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.’”