of the year may actually coincide with the horizon, so that
for several evenings she will rise at exactly the same hour. The name
‘Harvest Moon’ explains itself, since it always occurs at the time of
the autumn harvests and the vintage, and seems to supplement the fading
daylight for the benefit of late laborers in the fields. The ‘Harvest
Moon’ does not occur every year at precisely the same date. It is very
rare that Full Moon happens to fall just on September 22d. It usually
either precedes or follows that date. The ‘Harvest Moon’ is the Full
Moon which occurs nearest to the Autumn Equinox, either before or after.
The ‘Hunter’s Moon’ is the first Full Moon which follows the ‘Harvest
Moon.’ Like the former it rises for several successive evenings near the
same hour, but this phenomenon is less marked in the case of the
‘Hunter’s Moon,’ because it is farther from the Equinox.”
“Thank you, again,” said my friend. “I shall never henceforth look at
the moon without thinking of circles, straight lines, and arrows as well
as of ‘ring mountains’ and ‘seas.’”
“Then you are making good progress toward science,” I replied. “One last
look, now, at the photograph of the Old Moon’s sickle, and then we had
better postpone our examination of the large photographs, showing
certain particularly interesting districts on the moon, until to-morrow
morning. There is here another interesting point for artists to note.
The convex side of the sickle of the Old Moon, or the New Moon, is
always an arc of a circle, but the concave side is never circular
although it is often thus represented. The concave side, neglecting its
irregularities arising from the differences of level and of brilliancy
of the lunar surface, is elliptical in outline, that is to say, it is a
semicircle viewed obliquely.”
“Whatever its geometry may be,” replied my friend, “it is certainly very
beautiful. Good night, and I shall demand to see those large photographs
before the sun is very high to-morrow.”
------------------------------------------------------------------------
IV
GREAT SCENES ON THE MOON
------------------------------------------------------------------------
IV
GREAT SCENES ON THE MOON
MY friend did not leave me in doubt on the following morning as to the
genuineness of her interest in her new studies. The shadows of the trees
in the park were yet as long drawn out as the silhouettes of lunar peaks
at sunrise, when we resumed our place under the elm, and, at her
request, I opened once more my portfolio.
“The series of photographs that we are now about to examine,” I began,
“are on so large a scale that only a selected part of the moon is seen
in each of them. But within the restricted limits of these pictures the
amount of detail shown is truly astonishing, far more indeed than can be
found on the most elaborate lunar charts. These photographs were made by
Mr. Ritchey with the great 40-inch telescope of the Yerkes Observatory.
Many more besides those that we are going to look at were taken by him,
but I have selected, where choice was difficult, six which seemed to me
to be of special interest. We shall begin with one which covers the
larger part of the _Mare Nubium_, in the southeastern quarter of the
moon. You certainly must remember the _Mare Nubium_, for it forms the
head of the ‘dark woman’ whom you discovered in the moon last evening,
and if you will hold this photograph at arm’s length you will see that
her face is unmistakably stamped upon it.”
“I am greatly flattered,” she replied, “that you should remember my
discovery so well. I begin to feel hopeful that it may yet find a place
in the books.”
“It certainly is as deserving of such a place as many things that get
into books. You ought to find a suitable name for this woman in the
moon.”
“If I believed myself capable of rivaling the man who christened the
‘Marsh of a Dream,’ I should surely try my hand at lunar nomenclature,
but I fear that I should fall too far short of the ideal he has set up,
and so I shall leave her nameless.”
“Permit me then to continue to call her the ‘dark woman’ whenever a
reference to her may seem useful in fixing the localities that we shall
talk about in this photograph. The most striking object shown in the
picture is the great ring mountain Bullialdus which forms an
extraordinary ornament on the top of the ‘dark woman’s’ ear. This
photograph was taken when the line of sunrise ran just along the border
between the _Mare Nubium_ and the _Oceanus Procellarum_. The _Mare
Humorum_ is yet buried in night beyond the upper right-hand edge of the
picture, but some of its bordering mountains and craters have been
touched by the morning sunbeams. You will observe that a little more
than half of the interior of Bullialdus—which, by the way, I did not
mention by name when we were studying the series of phase photographs—is
yet filled with shadow, but its double-headed central peak rises clear
and bright in the sunlight. The shadow of this central mountain can be
seen projecting toward the east over the floor. The east wall, which is
distinctly terraced, lies in full sunshine, and the light streaming over
the lofty crest of the western wall touches the floor on its eastern
half. The steep outer slopes that lead up to the western rampart, and
the deep parallel ravines cut near the crest are clearly shown. The
distance across the ring from the summit of the wall on one side to that
on the other is 38 miles. The depth of the depression is 8,000 feet
below the crest of the walls, but the latter rise only 4,000 feet above
the level of the _Mare Nubium_ outside, so that Bullialdus is an
excellent example of the characteristic form of the lunar volcano, which
I tried to illustrate for you last evening. The central mountain is
3,000 feet high. East of the south point of the ring a shadow shows the
existence of a profound cleft in the wall, while a little west of south
appears a smaller crater ring very black with shadow, except on its
eastern side. If we stood on the _Mare Nubium_ and looked toward
Bullialdus and its neighbor from a distance of 25 or 30 miles they would
resemble a double, flat-topped mountain, with its serrated crests
connected by a high neck. The summit of one of the little peaks shown in
the photograph in the plain just west of Bullialdus would form an
excellent point of observation. Still farther south stands another
crater ring most of whose interior is also, at present, filled with
shadow. East of this, and a little farther south, is still a third ring
of similar aspect, from which a curious range of hills runs southward.
Returning to Bullialdus you will notice the radiating lines of hills
that surround it, and particularly a more lofty and broken range which
runs eastward.”
[Illustration:
BULLIALDUS AND THE _Mare Nubium_.
]
“Bullialdus verily frightens me!” exclaimed my friend. “What an
unearthly look it has! The longer I regard it the stronger becomes the
indescribable impression that it produces. I begin to understand now
what you meant when you promised to find a history in the moon. Truly
there never can have been such another history. I almost feel that I do
not care whether the moon ever had inhabitants or not. Its own story is
more fascinating than that of any puny race of beings, passing their
ephemeral lives upon its wonderful surface, could possibly be.”
“I am glad,” I replied, “that you have begun to enter into the spirit of
those who long and carefully study the earth’s satellite. You see now,
that it is not necessary to the astronomer to find evidences either of
former or of present life upon the moon in order to stimulate his zeal.
For him, as you have yourself intimated, the relics of its past history,
which this little world in the sky exhibits so abundantly, are of higher
interest than any story of human empire, for they have an incomparably
vaster theme. But to lighten our labor a little, let me once more refer
to the ‘dark woman,’ whose features, like the outlines of a
constellation, serve for points of reference. I began by remarking that
Bullialdus seems to be placed just over her ear. Observe now that, taken
together with its immediate surroundings, the great crater ring forms a
kind of barbaric ear-ornament of most extraordinary form and richness of
detail. The line of hills east of Bullialdus, of which I spoke a few
minutes ago, connects the ring with a tumbled mass of mountains on the
border of the _Mare Humorum_. These mountains run northward, or downward
in the picture, for a distance of perhaps 150 miles, and then turn
abruptly westward for a like distance; after which, in the form of a
broken chain, constituting the eastern walls of a row of half-submerged
ring plains, they change direction once more and run southward in the
_Mare Nubium_. The whole system bears some resemblance to a gigantic
buckle.”
“What is that curious object below Bullialdus which resembles an
old-fashioned gold earring?”
“I was about to speak of that. It is a ring plain named Lubiniesky,
about 23 miles in diameter with a wall a thousand feet in height, except
in the direction of Bullialdus where it is broken down. The interior is
very flat, and it forms a fine example of the half-submerged lunar
volcanoes which abound in this hemisphere. It may have had a central
mountain like Bullialdus, but if so it has been completely buried under
the influx of molten lava or whatever it was that covered this part of
the moon. The perfect form of Bullialdus in all its details when
compared with the mere outline that remains of Lubiniesky indicates that
the former probably burst forth after the inundation of liquid rock that
drowned the latter. Thus we have in these two neighboring formations two
chapters of lunar history which, like the monuments of Egypt, tell the
story of widely separated epochs. The row of still more completely
submerged crater rings westward from Lubiniesky and Bullialdus show by
their condition that the depth of the lava flood was probably greater in
their vicinity than it was farther eastward.
“Now look southward from Bullialdus, at a distance about twice as great
as that of Lubiniesky and you will see another partially submerged ring,
with a more serrated crest. The name of this is Kies. It is remarkable
for the lofty mountain spur which sets off from its southern wall, and
also for the fact that one of the bright streaks from Tycho—one of a
parallel pair that I pointed out to you last evening—traverses its flat
floor and continues on, broadening as it goes, to a deep crater ring
which we have already noticed, southeast of Bullialdus.
“South of Kies, at the edge of the _Mare Nubium_, is a lofty mountain
range whose summits and slopes are very bright in the sunrise. At one
point a great pass breaks through these mountains, leading to a sort of
bay shut in on all sides by precipices and the walls of gigantic crater
rings. The large crater ring at the eastern corner of this bay is
Capuanus. The smaller ring on its western side with a conspicuous crater
on its eastern wall is Cichus. Notice the fine shadow that Cichus casts,
whose pointed edge is evidently due to the little crater on the wall.
That ‘little’ crater is six miles across! The twin rings apparently
terminating the mountain mass northeast of the bay are Mercator and
Capuanus. Between these and Kies you perceive two short ranges of small
mountains and then a kind of round swelling of the surface of the plain
resembling a great mound. These formations are rare on the moon. They
look like bubbles raised by imprisoned gases. The United States
Geological Survey has discovered something similar in form, but
infinitely inferior in magnitude, in the great mud bubbles that rise to
the surface of the Gulf of Mexico off the mouth of the Mississippi
River. But I do not mean to aver that the two phenomena are similar in
origin.
“Near the southern shore of the _Mare Nubium_ appears a long, dark line
which starts at the edge of a crater ring, crosses the southern arm of
the ‘sea,’ evidently penetrates the bordering mountains, and reappears
traversing the dark bay near its northern edge, cleaving both walls of a
small crater ring in its way.
“I should weary you, perhaps, with too much detail if I undertook to
identify all of the prominent objects in this photograph. Returning to
the southern shore of the _Mare Nubium_, I shall simply call your
attention to the very large ring plain with terraced walls and a peak a
little east of its center. This is Pitatus. An enormous ravine breaks
through its eastern side and connects it with a smaller ring from which
the dark line already mentioned starts. This dark line represents one of
the most remarkable clefts on the moon. It looks as though the crust had
been split asunder there over a distance of at least 150 miles. It bears
some resemblance to the great cañon near Aristarchus and Herodotus,
except that the latter is very tortuous and this is nearly straight.”
“Have I not heard of something similar in connection with the California
earthquake in 1906?” asked my friend.
“No doubt you are thinking of the great ‘fault’ which geologists have
discovered off the Pacific coast of North America. There is perhaps some
resemblance between these phenomena. Pitatus, I may add, is 58 miles in
diameter. You will observe how its southern wall has apparently been
broken down by the deluge of lava which buried so many smaller rings in
the _Mare Nubium_. If you will now turn your attention to the left-hand
side of the photograph, somewhat above the center, you will perceive a
very strange object, the so-called ‘Straight Wall.’ It lies just west of
a large conical crater pit which has a much smaller pit near its western
edge. You might easily mistake the ‘Straight Wall’ for an accidental
mark in the photograph. It is not absolutely straight, and near its
southern end it makes a slight turn eastward and terminates in a
curious, branched mountain, whose most conspicuous part is
crescent-shaped. The wall is about 65 miles in length and 500 feet in
height. It is as perpendicular on its east face as the Palisades on the
Hudson. It is not a ridge of hills at all, but a place where the level
of the ground suddenly falls away. Approaching it from the west you
would probably be unaware of its existence until you stood upon its
verge. The dark line that we see in the photograph is the shadow cast by
the wall upon the lower plain. In the lunar afternoon the appearance is
changed, and the face of the cliff is seen bright with sunlight. This
curious object has attracted the attention of students of the moon for
generations, and many speculations were formerly indulged in concerning
its possible artificial origin. It has sometimes been called the ‘Lunar
Railroad.’ Manifestly, whatever else it may be, it is not artificial.
The closest analogy perhaps is with what we were speaking of a little
while ago, a geological fault, that is to say, a line in the crust of
the planet where the rocky strata have been broken across and one side
has dropped to a lower level.
“The crater pit in the _Mare Nubium_, east of the ‘Straight Wall,’ is
named Birt, and its twin, 75 miles farther east, is Nicollet. Look now
at the hooked nose of your ‘dark woman.’ The huge wart upon it is a
crater plain named Lassell. Between the lower end of the ‘Straight Wall’
and Lassell, and over the bridge of the ‘nose,’ a wedge-shaped mountain
runs out into the _mare_. This is called the Promontorium Ænarium, and
must have formed a magnificent outlook if ever a real ocean flowed at
the foot of its cliffs. The ring with a crater on its wall below Lassell
is Davy. You will note some very somber regions scattered over this part
of the _Mare Nubium_. One of them forms the ‘dark woman’s’ eye, and just
over it, like an eyebrow, is a curving range of hillocks, including some
little craters. On the ‘cheek’—I am still utilizing the ‘dark woman’ as
a kind of signboard—at the base of the ‘chin,’ appears a partly double
range of large ring plains. The greatest of these, at the bottom, is
named Fra Mauro, and you will notice within it a curious speckling of
small craters. Adjoining Fra Mauro on the south are two intersecting
rings, Barry being the name of the western and Bonpland that of the
eastern one. The partially submerged ring is nameless, as far as I know,
while the upper or southern member of the group, with a broad valley
shut in between broken mountain walls opening out of its northern side,
is Guerike. There is only one other object, on the extreme lower
right-hand corner of the picture, to which I will ask your attention. It
is a singular range of mountains thrown into a great loop at its
northern end, and known as the Riphæan Mountains.”
“It seems to me,” said my friend, putting her elbow on the table, and
leaning her head a little wearily on her hand, “that there is a great
sameness in these lunar scenes—always crater rings with or without
central mountains, always peaks and ridges and chasms and black shadows.
Truly variety is lacking.”
“But what could you expect?” I replied. “Is it not enough to stimulate
your curiosity that you are looking intimately into the details of a
foreign world? When you go to Europe you see there mountains, plains,
rivers, lakes, cities, people, absolutely identical in their main
features with what you see in America. But you find them endlessly
interesting because of their comparatively slight differences from
similar things with which you are familiar, because of the great age of
many of the objects to which your attention is directed, because of the
long course of history which they represent, and principally, perhaps,
because you are aware of the sensation of being far from home. It ought
to be the same for you here on the moon. These things that we are
looking upon belong to a globe suspended in space 239,000 miles from the
earth. If the features of our globe are practically the same everywhere,
differing only in the arrangement of their details, you should not be
surprised at finding that nature does not vary from her rule of
uniformity on the moon.
“In the next photograph of the series,” I continued, “we have a
marvelous specimen of the lunar landscapes. It is perhaps the most
rugged region on the moon. It includes two objects of supreme interest,
Tycho, the ‘Metropolitan Crater,’ and Clavius, the most remarkable of
the ring plains. You will no doubt recognize Tycho at a glance. It is
near the center of the picture. Like the last photograph this one
represents an early morning scene. The western wall of Tycho throws a
broad, irregular crescent of shadow into the cavernous interior, but all
of the eastern, northern, and southern sides of the wall are illuminated
on their inner faces. The central mountain group is emphasized by its
black shadow. A little close inspection reveals the existence of the
complicated system of terraces by which the walls drop from greater to
lesser heights until the deep sunken floor is reached. The diameter of
Tycho is 54 miles, and it is at least 17,000 feet deep, measured from
the summits of the peaks that tower on both the eastern and the western
sides of its wall. The vast system of bright streaks radiating from
Tycho is not seen here, the time when the photograph was made being too
near the sunrise on this part of the moon. The dish-shaped plains
crowded around Tycho form a remarkable feature of this part of the lunar
surface. It would be useless to mention them all by name, and I shall
ask your attention only to some of the principal ones.”
“Thank you for being so considerate,” said my friend, smiling. “I am
sure that I should forget the names as fast as you mentioned them.”
[Illustration:
TYCHO, CLAVIUS, AND THEIR SURROUNDINGS.
]
“Oh, I have no fault to find with your memory,” I replied. “I doubt if
many selenographers could recall them without referring to a chart. Let
us begin with the greatest of all, Clavius, which, you see, is near the
top of the picture. I think I told you before that Clavius is more than
140 miles across. The great plain within the walls sinks 12,000 feet
below the crest of the irregular ring, but the plateau outside, on the
west, is almost level with the top of the ring. It is difficult to
imagine a more wonderful or imposing spectacle than that which Clavius
would present to a person approaching it from the western side, and
arriving at about the time when this photograph was made, on the top of
the wall. Notice how in one place the summit of a ridge, standing off on
the inner side of the western wall, has come into the sunlight, and
think of the frightful chasm that must yawn between. Clavius is so
enormous that the two crater rings, each with a central mountain
standing on its wall, seem very small in comparison with the giant that
carries them, and yet they are 25 miles in diameter! Stretched out into
a straight line, the tremendous wall of Clavius would form a range of
towering mountains, extending as far as from Buffalo to New York. Look
at the curved row of craters, the smallest larger than any on the earth,
which runs across the interior. In addition to these there are many
smaller craters and mountains standing on the vast sunken plain, some of
them looking like mere pinholes, and yet all of really great size.”
“Truly,” interrupted my listener, “the giantism—I think that is the word
you employ—the giantism of the moon appalls me! How can I ever think,
again, that the so-called great spectacles of nature on the earth are
really great? You have destroyed my sense of proportion. Such immense
things standing on a world so small as the moon—why it seems contrary to
nature’s laws.”
“I have already told you that the very smallness of the moon may be the
underlying cause of the greatness of her surface features. And I may now
add that if your imagined inhabitants ever existed they, too, may have
been affected with ‘giantism.’ A man could be 36 feet tall on the moon
and well proportioned at that, without losing anything in the way of
activity.”
“Indeed! You almost make me hope that there never were such inhabitants,
for what beauty could there be in a human being as tall as a tree?”
“Very little to our eyes, perhaps. You recall the impressions of
Gulliver in the land of the Brobdingnags. However, they are not my
inhabitants but yours, and if the law of gravitation says that they must
have been twelve yards tall, then twelve yards tall they were. Take
comfort, nevertheless, in the reflection that, after all, we cannot
positively assert that gravitation alone governs the size of living
beings on any particular world. We have microscopic creatures as well as
whales and elephants on the earth, and human stature itself is very
variable.”
“Thank you, again. You have saved my lunarians. And now please tell me
what is that frightful black chasm above Clavius?”
“It is a ring plain named Blancanus, 50 miles in diameter, and
exceedingly deep. It is so black and terrible because complete night yet
reigns within it, except on the face of its eastern wall. It is really a
magnificent formation when well lighted, but like so many other great
things it suffers through its nearness to the overmastering Clavius.
When Goliath was in the field his fellow Philistines cut but a sorry
figure. Look at the marvelous region just below Blancanus and imagine
yourself entangled in that labyrinth! You would have but a small chance
for escape, I fancy.”
“I am sure I should never have the heart to even try to get out of it.
One might as well give up at once.”
“Yes, you are probably right. But I will direct you to something not
quite so frightful, although still very formidable in appearance. Still
farther below you observe a huge ring plain whose eastern wall is
brightly illuminated, while nearly all the interior plain, although
comparatively dark in tone, lies in the sunshine. It is Longomontanus. I
pointed it out to you in one of the smaller photographs. Longomontanus
is 90 miles across and 13,000 or 14,000 feet deep, measured from its
loftiest bordering peaks. The very irregular formation below it is
Wilhelm I. It is remarkable for the mountainous character of its
interior.”
“For what William was it named?”
“I do not know. We are now near the southern border of the region that
we inspected in the preceding photograph. In the lower part of this
picture you perceive some of the projecting bays of the _Mare Nubium_,
and you can see again the remarkable cleft of which I spoke. The large
ring near the bottom of the picture is Pitatus with its smaller neighbor
Hesiodus. It is from the eastern side of the latter that the cleft
apparently starts. Pitatus, you see, has a central peak, while Hesiodus,
as if for the sake of contrast, possesses only a central crater pit. The
ravine connecting the two is plainly visible. Toward the east you will
recognize again Cichus, with its crater on the wall and its broad shadow
with a sharp point, while still farther east, on the very edge of night,
yawns Capuanus. The two walled plains above Pitatus are Gauricus on the
left and Wurzelbauer on the right. The hexagonal shape of the former is
very striking. This is a not uncommon phenomenon where the lunar
volcanoes and rings are closely crowded, and it suggests the effect of
mutual compression, like the cells of a honeycomb. Away over in the
northwestern corner is a vast plain marked by a conspicuous crater ring
which bears the startling name of Hell. It borrows its cognomen,
however, from an astronomer, and not, as you might suppose, from Dante’s
‘Inferno.’
“Before quitting this photograph permit me to recall you to the
neighborhood of Tycho and Clavius. To the left of a line joining them
you will perceive a flat, oval plain with a much broken mountain ring.
This is Maginus. Last evening while we were looking at one of the
smaller photographs I pointed it out under a more favorable
illumination, telling you at the same time that it possessed the
peculiarity of almost completely disappearing at Full Moon. Already,
although day has not advanced very far upon it, you observe that it has
become relatively inconspicuous. This is a lesson in the curious effects
of light and shadow in alternately revealing and concealing vast objects
on the moon. You will notice that in many particulars Maginus resembles
a reduced copy of Clavius. But the walls of Clavius are in a
comparatively perfect condition while those of Maginus have apparently
crumbled and fallen, destroyed by forces of whose nature we can only
form guesses. Evidently the destruction has not been wrought, like that
of some of the rings in the _Mare Nubium_, by an inundation of liquid
rock from beneath the crust. It resembles the effects of the
‘weathering’ which gradually brings down the mountains of the earth, but
if such agencies ever acted upon the moon, then it must have had an
atmosphere and an abundance of water. In any event, here before us is
another page of lunar chronology. Maginus is evidently far older than
Clavius; Clavius is older than the craters standing on its own walls.”
We now took up the third of the large photographs representing a part of
the southwestern quarter of the moon, more extraordinary for its
mountains, plateaus, and extinct volcanoes than the famous southwestern
region of the United States.
“Here is something that you will surely recognize without any
assistance,” I said. “In the lower left-hand corner of the picture is
the great three-link chain of crater rings, of which Theophilus is the
principal and most perfect member.”
“Oh, I recall them well,” replied my friend. “And yet they do not appear
to me exactly the same as when I saw them before.”
[Illustration:
THE GREAT SOUTHWEST ON THE MOON.
]
“One reason for that is because this photograph represents them on a
much larger scale, and with infinitely more detail. Another reason is
that now we are looking at them in the lunar afternoon instead of the
lunar morning. We are going to see them represented on a still larger
scale, presently, but there are many things in this picture well worthy
of study. Advancing from the west, the line of night has fallen over the
extreme eastern border of the _Mare Nectaris_, and the shadows thrown by
the setting sun point westward. Observe how beautifully the brightly
illuminated terraces and mighty cliffs of the western wall of Theophilus
contrast with the black shadow that projects over half of the interior
from the sharp verge of the eastern wall. The complicated central
mountain is particularly well shown. The loftiest peak of this mountain
mass, which covers 300 square miles, is 6,000 feet in height. You will
see its shadow reaching the foot of the western wall. Theophilus is 64
miles in diameter, ten miles more than Tycho, and it is deeper than
Tycho, the floor sinking 18,000 feet below the top of the highest point
on the western wall. If it were the focus of a similar ray system it
would deserve to be called the ‘Metropolitan Crater’ rather than Tycho.
Plainly, Theophilus was formed later than its neighbor Cyrillus, because
the southwestern wall of the latter has been destroyed to make room for
the perfect ring of Theophilus.
“The interior of Cyrillus, you will observe, is very different from that
of Theophilus. The floor is more irregular and mountainous. The wall,
also, is much more complex than that of Theophilus. The broken state of
the wall in itself is an indication of the greater age of Cyrillus. On
the south an enormous pass in the wall of Cyrillus leads out upon a
mountain-edged plateau which continues to the wall of the third of the
great rings, Catharina. This formation seems to be of about the same age
as Cyrillus, possibly somewhat older. Its wall is more broken and worn
down, and the northern third of the inclosure is occupied by the wreck
of a large ring. Observe the curious row of relatively small craters,
with low mountain ranges paralleling them, which begins at the
southwestern corner of Cyrillus and runs, with interruptions, for 150
miles or more. South of this is a broad valley with small craters on its
bottom, and then comes an elongated mountainous region with a
conspicuous crater in its center, beyond which appears another valley,
which passes round the east side of Catharina, where it is divided in
the center by a short range of hills. The southeastern side of this
valley is bounded by the grand cliffs of the Altai Mountains, which
continue on until they encounter the eastern wall of the great ring of
Piccolomini, whose interior appears entirely dark in the picture, only a
few peaks on the wall indicating the outlines of the ring. The serrated
shadow of these mountains, thrown westward by the setting sun, forms one
of the most striking features of the photograph. The northeastern end of
the chain also terminates at a smaller ring named Tacitus. You see that
Riccioli was rather cosmopolitan in his tastes, since he has placed the
name of a Roman historian also on the moon. Beginning at a point on the
crest of the Altai range, south of Tacitus, is a very remarkable chain
of small craters, which extends eastward to the southern side of a
beautiful ring plain with a white spot in the center. This ring is named
Abulfeda. The chain of small craters or pits to which I have referred
continues, though much less conspicuous, across the valley that lies
northwest of the Altais. It is a very curious phenomenon, and recalls
the theory advocated by W. K. Gilbert, the American geologist, that the
moon’s craters were formed not by volcanic eruptions but by the impact
of gigantic meteorites falling upon the moon, and originating, perhaps,
in the destruction of a ring which formerly surrounded the earth,
somewhat as the planet Saturn is surrounded by rings of meteoric bodies,
which may eventually be precipitated upon its surface. The moon is more
or less pitted with craterlets in all quarters, but there are places
where they particularly abound. On inspecting this photograph carefully
you will perceive several rows of much larger pits, two or three of them
in the upper half of the picture, and one below the center, crossing the
little chain of pits that I have just mentioned. The linear arrangement
of some of the ring plains is also very striking. In regard to the
theory that the lunar craters were formed by the impact of falling
masses I may mention that two distinguished French students of the moon,
Messrs. Loewy and Puiseux, have lately expressed the opinion that all of
the features of the lunar surface are most readily explicable as the
result of causes similar to those which have produced the topography of
the earth. If that is so there is no need for us to invoke the agency of
meteorites in pitting the surface of the moon. South of the Altai
Mountains you will see a singular collocation of ring plains and craters
which somewhat resemble in their arrangement Theophilus and its
neighbors. First comes a large sunken plain just above the mountains. In
fact the Altai range constitutes the northwestern wall of this
formation, which you may recognize by a conspicuous oval crater near its
upper side. Above this broken ring appear three other smaller ones,
grouped at the corners of a triangle. The one on the right, with a
central pit and a small ring plain on the inside of its western wall, is
called Zagut. Its close neighbor on the west with most of its interior
in shadow, is Lindenau, remarkable for its depth. The most southerly and
largest of the group, with four or five large crater pits forming a
curved row across its interior, is named Rabbi Levi. Starting from the
east side of Rabbi Levi there is a long row of similar craters rather
larger than those in its interior, which runs eastward almost to the
edge of the photograph. North of these, parallel with and, in some
instances, touching the crater pits, is an equally remarkable row of
flat, smooth, walled valleys, which seem to overlap one another on their
western sides, and which increase in size the farther east they go. The
largest of these, with a very irregular wall, and having a smaller ring
with a central peak apparently attached to its northern side, is Gemma
Frisius.”
My friend had listened to me in silence for a long time, following my
finger as it pointed out the various objects on the photograph, but now
she interrupted again: “You were pleased to compliment my memory a
little while ago,” she said, “but do you really think that I can ever
recall all this that you have been saying, with theories about huge
flying stones hitting the moon, and a string of the strangest names that
I have ever heard applied to objects that are no less bizarre?”
“Pardon me,” I replied, “but you will remember more than you think you
will. The very oddity of these Hebraic and Arabic names will serve to
fix them in your memory, so that you will at least recognize them when
you see them again. Those curious objects will also come before your
mind’s eye whenever you think of, or look at, the moon. Trust me when I
tell you that you are forming a better acquaintance with selenography
than you are aware of. As to the theory that I have mentioned, what can
appeal more powerfully to the imagination than the idea of the moon
being bombarded by the fragments of an immense ring falling from the
sky? The fact that men of science have believed such a thing possible
ought to form a strong appeal to your lively fancy. In any case, I am
disposed to be merciless just now, like a man who has found a patient
listener to his hobby, and I am going to trouble you with a few more odd
names and singular facts.”
“Well,” she replied, with a sigh, ending with a smile, “go on. After all
I believe I am really interested.”
“I am sure you are, for who could fail to be interested by things so
remarkable in themselves, and so vastly beyond all human experience, as
those that this photograph shows? We stopped at Gemma Frisius. Let us
use that for a new starting point. A considerable distance south, say
about a hundred miles, is an old friend of ours, Maurolycus. It is the
large ring plain, with another half obliterated, on its southern side,
in the upper part of the picture. Notice the row of wrecked rings,
beginning at a great crater on its northeast wall and running westward.
The broad, flat plain directly east of Maurolycus is Stöfler, whose name
you will also recall. I shall not trouble you with the names of all the
rings south of Stöfler and Maurolycus, but simply ask you to observe
that they form a winding row which leads to a very grand ring almost
entirely buried in night, the inside of its western wall alone being
bright with sunshine. This wall, and some mountain peaks near it,
resemble brilliant islands lying in the edge of the Cimmerian ocean
whose ethereal waves wash the broken coast of the moon. Follow the
ragged sunset line downward, and all along you will see these islands of
light in the darkness; tips of mountains still shining while the sun has
set upon all the valleys around, somewhat as you have seen the snowy top
of Mont Blanc and the pinnacles of its attendant giants glowing after
the shades of night have fallen deep upon Chamounix.
“Look next, if you please, at the right-hand side of the photograph.
Somewhat above the center, three conspicuous dish-shaped ring plains are
seen, two near together, the third farther away toward the left and
downward. The largest of these is Aliacensis, its near neighbor is
Werner, and the third is Apianus. They are from 40 to 50 miles in
diameter. Still lower, and nearer the middle line of the picture, is a
row of four or five ring plains, varying from 30 to 40 miles in
diameter. The uppermost, or most southerly of these is double, or, in
fact, partly triple, for the lower member of the pair has a broken plain
attached to its southeastern side. This one, with a small central peak,
is named Abenezra. Its close neighbor on the southwest is Azophi. You
notice the singularity of the names. The next one below, with a small
crater on its east side, is Geber. Then comes Almamon, and finally,
largest of all, Abulfeda, which I pointed out to you as marking the end
of the curious row of little crater pits, running eastward from the
Altai Mountains. There is just one other formation to which I wish to
call your attention in this remarkable photograph, and then we shall
turn to the next in the series. West of Abenezra and Azophi, about half
way to the Altai Mountains, you will notice a very irregular depression
with three strongly marked craters within it. This bears the name of
Sacrobosco, an old-time astronomer. Its eastern wall with its shadow
looks like an elongated letter W standing on end. Sacrobosco and its
surroundings constitute one of the most intricate regions on the moon,
high plateaus alternating with great sunken valleys, rings, craters, and
crater pits. The wall of Sacrobosco is extremely irregular in height,
shooting up in some places with peaks of 12,000 feet elevation, and
sinking in others almost to the level of the surrounding plateaus.”
We now took up the next photograph representing Theophilus and its
companions on a greatly enlarged scale. My friend uttered a cry of
astonishment upon seeing it.
“Dear me,” she exclaimed, “the moon becomes more terrible every moment!
Positively, I almost shrink from the sight.”
[Illustration:
THE GIANT RING MOUNTAINS, THEOPHILUS AND ITS NEIGHBORS.
]
“Yes,” I assented, “it surely is terrible here. In a little while,
however, I shall show you a lunar scene of surpassing beauty. But study
this spectacle with an inquiring mind and you will find that it, too,
has its attractions. You are now looking upon Theophilus, Cyrillus,
Catharina, and the surrounding region as the astronomer sees them with
the most powerful telescopes. Indeed, with the telescope he sees the
details more sharply than they are visible here, for the best
photographs still lack something in distinctness. The illumination when
this picture was taken was practically the same as in the last that we
examined, but the magnification is much greater. Look, now, at the
central mountain in Theophilus. Its great buttresses cast their shadows
into profound ravines and chasms, imparting to it a most singular
outline. Observe the tooth-shaped shadows of its two principal peaks,
thrown westward across the floor, while the broad shadow of the western
wall emphasizes the immense depth of the depression. The glare of the
afternoon sun on the cliffs of the inner side of the eastern wall is so
brilliant that the details are obscured. But the surface of the moon
outside, particularly toward the north and the west, is beautifully
brought out with all its wonderful modulations and irregularities.
Judging by appearances, those who hold that Theophilus and similar
formations, notwithstanding their enormous magnitude, are really of
volcanic origin, have the strongest reasons for their opinions. Immense
flows of lava seem to have taken place on all sides of the great ring,
entering the _Mare Nectaris_ on the west. Notice the huge mountain fold
which runs from the parallel ridges on the southwestern side of
Theophilus to the crater ring Beaumont, lying west of Catharina.
Observe, also, the complicated form of the wall dividing Theophilus and
Cyrillus. Two deep ravines, shown by the shadows that fill them, cross
one another like the arms of a flat letter X. One of these ravines turns
northward along the wall and re-enters Theophilus, while the other
continues for a long distance within the western side of Cyrillus. I
cannot imagine a more interesting or a more stupendous excursion for a
geologist, a mountaineer, or a seeker after wonderful and sublime
aspects of nature, than a climb around the crest of the wall of
Theophilus—if indeed such a climb can be regarded as humanly possible.
“Now, again, I am reminded of what I once told you about the amazing
contrasts of light and darkness, and of heat and cold, upon the moon.
Suppose yourself standing on the verge of the eastern wall of Theophilus
where the edge seems sharpest, and looking down into the abyss at your
feet. The sun’s rays would be unbearably hot where they touched your
face and hands, but if you let yourself down a little way into the
blackness beneath you would not only pass instantly into night, but you
would shiver and shrink with cold so frightful that no winter experience
that you have ever had could give an idea of its intensity. From that
point of observation you would look across a chasm of inky darkness, 25
miles broad, and see, towering up from the illuminated plain afar off,
with their summits more than two miles below your level, the brilliant
group of the central peaks, while behind them the crest of the western
wall would appear like a bright line on the horizon 60 miles away.
Changing your place to one of the peaks on the dividing wall you would
look down into Theophilus on one side and Cyrillus on the other. Then
upon lifting your eyes to the black, airless sky you would see the stars
sparkling on all hands, and, hanging in the heavens like a portentous,
strangely colored moon many times larger than the disk of the sun, would
appear the mottled orb of the earth. The terrific nature of the scenery
around you, the meeting of day and night at your feet, and the
incredible blending together of their characteristic aspects in the sky
above you, the startling magnitude of the suspended earth—all these
things combined would make you feel as if you were not only in another
world but in another universe.”
“I no longer wish to visit the moon,” interrupted my friend, shaking her
head.
“Not if you were assured of a safe return?”
“No, it would upset my mind. I am certain that I should go crazy in such
a world where everything seems to be topsy-turvy.”
“Wait until we arrive at the ‘Sea of Serenity’ once more, and perhaps
you will think better of it. Notwithstanding the increased
magnification, the details in Cyrillus and Catharina are hardly better
seen in this photograph than in its predecessor, but the increase of
size is very effective in emphasizing some of the features of the
surrounding district. Cyrillus is seen to have a decided hexagonal
outline, and west of its southern corner is an exceedingly curious
formation, approaching closely to a square shape. The wall is
illuminated within on all four sides, and out of the midst of the
lozenge-shaped shadow resting over the bottom of the included valley,
rises a mountain which, like the walls, is bright with sunshine. On the
southwest a semicircular ridge runs out into the darkness, its top
brightly illuminated. The general effect of the entire formation is
fantastic. And could you imagine a wilder scene than that presented by
the elongated mountain mass, which starts from the southwestern side of
Cyrillus, skirts the border of Catharina, and continues on along the
northwestern side of the broad valley in the upper part of the picture?
See how it has, apparently, been rent apart by tremendous forces and
torn by volcanic outbursts, which have left yawning craters everywhere.
Even the valley itself seems to be simply a chain of wrecked crater
rings of vast size, the cross walls having nearly disappeared. Observe,
too, the immense number of crater pits of all sizes scattered
everywhere, both inside the ring plains (Theophilus alone having few of
them) and over the surrounding country. We shall see a still more
remarkable example of this pitting of the lunar surface in the
neighborhood of Copernicus, which is the chief object in the next
photograph that we take up.”
We came now to the large picture of Copernicus, and my friend took it in
her hands to examine it.
“It is a marvelous thing to look upon,” she said, “but it doesn’t
frighten me as Theophilus did.”
“No, Copernicus is rather sublime than terrifying in aspect. Its
comparatively lone situation, with the _Mare Nubium_, the _Oceanus
Procellarum_ and the _Mare Imbrium_ surrounding it on all sides with
their broad, level expanses, gives it an appearance of solitary grandeur
belonging to no other single formation on the moon. ‘The monarch of the
lunar ring mountains,’ Mr. Elger has termed it. First let me tell you
the principal facts known about Copernicus. It is 56 miles in diameter,
two miles more than Tycho, and eight less than Theophilus. It is not as
deep as either of those formations, the highest points on its walls
being 12,000 feet. But the walls are more uniform in height than is
usual with so extensive a ring. They are very steep on the inside,
especially near the top, where their slope has been estimated by Neison
at from 50° to 60°. To a person standing on their verge they would seem
almost perpendicular. The central mountain consists of five principal
peaks. The outer slopes of the ring are also steep, but its maximum
height above the surrounding surface does not exceed 3,000 or 4,000
feet, so that Copernicus, like the other great ring mountains, is, in
reality, a vast sink, encircled with a mountain ridge. You will note
that Copernicus clearly exhibits the tendency to a hexagonal form which
we have observed elsewhere, although it stands alone with no other great
rings pressing against its walls. Curiously enough the form of
Copernicus is very closely repeated in the small crater ring Gay Lussac,
situated in the mountains on the lower (north) side. This picture, I
should remark, unlike the last two preceding it, was taken near lunar
sunrise, and accordingly the light comes from the west. This is the best
illumination for studying Copernicus and its vicinity. Of all the great
ring plains Copernicus perhaps gives the most striking testimony in
favor of the view of those who hold that the lunar volcanoes were once
the actual centers of volcanic action, resembling the volcanoes of the
earth in the ejection of vapors, ashes, stones, and streams of lava. The
slopes around Copernicus for many miles look as though they had been
covered with lava and pitted with minor craters such as appear on the
shoulders and in the vicinity of many of our volcanoes, while the
appearance of the great ring does not contradict the theory of Nasmyth
and Carpenter, which I have previously mentioned, that it was built up
by ejections from a central crater now more or less completely filled.
As I have already told you the lunar volcanoes differ essentially from
those of the earth in that their central depressions lie deep beneath
the level of the surrounding surface of the moon. This is strikingly
true of Copernicus, and it is a result that might have been foreseen
from the enormous size of the craters. A mountain of sufficient
magnitude to carry the vast cup of Copernicus on its head, as Vesuvius,
Etna, Cotopaxi, and Popocatepetl carry their craters, could not stand
even on the moon. Observe the generally radial arrangement of the lines
about Copernicus, recalling the similar arrangement of lava flows about
terrestrial volcanoes. Some of these lines, as you will see, consist of
long rows of pits. Similar phenomena may be seen along the lava streams
that we are familiar with on our planet, where small craters break forth
one after another. A striking example of this arrangement is visible in
the photograph on the northeastern slope leading up toward the
Copernicus ring. But you will also see many very remarkable rows of pits
in the vicinity of Copernicus which are not radial in arrangement with
respect to the ring. The most conspicuous of these is on the
northwestern side, about half way between Copernicus and the ring of
Eratosthenes, which standing at the upper end of the chain of the
Apennines appears at the left-hand edge of the picture. There are
hundreds, probably thousands, of these pits on all sides of Copernicus.
“One of the explanations that has been suggested for them is that they
were produced by the fall of enormous volcanic bombs thrown from
Copernicus when it was in eruption. I wish merely to mention this idea
without comment. It however calls up another interesting theory, which
has not met with much acceptance, to the effect that such lunar
volcanoes as Copernicus may have been powerful enough to eject masses of
lava and rocks with a velocity sufficient to enable them to escape from
the attraction of the moon, whereupon they became meteorites traveling
in independent orbits around the sun. Some of these, the theory
suggests, may be among those that have fallen upon the earth. A velocity
of a mile and a half per second would be sufficient to overcome the
gravitation of the moon. That is only three or four times the initial
velocity which some modern guns are capable of imparting to their
projectiles.”
“I am sorry,” explained my friend, “that you seem to attach little
importance to so interesting a theory. It stirs my imagination to think
of the moon sending bits of herself back to her mother planet. For my
part, the theory does not seem to be any harder to believe than that of
your Professor Darwin that the whole moon was thrown off from the earth.
Besides, it intensifies my appreciation of the grandeur of Copernicus
when I am told that that great volcano could once bombard the earth
across—what is it, 240,000 miles?—of space.”
“As you always choose the most picturesque theories to rest your belief
upon, I shall not complain if you accept the lunar volcano theory of
meteorites,” I replied. “But, for the present, we have done with it, and
I am now going to ask you to inspect the photograph for other
interesting objects. East and north of the great ring you will see an
extensive mass of mountains. Those on the north, with immense buttresses
projecting into the _Mare Imbrium_, are the lunar Carpathians. I have
already directed your attention to a comparatively small crater ring
which resembles a reduced copy of Copernicus, situated in these
mountains at the head of a bay which penetrates southward between high
ridges, for about 30 miles. This crater is named Gay Lussac. It has a
small deep neighbor on the southwest. West of Gay Lussac the Carpathians
gradually dwindle away until they sink to the level of the plain. Toward
the east they project in several bold headlands, terminating with
towering peaks into the ‘sea.’ Lying off the point of the headland on
the western side of the bay that leads to Gay Lussac you will perceive
two charming little craters, almost perfect twins. Much farther toward
the north and west is a larger crater, more than half of whose interior
is black with shadow. This is Pytheas. Its lonely situation is very
striking, but upon close inspection you will notice that a low range of
hills appears to connect it with the twin craters that I have just
pointed out. This range of hills, lying on the ‘sea’ bottom, is
curiously forked, the other branch leading to a pair of small peaks in
the ‘sea,’ which possess no craters. The little crater east of Pytheas
is also a beautiful object in the picture.
“Near the eastern end of the Carpathians the mountains make their
greatest advance into the _Mare Imbrium_, leaving a large
square-cornered bay on the west. From this point they turn southward,
forming a complicated mass of peaks and ridges interspersed with craters
and pits. These mountains east of Copernicus are among the most singular
upon the moon, for they inclose a group of irregular-shaped plains, the
walls of which consist of immense, more or less separate, masses. Look
at the one nearest to Copernicus, which has somewhat the form of a
starfish, and observe how curiously its southern border reflects, on a
smaller scale, the forms characteristic of the headlands and bays along
the shore of the _Mare Imbrium_ below.
“Above Copernicus you see a large crater ring more than half in shadow,
with a plain of an irregular hexagonal shape, northwest of it. The large
ring is named Reinhold. A broken mass of mountains extends from its
southern side far into the _Mare Nubium_. In the upper right-hand corner
of the picture is another large ring called Landsberg. In the upper
left-hand corner you see a roughly hexagonal ring plain, level on the
interior, named Gambart. Mountains break the level of the _mare_ both
south and north of Gambart. Those on the north are remarkable for the
darkness of the surface, especially in the northwestern part.
“Almost directly west of Copernicus lies an exceedingly singular object.
It is a part of the underworld of the moon, the buried moon, which was
covered up ages ago by that immense outgush of lava of which I have so
often spoken. Once evidently it was a ring larger than Eratosthenes.
Now, only its outlines can be traced, the whole immense depression of
the interior and the surrounding walls to their very top having been
covered up. It is pitted and surrounded with little craters of a later
date. I have already told you that Eratosthenes, the ring at the
left-hand edge of the photograph, marks the termination of the great
range of the lunar Apennines. But these mountains seem to be continued
beyond Eratosthenes in two short branches, one turning eastward toward
the Carpathians, and the other reaching to the highest part of the
buried wall of the submerged ring that we have been talking about and
which bears the name of Stadius. You will be interested in knowing that
southwest of Stadius, but off the edge of the picture, there is a place
in which low hills and ridges abound, where the German astronomer
Schröter imagined that he had discovered a lunar city! His mistake was,
perhaps, natural, considering the slight power of his telescope and the
strangely regular arrangement of the lines of hills which he mistook for
streets.”
“I regret that he was deceived.”
[Illustration:
TWO GREAT LUNAR “SEAS”
The _Mare Serenitatis_ and a Part of the _Mare Imbrium_.
]
“So do I. We shall now leave Copernicus and its marvelous surroundings,
and turn to the last photograph in our series, representing the _Mare
Serenitatis_ in its full extent, and a large part of the _Mare Imbrium_.
Is it not a beautiful picture?”
“It is, indeed, but so strange!”
“There is, I believe, nothing in the lunar world that would not seem
strange to our eyes. To understand just what this picture means you
should imagine yourself floating in an airship at an immense height
above the surface of the moon. The _Mare Serenitatis_ you will recognize
as the great oval plain occupying the upper left-hand part of the
photograph. It is entirely encircled by mountains except in three
places—at its eastern end, where a broad strait opens between the
Apennines on the south and the Caucasus on the north, leading into the
_Mare Imbrium_; on the northwest, where another strait opens into the
_Lacus Somniorum_, the ‘Lake of the Sleepers,’ or ‘The Dreamers,’ and on
the southwest, where a third strait with a conspicuous crater in its
center leads into the _Mare Tranquillitatis_. The _Mare Serenitatis_ is
430 miles long and nearly as broad, and covers an area of about 125,000
square miles. A great many details are visible on its floor. Even if it
were covered with water we might see these, for, as you have probably
heard, the bottom of deep lakes is visible when one looks down upon them
from a great height. The surface of water, however, at certain angles of
view and of illumination, would produce flashes and glares of light
which are never seen on this vast lunar plain.”
“Oh, but it _must_ once have been a sea,” said my friend, poring over
the photograph. “I cannot give up that idea. It gives the interest of
life to the moon, if not now at least in the past.”
“You are by no means compelled to give up your idea,” I replied. “On the
contrary you are supported by the opinion of many astronomers, including
Messrs. Loewy and Puiseux, whom I quoted a little while ago. They aver
that the resemblances between the lunar _mares_ and the beds of our
terrestrial oceans are too numerous and too decided to permit any other
conclusion than that in the one case as in the other a deep covering of
water has produced the characteristic features. One striking resemblance
that they note is in the surface contours. The lunar sea beds are
generally deepest along the shores; the same is true of the terrestrial
seas. Continents on the other hand are characterized by concave
surfaces. But before we study the two lunar ‘seas’ in detail let us
first look at their shores and surroundings. The upper and right-hand
sides of the _Mare Serenitatis_ are bordered by hundreds of miles of
magnificent cliffs, which in many places are very steep and of great
height. These form what we may call the sea front of the Hæmus
Mountains, which join the lunar Apennines on the southern shore of the
strait leading into the _Mare Imbrium_. These mountains possess one
conspicuous crater, set like a gem in the chain, at about a third of its
length from the western end. This crater is Menelaus, which we saw in
one of the smaller photographs. It is characterized by its exceptional
brilliance as well as by the fact that the longest of the bright bands
that start from Tycho passes through it, and then continues on across
the _Mare Serenitatis_ and the _Lacus Somniorum_, to the _Mare
Frigoris_. This band, more than 2,000 miles long, has come all the way
from Tycho, high in the southern hemisphere, never turning aside to
avoid anything in its path. Mountains, craters, and ring plains are
equally indifferent to it. It is like a Roman road, and like that, too,
it suggests for its creation a power that knew no master, and admitted
of neither rivalry nor opposition. The existence of this mysterious band
increases the difficulty of finding a satisfactory explanation of the
Tychonic rays. In the midst of the _mare_ the band or ray crosses
another lone crater, 14 miles in diameter, named Bessel. The full length
of the ray is not shown in this photograph, but on its way from Bessel
it touches two other small craters in the ‘sea.’
“That portion of the Hæmus range in which Menelaus is set is a very
attractive scene on account of the bow shape of the mountains, and the
situation of the bright crater just in the center of the bow. Menelaus
and the streak from Tycho can be seen at Full Moon with no greater
optical aid than that of a good binocular. On the edge of the ‘sea,’ off
a lofty headland of the Hæmus chain, another lone little crater is
visible, Sulpicius Gallus by name. It, too, is remarkable for its
brilliant reflective power. Behind the mountains, directly back of
Sulpicius Gallus, and lying in an upraised part of the _Mare Vaporum_,
is a larger, and even brighter, crater ring than Menelaus. It is named
Manilius, and is likewise a conspicuous object for a binocular at Full
Moon. Below Sulpicius Gallus the Hæmus Mountains broaden out and assume
a curious somber tone, until, in the form of a rough plateau, they blend
with the wide-expanded southwestern slopes of the Apennines. The latter
rise gradually to the chain of huge peaks fronting the _Mare Imbrium_.
They contain one notable crater ring named Marco Polo, which lies just
above a great square _massif_, which breaks the narrow chain of the
illuminated summits of the Apennines. The precipitous front of this
range appears very brilliant in the afternoon sun, for here again we
have a photograph made after the time of Full Moon. The end of the
Apennines touching the strait, of which I have previously spoken,
terminates with a high cape called Mount Hadley. In the strait, off this
cape, is an array of small mountain peaks, which must have been islands,
if the lunar ‘seas’ were once true seas.
“Across the strait, on the northern side, stand the lunar Caucasus
Mountains. They run out to a point in a long, irregular, broken ridge.
The distance from Mount Hadley across the strait to the projecting point
of the Caucasus range is about 50 miles. The islands narrow the main
opening to a width of 30 miles. In strict fact the Caucasus range is not
continuous. The point fronting the strait is, in reality, the end of a
large irregular ‘island,’ with intricate channels separating it from the
mainland. Still farther north the photograph shows a broad valley
severing the mountain range from side to side. The main mass of the
Caucasus continues northward to the great ring mountains Eudoxus and
Aristoteles. In the center of the range, opposite the lower corner of
the _Mare Serenitatis_, is an irregular ring plain, Calippus. West of
this the mountains break down in great precipices to the level of a
plain that might be compared with one of the ‘parks’ of Colorado. Beyond
this, in the shape of a broad mass of hills, it skirts the border of the
_Mare Serenitatis_ for nearly 200 miles to a sharp promontory which
shuts off the _Lacus Somniorum_ on one side from the _mare_. West of
Aristoteles and Eudoxus the mountain mass extends to a curious
sharp-angled plain, which it skirts on the north and south.
“The western shore of the _Mare Serenitatis_ beyond the strait opening
into the _Lacus Somniorum_ is bordered by a series of alternating ring
plains and connecting mountains. The first and largest of the rings is
Posidonius, an immense formation 62 miles in diameter, with a central
crater and curious ridges within the inclosure. Above Posidonius is Le
Monnier, a ring plain whose ‘seaward’ wall has been broken down. Above
that, again, is a mountain range terminating with broken crater rings.
Then we arrive at the strait opening into the _Mare Tranquillitatis_,
which is twice as broad as that between the Apennines and the Caucasus,
and just in the middle of it stands a very perfect crater ring named
Dawes. On the eastern side of this strait the Hæmus Mountains begin with
a long cape called the Promontory Acherusia. Above this promontory, at
the edge of the picture, appears the ring plain Plinius, with a distinct
central peak. This completes the circuit of the _Mare Serenitatis_.
“We return to the Caucasus region. These mountains front the _Mare
Imbrium_ along the upper part of their course with sharp slopes and
cliffs. In the ‘sea,’ nearly opposite the deep, broad valley which I
pointed out as dividing the range completely across, stands a
triangular-shaped ring plain dark with shadow on one of its sides. This
is Theætetus, interesting as the scene of an alleged display of ‘smoke,’
reported to have been witnessed by a French observer with his telescope
a few years ago. Several occurrences of this kind have been reported on
the moon, but more or less doubt attaches in every instance the accuracy
of the observations, or at least to that of the conclusions drawn from
them. Below Theætetus is an oval ring almost entirely filled up, with
two craters within it. This is named Cassini. Below Cassini begins
another mass of mountains, the lunar Alps. These are by no means as
extensive as the Caucasus, but they contain some lofty peaks, and are
traversed by one of the most remarkable valleys on the moon. It is not
very distinctly shown in this picture, but you may recognize it by a
dark band commencing opposite a small bay which sets back into the
mountains. The valley continues through the mountains and the adjoining
hilly regions nearly to the shore of the narrow _Mare Frigoris_, which
runs in a sloping direction from beyond Aristoteles to the bottom edge
of the picture. The Alps spread eastward, broadening out with many
separate peaks, and skirting the _Mare Imbrium_, until they reach one of
the most singular and interesting of lunar formations, the oval ring
plain Plato. This looks like a dark lake surrounded by high cliffs. In
the photograph all of the encircling wall is illuminated on the inner
side except at the east end, where the shadows extend a short distance
upon the floor. Plato looks as though it might once have been a ring
mountain of the usual type, which has been partly filled in the interior
by a local uprush of molten lava. The diameter of the ring is 60 miles,
but the inclosure sinks only about half as deep beneath the crest of the
wall, as is the rule with formations of similar outline. A central peak,
a group of mountains, may be buried there.
“It is within this ring of Plato that some of the strongest evidences of
continued change, and possibly of continued life upon the moon, have
been found. Prof. William H. Pickering, after long and careful studies
of this remarkable plain, says of it:
“‘Plato is, I believe, more active [in a volcanic sense] than any area
of similar size upon the earth. There seems to be no evidence of lava,
but the white streaks indicate apparently something analogous to snow or
clouds. There must be a certain escape of gases, presumably steam and
carbonic acid, the former of which probably aids in the production of
the white markings.’
“The white marks to which Professor Pickering refers are but faintly
indicated in the photograph before us, but with the telescope, when the
illumination is favorable, they are plainly seen. There are a number of
very small crater pits scattered over the floor of Plato, and around
these changes of color occur which have been ascribed to the emission of
some substance from the pits and to the presence of vegetation,
nourished by the gases and vapors, and springing into renewed life every
time the sun rises upon the plain. Broad areas of the inclosure
gradually change color as the sun rises, and again as the sun sets, and
these phenomena have also been ascribed to the presence of vegetation.
You may, if you wish, regard Plato as a kind of mountain-ringed prairie,
covered with something analogous to prairie grass and shrubs, which
depends for its existence, partly, upon the supply of gases spreading
over the surface from the crater pits.”
“So this, then, is your ‘lunar grass’?”
“Yes, but not all of it. Mark, I do not aver that it actually exists; I
only say that it has been suspected to exist. On some of the _mares_
similar appearances are seen, as I have already told you, on a much more
extensive scale, and I may again quote Professor Pickering, who says
that some of his observations ‘point very strongly to the existence of
vegetation upon the surface of the moon in large quantities at the
present time.’”
“Does this vegetation resemble that of the earth?”
“I cannot tell you.”
“But where vegetation exists animal life is possible, is it not?”
“Yes, it is _possible_.”
“What forms would it have?”
“I cannot tell you. But I certainly should not expect to find manlike
creatures there.”
“Oh, men are not _necessary_ everywhere,” said my friend, laughing. “I
am content if you admit that there may be living creatures of some kind.
Henceforth I shall never forget Plato and the other places on the moon
where such significant changes are seen.”
“I shall presently point out to you one of the most notable of those
other places,” I replied. “Let me now fulfill my promise to tell you
more about the lunar atmosphere. I have told you already that there are
strong reasons for supposing that the moon once had a far more dense
atmosphere than she possesses at present, and I have mentioned some of
the ways in which this atmosphere is supposed to have disappeared. I
think that it is worth our while to refer to them again. In the first
place the moon’s atmosphere may have been withdrawn into vast internal
cavities formed by the gigantic volcanic eruptions. Secondly, it may
have been absorbed both mechanically and chemically by the core of the
moon as it cooled off. We know that cooling rocks absorb immense
quantities of the gases constituting the air we breathe. In fact we may
look forward to a time, fortunately for us extremely remote, when the
interior rocks of the earth will, in this manner, absorb perhaps all of
its atmosphere.”
“But if the air of the moon has gone into great cavities in the
interior, why might not the living beings of the moon have followed it
there?”
“According to some of the theorists,” I answered, “that may really be
what has occurred, and thus the moon has become a ‘cavern world’ on a
gigantic scale. But science does not regard seriously these speculations
about ‘cave life’ in the moon. A third hypothesis is that which I have
mentioned concerning the escape of the atmospheric gases from the moon
on account of its attraction being insufficient permanently to retain
them. This process would be gradual, because the molecules of a gas fly
in _all_ directions, only a small proportion having their trajectories
directly away from the center of the globe on which they are held. But a
singular consequence of this theory is that interplanetary space must
contain an enormous number of such wandering molecules, and every
attracting body must draw more or less of them to its surface, thus
forming an atmosphere for itself. As Professor Young has remarked, if as
many of these molecules enter a planet’s atmosphere in a day as escape
from it there can be no decrease of the total amount of air. If more
escape than enter, the atmosphere will diminish. If more enter than
escape, the atmosphere will grow. Finally if none escape the atmosphere
may increase indefinitely. This, as far as the effect of gravitation is
concerned, should be the case on the sun, for the solar attraction is
more than sufficient to retain any gas known to us. In consequence, the
sun’s atmosphere may be increasing in extent and density. Even the
earth’s atmosphere may be slowly increasing from this cause, and herein
may lie the explanation of the enormous atmosphere surrounding the great
planet Jupiter.
“In view of what I have said it is evident that the moon cannot be
entirely airless. Recent observations have confirmed this conclusion,
and some observers have thought that they could detect the presence of
something resembling clouds occasionally creeping like low fogs over
certain places on the moon. All this, you will observe, has an important
bearing upon the question of life on the moon at the present day.
Certain forms of plant life and low animal organizations might exist in
such an atmosphere as the moon still possesses.”
“But,” interjected my friend, “is not this that you have been telling me
in contradiction to what you said about the cause of the sharp division
between day and night on the moon, and about the visibility of the stars
there in the daytime?”
“Not at all,” I replied, “for the effects of which I spoke are relative.
In any case the atmosphere of the moon must be too rare to diffuse any
perceptible amount of light into the shadows, or to illuminate the sky
sufficiently to render the stars invisible. The same reasoning applies
to what I have told you about the contrasts of cold and heat on the
moon.
“But we have not yet finished with our photograph. We were looking at
the plain of Plato, you will recollect. Notice, now, the _Mare Imbrium_
off the coast that adjoins Plato on the south. You see there several
bright spots resembling islands. Islands they must have been if the
_mare_ once had water covering it. One of these, standing by itself, an
irregular, bright clump with a distinct shadow on the western side,
bears the name of Pico, taken from the sharp peak in the Azores Islands.
The broken mass southeast of Pico, and nearer the coast, constitutes the
Teneriffe Mountains. You will notice that terrestrial geography has been
drawn upon in this case also to supply a name. Still farther east is a
long ‘island’ named the Straight Range. Beyond that, at the edge of the
picture, appears Cape Laplace, at the western end of the ‘Bay of
Rainbows.’
“We now turn to the southwestern border of the _Mare Imbrium_, in the
upper part of the photograph. This, as I have already pointed out, is
skirted by the steep cliffs of the Apennines for a distance of more than
400 miles. Opposite the crater ring Marco Polo, in the Apennines, you
will notice how the floor of the ‘sea’ is upheaved, containing a great
number of irregularities, and some small peaks. This would have been a
dangerous part of the ‘Sea of Rains’ for the lunar navigators. At the
northwestern corner of this region lies a large ring plain, with
indefinite light stripes crossing its floor, which is named Archimedes.
It is about 50 miles in diameter. Northwest of it are two smaller ring
mountains, Aristillus (the larger) and Autolycus. If we could suppose
these immense volcanoes to have been in eruption when these seas were
navigable, imagine the magnificent spectacle that they would have
presented to anyone approaching in a ship from the direction of the
strait between the Apennines and the Caucasus.
“Let us now pass this strait and enter the _Mare Serenitatis_. You will
admire the beautiful modulation of the bottom, as shown in the
photograph. Lighter and darker regions are curiously interspersed, and
in some places there are faint indications of that wonderful lunar world
of remote antiquity which lies buried in the grave of a planet. Directly
opposite the opening of the strait, a small, round, light spot is seen
in the midst of the sea. This is Linné, very famous for its strange and
suggestive history. Here, if anywhere on the moon, changes visible to
human eyes have taken place, and, in the opinion of Professor Pickering,
are still taking place every fortnight. In the center of the light spot
is a minute crater, and from this crater there seems to issue some kind
of vapor which spreads over the surrounding surface, alternately
expanding and shrinking in extent. A remarkable change in the form and
appearance of Linné was recorded by the astronomer Schmidt, at Athens,
in 1866. What had occurred has been explained by some as the falling in
of a crater floor some six miles in diameter. But the observations of
Professor Pickering are more interesting and suggestive. According to
him the bright patch about the crater pit extends during the lunar night
and diminishes by day, indicating that something issues from the pit and
is deposited over the surrounding plain in the form of hoar frost, which
melts away in the sunshine. He has even recorded an apparent expansion
of the white area during a lunar eclipse when the cold shadow of the
earth tends to condense the vapors. If this is true it seems rather
surprising that many more similar phenomena are not visible elsewhere.
“Among the most remarkable and beautiful features of this photograph are
the winding ridges like half-submerged mountain ranges that appear on
the sea bottom in various places. Notice particularly the long twisted
chain that lies across the western part. Between this and a shorter
range, close to the west shore, runs a broad, dark valley, with the
crater Dawes lying in the middle of it at the upper end. Some of these
winding ridges suggest by their shape and modulation the action of
water. Finally, let us return to the strait through which we recently
passed. Notice that the Apennines and the Caucasus look as though they
had once formed a continuous line of mountains, which has been broken
through in its center, leaving huge buttresses on each side, like the
Pillars of Hercules at the Strait of Gibraltar?”
“That place has an irresistible attraction for me,” said my companion.
“I cannot withhold my imagination from picturing the scene there when
the waters rolled deep over those great bottoms, and when white-sailed
ships were passing and repassing between the towering capes, carrying
the commerce of opulent cities situated along those marvelously
picturesque shores.”
“Perhaps,” I suggested, “the lunarians, whom you have reconstructed in
your fancy, reached, before the catastrophe came that ended their
existence, a higher state of civilization than ours, and learned to
substitute electrically driven vessels for white-winged ships.”
“That would be like the introduction of vulgar steamboats on the canals
of Venice,” she replied.
“Well,” I said, “this ends our survey and one month of photographic
journeying on the moon, and I am glad that you have finished it with so
pleasing a vision.”
* * * * *
Upon parting from my friend I left the photographs in her possession. A
few weeks later I received a letter from her in which she said:
“I have been studying and restudying those wonderful pictures of the
moon. I have ordered a telescope to be set up in my park near the elm,
and when it is ready I wish you to come and instruct me how to view the
moon for myself. I believe that I am becoming a learned and enthusiastic
selenographer, and those strange names—Gemma Frisius, Bullialdus,
Abulfeda, Abenezra, Rabbi Levi, Maurolycus, Fra Mauro, Sacrobosco,
Zagut, Cichus, Sulpicius Gallus—have established their fascination over
my mind. Theophilus no longer terrifies me with its formidable aspect,
and I spend hours poring over the _Mare Serenitatis_. But my fancy
remains faithful to the ‘Marsh of a Dream,’ which still represents for
me the culmination of lunar ideality.
“As to life on the moon, I find that I cannot be satisfied with a mere
grass theory. I am so well convinced that there must be something more,
that I no longer relegate my lunarians to an age antedating the
volcanoes. On the contrary, as soon as I get my telescope I am going to
look for signs of them and their doings in the present day, and willy
nilly, sir, you have got to aid me in the search.”
------------------------------------------------------------------------
APPENDIX
------------------------------------------------------------------------
APPENDIX
DATES, and age of the moon, when the twenty-one serial photographs were
made at the Yerkes Observatory, by Mr. Wallace, with the 12-inch
telescope and a special color filter constructed by him:
No. 1, February 19, 1904; Moon’s Age 3.85 Days
No. 2, September 24, 1903; Moon’s Age 3.87 Days
No. 3, July 29, 1903; Moon’s Age 5.54 Days
No. 4, November 24, 1903; Moon’s Age 5.74 Days
No. 5, July 1, 1903; Moon’s Age 6.24 Days
No. 6, November 26, 1903; Moon’s Age 7.75 Days
No. 7, July 2, 1903; Moon’s Age 7.24 Days
No. 8, August 31, 1903; Moon’s Age 9.22 Days
No. 9, August 2, 1903; Moon’s Age 8.97 Days
No. 10, November 30, 1903; Moon’s Age 11.78 Days
No. 11, December 1, 1903; Moon’s Age 12.98 Days
No. 12, September 4, 1903; Moon’s Age 13.27 Days
No. 13, September 5, 1903; Moon’s Age 14.40 Days
No. 14, August 26, 1904; Moon’s Age 15.65 Days
No. 15, August 28, 1904; Moon’s Age 17.41 Days
No. 16, August 29, 1904; Moon’s Age 18.62 Days
No. 17, October 10, 1903; Moon’s Age 20.06 Days
No. 18, September 29, 1904; Moon’s Age 20.50 Days
No. 19, August 16, 1903; Moon’s Age 23.81 Days
No. 20, August 17, 1903; Moon’s Age 24.84 Days
No. 21, August 19, 1903; Moon’s Age 26.89 Days
------------------------------------------------------------------------
INDEX
------------------------------------------------------------------------
INDEX
Abenezra, 205.
Abulfeda, 200.
Air on the moon, 230.
Albategnius, 78.
Aliacensis, 78, 205.
Almamon, 205.
Alps, 97.
remarkable valley in, 225.
Altai Mountains, 72, 200.
chain of small craters near, 200.
Animal life on the moon, 228.
Apennines, 10, 96, 104, 137, 143, 222, 232.
Apennines and Caucasus, strait between, 234.
Apianus, 205.
Archimedes, 143, 232.
Aristarchus, 117, 153.
astonishing brilliance of, 112.
cause of brilliance of, 155.
possible composition of, 113.
Aristoteles, 76, 138.
Atlas, 70.
Atmosphere, lunar, 31.
disappearance of, 228.
what has become of it, 32.
Atmospheres, how they may grow or decrease, 230.
Azophi, 205.
Barry, 189.
“Bay of Rainbows,” 109, 111, 142, 157, 158.
Bessel, 221.
Bianchini, 110.
Birt, 188.
Blancanus, 78, 194.
Bonpland, 189.
Bubbles on the moon, 186.
Bullialdus, 180-182, 183.
mountains near, 183.
Byrgius, 147.
Calippus, 223.
Campanus, 185.
Capuanus, 185, 196.
Carpathian Mountains, 215.
Cassini, 225.
Catharina, 72, 138, 199, 206.
remarkable valleys near, 199-200.
Caucasus Mountains, 97, 223.
Caucasus and Apennines, strait between, 234.
“Cavern life” on moon, 229.
Cichus, 185, 196.
Clavius, 99, 101, 146, 192.
length of great wall of, 193.
Cleft near shore of _Mare Nubium_, 186.
Clefts on moon, resemblance of, to geological “faults,” 187.
Cleomedes, 56.
Color on moon, 155.
Colorado Cañon, possible likeness of, to cleft on moon, 118.
Copernicus, 102, 211.
an argument for the volcanic theory, 212.
buried ring near, 217-218.
crater pits near, 213.
hexagonal form of, 212.
Craters, lunar, formation of, ascribed to falling masses, 201.
theory of, advanced by W. K. Gilbert, 200.
Cyrillus, 72, 138, 199, 206, 210.
“Dark woman” in moon, 133.
Darwin, Prof. George, 6.
Dawes, 224.
Day and night on moon, 63.
Day on moon, length of, 95.
Earth, light of, on moon, 50.
shadow of, effects of, as shown by Prof. William H. Pickering, 28.
shadow of, its size and length, 18.
Eclipse, lunar, 17-18.
earth’s atmosphere and, 27.
moon visible during, 26.
Eclipses, lunar, colors visible during, 29.
fix historic dates, 29.
number of, in year, 42.
when annular, 41.
Eclipses, why they do not occur every month, 38.
solar, different from lunar, 27.
number of, in year, 42.
why of greater scientific value than lunar, 28.
Endymion, 70, 132.
Eratosthenes, 104.
Eudoxus, 76, 138.
Fracastorius, 73.
Fra Mauro, 189.
Full Moon in winter, why it runs high, 170.
Furnerius, 63.
Gases, escape of, from moon, 229.
from planets, 33.
molecular velocity of, 33, 229.
Gassendi, 117.
Gauricus, 196.
Gay Lussac, 216.
Gemma Frisius, 202.
“Geology” of moon, 67.
“Giantism” on the moon, 193.
Gilbert, W. K., theory of lunar craters advanced by, 200.
Gravitation on moon, 34-36.
Grimaldi, 121, 132.
Hæmus Mountains, 92, 220.
Hansen, theory of, in regard to the other side of moon, 139.
Harbinger Mountains, 118.
“Harvest Moon,” 171.
Heat of moon compared with that of sun, 164.
Helen of Troy, 92.
Hell, 196.
Hercules, 70.
Herodotus, 112, 158.
cleft near, 118.
Herschel, Sir William, his supposed discovery of active lunar volcano,
112.
Hipparchus, 78.
Hoar frost on moon, 34.
“Hunter’s Moon,” 171.
Inhabitants of moon, 31-32, 88, 95.
would not be visible with most powerful telescope, 120.
why they might be giants, 193.
Kepler, 114, 141, 152.
Kies, 185.
_Lacus Somniorum_, 93, 219.
Landsberg, 217.
Langrenus, 56, 123, 132.
Lava floods on moon, 73, 152.
Libration, effects of, on visibility of parts of moon, 57-58, _Note_.
Life on the moon, 230.
Light of moon compared with that of sun, 163.
Lindenau, 202.
Linné, changes in, 233.
observations of, by Professor Pickering, 233.
observations of, by Schmidt, 233.
Loewy and Puiseux, opinion of, on lunar “seas,” 220.
Longomontanus, 101, 146, 150, 195.
Lubiniesky, 184.
Lunar charts, their relative accuracy, 161.
Lunar history, 62.
Lunar month, 12.
Lunar nomenclature, 59, 147.
number of objects bearing names, 161.
singularity of, 203.
Lunar plains, darkening of, near sunset, 144.
Lunar sickle, rule to determine position of, 168.
Lunar vegetation, 32.
Maginus, 100, 146, 196.
Manilius, 122, 222.
Marco Polo, 222.
_Mare Crisium_, 52, 55.
_Mare Fœcunditatis_, 65.
_Mare Frigoris_, 76.
_Mare Humorum_, 106, 117, 150.
_Mare Imbrium_, 103, 142, 144, 224.
irregularities in the floor of, 232.
islands in, 231.
_Mare Nectaris_, 68, 135.
_Mare Nubium_, 102, 148, 180.
_Mare Serenitatis_, 74, 135, 219, 232.
bordering cliffs of, 220.
ridges in the floor of, 234.
_Mare Tranquillitatis_, 74, 135.
_Mare Vaporum_, 103.
“Marsh of a Dream,” 68, 91.
Mass of moon, 35.
Maurolycus, 75, 142, 204.
Menelaus, 92, 221.
Mercator, 185.
Month, lunar, 12.
Moon, birth of, and Pacific Ocean, 6.
distance of, from earth, 8.
greatest distance of, from earth, 8.
influence of, on growth of vegetation, 163.
least distance of, from earth, 8.
life on, 230.
other side of, 139.
seen by daylight, 149.
separation of, from the earth, 6.
turning of same face of, toward earth, 19.
Moon and earthquakes, 165.
Moon and the weather, 164.
Moon and volcanoes, 165.
“Moon Maiden,” 10, 96, 103, 122.
Mount Hadley, 222.
New Moon, photograph of, 49.
New Moon and the weather, 169.
Newton, deepest crater on moon, 107.
Nicollet, 188.
Night, advance of, over moon, 131.
length of, on moon, 95.
lunar, coldness of, 136.
Nodes of moon and their revolution, 38-39.
_Oceanus Procellarum_, 105, 150.
submerged rays in, 151.
Orbit of moon, 8.
Origin of moon, 6.
Pacific Ocean and birth of moon, 6.
_Palus Somnii_, 68.
Path of moon about sun, 16-17, _Note_.
Petavius, 62, 123.
Phases of moon explained, 13-16.
Phlægrean Fields, 60.
Photographs, lunar, 47-49.
in series, 57.
peculiar tone of, 145.
why they are reversed, 52.
Photometry, lunar, 153-154.
Piccolomini, 72, 200.
Pickering, Prof. William H., observations of Linné by, 233.
on effects of earth’s shadow on moon, 28.
opinion of, regarding changes in Plato, 226.
theory of Tycho’s rays by, 126.
Pico, 231.
Pitatus, 186, 195.
Plato, 105, 114, 132, 225.
changes observed in, opinion of Prof. William H. Pickering regarding,
226.
vegetation in, 227.
Plinius, 224.
Poetry of the moon, 96.
Posidonius, 224.
Prism binoculars for viewing moon, 122.
Proclus, 68.
remarkable brilliance of, 90.
Projectile force on moon, 37.
Promontorium Ænarium, 189.
Promontory Acherusia, 224.
Promontory Heraclides, 109.
Promontory Laplace, 109.
Ptolemæus, 98.
Pytheas, 216.
Rabbi Levi, 202.
Reinhold, 217.
Riccioli, nomenclature of, for lunar objects, 69.
Rings, ranges of, on moon, 97-98.
Riphæan Mountains, 190.
Rotation of moon “braked” by tides, 20.
Rotation of moon, illustrated, 19-20.
Sacrobosco, 206.
Schickard, 115.
Schmidt, observation of Linné by, 233.
Schröter, “lunar city” discovered by, 218.
“Seas,” lunar, character of beds of, 142.
Shadow of moon during eclipse, 40.
length of, 40.
Shadows on moon, topography revealed by, 100-101.
used to measure heights on moon, 63.
Sidereal revolution of moon, 11.
_Sinus Æstuum_, 103.
_Sinus Iridum_, 109.
_Sinus Medii_, 103.
_Sinus Roris_, 159.
Size of moon, 8.
Sky as seen from moon, 29.
Snow, can it exist on the moon? 69.
Snow, non-existence of, on moon, 69.
South polar region of moon, 77.
South pole of moon, 106.
Stadius, 218.
Stöfler, 75.
row of rings near, 204.
“Straight wall,” 187.
Strait between Apennines and Caucasus, 234.
Sulpicius Gallus, 222.
Sunrise, slow progress of, on moon, 94.
Sunshine, effects of, when vertical on moon, 100.
Surface of moon, nature of, 9.
size of, 9.
Synodic revolution of, 11-12.
Tacitus, 200.
Telescopic power, limits of, 120.
Teneriffe Mountains, 231.
“Terminator,” meaning of, 63.
Theætetus, 225.
Theophilus, 71, 138, 198, 206.
crater pits near, 211.
grandeur of appearance of, when highly magnified, 207.
great mountain fold near, 207-208.
what would be seen from wall of, near sunset, 208.
Tidal attraction, forcing moon away in reaction of, 6.
“Tidal friction,” 20.
Tides, causing of, by moon and sun, 21.
effect of earth’s rotation on, 25.
service rendered by, 22.
simultaneous occurrence of, on opposite sides of earth, 23.
when highest, 21.
when lowest, 22.
Twilight, why none on moon, 64.
Tycho, 10, 98, 106, 146, 191.
longest ray of, 93, 221.
ray system of, 124.
rays of, 99.
theory of Nasmyth, 124-125.
theory of Pickering, 126.
Vegetation on moon, 32, 115, 142, 144, 227.
Vendelinus, 59, 123.
Venus seen by daylight, 149.
Verne, Jules, “Trip to the Moon,” 4.
Volcanic craters on moon, 60.
Volcanoes, lunar, 54.
contrasted with those of earth, 84.
effects of gravitation on size of, 86.
Water on the moon, 53.
“Weathering” on the moon, 197.
Weight on the moon, 36.
Werner, 78, 205.
Wilhelm I., 102, 195.
Zagut, 202.
------------------------------------------------------------------------
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