THE MOON
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[Illustration:
COPERNICUS AND THE CARPATHIAN MOUNTAINS.
]
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+THE MOON+
+A POPULAR TREATISE+
_By_
GARRETT P. SERVISS
[Illustration]
+D. APPLETON AND COMPANY
NEW YORK+
1907
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COPYRIGHT, 1907, BY
D. APPLETON AND COMPANY
_Published October, 1907_
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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.
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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
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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
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INTRODUCTION
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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.”