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.
]