days. The sunset line has advanced to the borders of the _Mare Nectaris_
and the _Mare Tranquillitatis_. Toward the south a vast region which was
very brilliant in the morning and midday light with the reflections from
mountain slopes and the rays of Tycho, has passed under the curtain of
night. The great crater rings on the eastern border of the _Mare
Nectaris_, and thence upward to the South Pole, are beginning to
reappear, but with the shadows of their walls thrown in a direction
opposite to that which they assumed before. By a little close inspection
you will recognize Theophilus and its neighbors which were so
conspicuous for many days while the sunrise was advancing, but which
have been almost concealed in the universal glare of the perpendicular
sunshine since the Full Moon phase was approached. On the _Mare
Tranquillitatis_ and the _Mare Serenitatis_ it is late afternoon, and
your favorite ‘Marsh of a Dream’ has become a true dreamland.”
“This oncoming of night,” said my friend, “seems to me more imposing,
and more suggestive of mystery than was the advance of day.”
“Surely it is. Do we not experience similar sensations when night
silently creeps over the earth? But it imparts a feeling of loneliness
and desolation when we watch it swallowing up the barren mountains and
plains of the lunar world that we do not experience in terrestrial life.
There are no cheerful interiors on the moon to which one can retreat
when darkness hides the landscapes. There is another thing about the
lunar night to which I have made but scant reference thus far. I mean
it’s more than Arctic chill. Imagine yourself standing there in the
midst of the broad plain of the _Mare Tranquillitatis_. Toward the east
you would see the sun close to the horizon, yet blazing bright and hot,
without clouds or mists to temper its rays. The rocks or soil beneath
your feet would perhaps be cold to the touch, because the surface of the
moon radiates away the heat very quickly, but your face and hands would
be almost scorched by the intense solar beams. Looking toward the west
you would see the shining tips of mountains suddenly extinguished, one
after another, and when the sharply defined edge of the advancing night
passed over you it would be as if you had plunged into a cold bath. In a
little while, if you remained motionless, you would be frozen. No
clothing would suffice to keep you warm. Nothing that polar explorers
have ever experienced can be likened to the cold of the lunar night.
Only the apparatus of the laboratories for producing temperatures,
capable, when combined with pressure, of liquifying and solidifying the
air itself, can bring about upon the earth a lowering of temperature
comparable with that which occurs during the lunar night.”
“But I do not exactly see why night should be so much colder on the moon
than on the earth. She is not farther from the sun.”
“No, her average distance from the sun is the same as that of the earth.
The reason why her nights are so cold is to be found in the absence of
an atmosphere like ours. The air is the earth’s blanket, which serves a
double purpose, tempering the heat by day with its vapors and winds, and
keeping the earth warm at night by preventing the rapid radiation into
space of the heat accumulated during the daylight hours. If there is any
atmosphere at all upon the moon—and I shall tell you by and by what has
been learned on that subject—it is so rare as compared with ours that it
can exercise very little effect upon the temperature of the lunar
surface.
“Now, look at the great range of the lunar Apennines. You will see that
the eastern faces of these mountains are in the sunlight, and they cast
no shadows, as they did in the lunar morning, over the _Mare Imbrium_.
The same is true of the lunar Caucasus, and the lunar Alps. All of these
mountains are very steep on the side facing the plains, and that is the
side presented sunward in the lunar afternoon. By turning to photograph
No. 16, we shall see this phenomenon more clearly displayed. This
photograph, measured by the age of the moon when it was taken, is more
than a day older than the other, but once again the effect of libration
has, in part, counteracted for us the advance of the line of sunset.
Still it has distinctly advanced. You will observe that it has now
passed completely across the _Mare Nectaris_, and more than half across
the _Mare Tranquillitatis_, while only the mountain tops along the
western edge of the _Mare Serenitatis_ remain to indicate its outlines
in that direction. Theophilus, Cyrillus, and Catharina, on the eastern
border of the _Mare Nectaris_, have again become very conspicuous, but
this time in evening instead of morning light. See how sharply the
western wall of Theophilus stands out against the darkness of night
behind it, and how its central peak glows in the setting sun while all
the vast hollow beneath it is black. The floors of Cyrillus and
Catharina, being less profoundly sunken, are still illuminated. Below
the _Mare Serenitatis_, the twin rings, Aristoteles and Eudoxus, are
very conspicuous, and they show the same change of illumination as
Theophilus, their western sides being strongly illuminated on their
inner faces, while the eastern walls cast shadows into the interior. The
mountainous character of the surface in the neighborhood of the North
Pole of the moon seems to be more clearly brought out in evening than in
morning light. In this picture the North Polar Region seems to be almost
as much broken up with gigantic rings as is that surrounding the South
Pole. In both cases, you observe, many of the rings are poised just on
the edge of the lunar disk, and their libration alternately swings them
in or out of view.”
[Illustration:
NO. 16. AUGUST 29, 1904; MOON’S AGE 18.62 DAYS.
]
“Then the other side of the moon may not be very different from the side
that is turned toward us.”
“In its general features I doubt if it is at all different. There was
once a theory, which had considerable vogue, that the side of the moon
turned away from the earth presented a great contrast with its earthward
side. A German mathematician, Hansen, drew conclusions, which are no
longer accepted, as to the form of the moon. He thought that the moon
was elongated in the direction of the earth, somewhat like an egg, her
center of figure being about thirty miles nearer to us than her center
of gravity. This, if true, would make the part of the lunar surface that
we see lie at a great elevation as compared with the other part, and the
center of gravity being toward the other side would cause the atmosphere
and water to gravitate in that direction.”
“What a pity that so interesting a theory should have been abandoned!”
“If interest were the only test of the value of a scientific theory
knowledge would not advance very fast. Notice how this very photograph
before us vindicates the true scientific attitude toward nature. It
records all the facts within its range, and leaves the theories to us.
The features of your ‘dark woman’ are, in their way, as clearly marked
in the photograph as is the range of the lunar Apennines. It is for us
to recognize the essential difference between the interpretations which
we choose to put upon these two phenomena. Giving play to fancy, we see
the figure of an old woman in the one case, and employing our reason we
find a chain of unmistakable mountains in the other.”
“But surely you do not mean to aver that science has no other business
than that of recording facts.”
“By no means. It is also the business of science to find hypotheses and
to build up theories that will explain its facts and connect them
together systematically, according to some underlying law. But as I have
just intimated it is the mark of true science that it never retains a
theory merely because it is interesting. The truth is the only
touchstone. Still, even the most conscientious scientific investigator
may be misled by his imagination. His greatest virtue is that he never
lets his fancies deceive him after he has recognized their false
character. Point out your ‘dark woman’ to the child, or the savage, and
it will be in vain afterward to explain that her profile is made up of
plains and mountains. The child and the savage are not scientific but
imaginative, and only after a long education will they abandon the
apparent for the real.
“I will ask you now to take up photograph No. 17. The age of the moon
here is twenty days. Comparing it with the last photograph we see that
Theophilus has disappeared, although Cyrillus and Catharina, being a
little farther east, are yet visible. Half of the _Mare Serenitatis_ is
buried in night, and only a little of the eastern edge of the _Mare
Tranquillitatis_ remains visible. Aristoteles and Eudoxus are now very
close to the terminator, and the shadows of their eastern walls are
spreading farther over their floors. Aristarchus is very brilliant, as
it is still early afternoon on that part of the moon, and the sunshine
is intense. Observe that Kepler, the crater ring directly east of
Copernicus, has become more conspicuous than we have seen it in any
preceding photograph. This is especially true of the system of bright
rays surrounding it, and it is due to the change of illumination. In the
southern part of the moon, west of Tycho, you will now recognize many
gigantic formations which we first saw when the sun was rising over
them. Some of them are even more prominent in the sunset light. Among
these is our old acquaintance Maurolycus, whose western wall is so
brilliant that it resembles a tiny crescent moon. The double row of
broad, dish-shaped walled plains along the central meridian has also
become visible once more. In fact the amount of delicate detail and the
sharpness of the definition in these photographs are very remarkable.
Observe the curious mottling of the ‘seas.’ It is in some of the
differences of tint, which correspond in telescopic views of the moon
more or less closely with the varying shades in the photographs, that
some selenographers have thought they could detect evidences of the
presence of vegetation on the moon. We shall talk about that more in
detail another time. It is sufficient just now to notice that the beds
of the _mares_ are by no means uniform either in tint or in level. All
of them are more or less ‘rolling,’ like many of our prairies, and often
winding chains of hills and huge cracklike ravines are visible in them.
In this photograph the amount of detail shown in the _Mare Imbrium_ is
particularly striking. Notice how some of the crinkled rays from
Copernicus extend almost to the center of the ‘sea,’ and how in front of
the precipitous base of the Apennine range the lighter-colored ground,
with three prominent ring plains in it, presents the appearance of
shallows. Lying off the shore south of Plato and the Alps a number of
isolated mountain peaks are seen, mere white specks on the gray
background. The undulating character of the ‘bottom’ of the ‘Bay of
Rainbows’ is also distinctly indicated. By the way, I should perhaps
mention the names of the three rings lying off the front of the
Apennines, for although they are among the most interesting on the moon
they have hitherto escaped our special attention. The largest of the
three is Archimedes, the second in size is Aristillus, and the smallest
is Autolycus. You will hear of them again when we come to the large
photograph of the _Mare Imbrium_ and the _Mare Serenitatis_.
[Illustration:
NO. 17. OCTOBER 10, 1903; MOON’S AGE 20.06 DAYS.
]
“Let me now prepare you for an almost dramatic change in the appearance
of some of the most conspicuous lunar features which will take place
when we pass from this photograph to No. 18. Direct your attention
particularly to the chain of the Apennines. In No. 17 it lies very
brilliant in the sunlight, with its western slopes distinctly visible,
rising gradually from the shores of the _Mare Serenitatis_ and the _Mare
Vaporum_, while the ‘sea’ along its eastern front is bright with day. In
No. 18 the Apennines have become simply a chain of illuminated mountain
tips with comparative darkness all around them. Their western slopes are
practically invisible, the _Mare Imbrium_ on the east has turned dark,
as if twilight had fallen over it—although as I have told you there is
no twilight on the moon—and at its northern end the great range, with
only its summits illuminated, projects like a row of electric lights far
into the black night that has covered the plains beneath.
“Yet, although the _Mare Imbrium_ has turned so dark as to be barely
visible over its western half, the sun has by no means set upon it, and
the darkness is perhaps greater than it should, theoretically, be under
the circumstances. This phenomenon of the rapid darkening of the great
lunar levels as the sun declines is one of the arguments that have been
found to favor the hypothesis of the existence of vegetation. If, for
the sake of discussion, we admit the possibility of vegetation growing
on the lunar plains, it will be interesting once more to compare
photographs Nos. 17 and 18.”
“Don’t say that it is merely for the sake of a discussion,” interrupted
my friend. “I shall be far more deeply interested if you will simply say
that it may be true.”
[Illustration:
NO. 18. SEPTEMBER 29, 1904; MOON’S AGE 20.50 DAYS.
]
“Very well, let us put it that way, then. As I was remarking, if we
again compare the two photographs, keeping the vegetation hypothesis in
view, we may ascribe at least a part of the rapid darkening of the plain
of the _Mare Imbrium_ to a change in the color of the—what shall I say,
grass?—covering it.”
“Good! good!” exclaimed my friend, clapping her hands. “Just listen to
him! After gravely rebuking me so many times for my unscientific faith
in the lunar inhabitants of a long past age, now you are talking of
‘grass’ on the moon.”
“You are hardly fair,” I protested. “It is you who have just led me to
make an admission which many astronomers would laugh at, and you ought
to support me with all the brilliance of your imagination when I try to
picture a state of things so consistent with your predilections about
the moon.”
“Oh, I do support you with all my heart!” she replied. “Pray go on, and
tell me about the lunar grass.”
“Not just at present,” I said. “We are going to take that subject up
again, and I may then succeed in convincing you that there is far more
evidence for believing that vegetation exists on the moon in the present
day than for believing that intellectual beings inhabited it at some
unknown former period. I should warn you, too, that I have been using
the contrasts of light and darkness between these two successive
photographs simply as an illustration of what occurs in visual
telescopic views; but that, for some reason, the lunar plains nearly
always appear darker in photographs when contrasted with the mountainous
regions than they do when viewed with the eye. Owing, also, to a variety
of influences two successive photographs of the moon may differ in tone
when the eye would detect no corresponding difference. All this,
however, does not invalidate what I have said about the lunar ‘seas,’ or
plains, darkening near sunset more rapidly than we should expect them to
do, as a simple result of the low angle at which the sunlight strikes
them.
“You will notice that the waning of day between photographs Nos. 17 and
18 has produced a remarkable change in the appearance of Tycho. Since
the Full Moon phase Tycho has resembled a button rather than a volcanic
crater, but now it has once more assumed the form of a very beautiful
ring with its central peak clearly shown, its western wall, bright and
its eastern wall casting a broad, black shadow. Most of the rays have
now disappeared, only two or three, running over the eastern hemisphere,
remaining visible. The immense walled plains near Tycho have again
become prominent, Maginus toward the southwest, Clavius toward the
south, and Longomontanus toward the southeast being the most
conspicuous. Clavius is always a wonderful object for the telescope, but
it is rather more interesting in the lunar morning than in the evening.
Away over near the eastern limb, where the sun is still high, Grimaldi
shows its dark oval, with a couple of mountain peaks on its western
rampart shining brilliantly. The small, dark spot below it, toward the
east, is in the walled plain, Riccioli. The bright spot with starlike
rays, a long way south of Grimaldi, and east of the _Mare Humorum_, is
Byrgius, a walled plain near which exists a small system of bright
streaks resembling those surrounding Copernicus and Kepler, but much
less extensive.”
“Do you recall my expression of impatience this morning when you were
giving me the names of a long string of crater rings?” said my friend,
smiling. “Well, I am now going to make a confession. Perhaps it is
slightly of a penitential nature. I find now that these names, although
they certainly are far from picturesque in most cases, begin to interest
me, because, I suppose, I understand better the character and meaning of
the things that they represent. The ceaseless Latin terminations no
longer annoy me, for I do not think of them, but of the things
themselves.”
“It is always so,” I replied, “whenever one takes up a new study. I know
that you have dipped a little into botany, and I am sure that the Latin
names which abound in that science must have repelled you at first. But
after a time, when you had begun to recognize the beautiful flowers and
the remarkable plants for which they stood, you found that even these
names assumed a new character and became interesting and memorable. You
will find it the same if you continue to study the moon. The most stupid
designations will derive interest from their applications.”
“Yes, that is no doubt true. Still, I wish that Riccioli had possessed a
little more imagination.”
“Be thankful, then, that he did not name the lunar ‘seas’ and ‘bays.’
You must now bid good night to your ‘dark woman.’ You observe that the
_Mare Nubium_ is beginning to fall under the shadow, and that her
features are growing indistinct. If you will turn the photograph upside
down you will find that the Moon Maiden has retired. She belongs
exclusively to the western hemisphere, and it is only the eastern
hemisphere of the moon that now remains visible to us, for we are close
to the phase of Last Quarter. This is an aspect of the moon with which
you may not be very familiar. To see the moon at Last Quarter, and
particularly after she has passed that phase, we must rise near midnight
and devote the early morning hours to observation. During these later
phases, however, one may see the moon in the heavens during the daytime
all through the forenoon and a part of the afternoon. She is a very
beautiful object then, although few persons, I fear, ever take the
trouble to look at her. The lighter parts of her surface assume a
silvery tint in the daylight, and the dark plains seem suffused with a
delicate blue from the surrounding sky. Exquisite views of the moon may
then be obtained with a telescope. The glare of reflected light from the
mountains and crater rings, which dazzles the eye at night, is so
reduced that the telescopic image becomes beautiful, soft, and pleasing.
The same principle has been very successfully applied in recent years to
the study of the planet Venus. Her atmosphere is so abundant, in
contrast to what we find on the moon, that she is as blinding in a
telescope as a ball of snow glittering in full sunshine; but when seen
in the daytime, her features, indistinct at the best, may be more
clearly discerned.”
“Oh, you interest me deeply! If Venus is supplied with such an abundance
of air, I suppose she is inhabited?”
“It is not exactly orthodox among those calling themselves astronomers
to talk of inhabitants on the planets, but I do not mind telling you
privately that I think that Venus is most likely a world filled with all
kinds of animate existences. Our present business, however, is with the
moon, and I must recall your attention to the photographs. We shall next
take up No. 19. Here the crescent shape becomes again evident, but
reversed in position as compared with the crescent of the new and waxing
moon. Only two of the ‘seas’ now remain completely in view—the _Mare
Humorum_ and the _Oceanus Procellarum_.”
“That term I think you have translated as the ‘Ocean of Tempests.’ Pray,
do you know any reason why it should have been thus named?”
[Illustration:
NO. 19. AUGUST 16, 1903; MOON’S AGE 23.81 DAYS.
]
“There is not the slightest reason that I know of. You must ascribe it
to the vivid imagination of that old astronomer whom you so greatly
admire. I regret, sometimes, that he cannot be here to explain to you
the thoughts that occupied his mind. They must surely have been very
captivating, even though not very scientific. Remark that there are many
of the features of the eastern part of the moon which we can now discern
more clearly than in any of the preceding pictures. Beginning at the top
we see the vast inclosure of Longomontanus with the top of its
encircling walls illuminated, while the interior is all in deep shadow.
Its western rampart projects into the night and seems detached from the
main body of the moon. Along the terminator below Longomontanus, what
appears to be another immense walled plain presents a similar aspect.
This, however, consists of several smaller formations grouped near
together, only their loftiest points being illuminated. The steep
borders of the _Mare Humorum_ are finely shown. Notice how the floor of
that little ‘sea,’ which is about the size of England, as Mr. Elger has
remarked, is mottled with whitish spots, and how distinct the ring of
Gassendi appears at the northern end of the _mare_. You can even see the
comparatively small crater that crowns the northern wall of the ring.
Southeast of the _Mare Humorum_ are visible the great flat plains of
Schiller and Schickard. Notice also how all the surface of the moon in
that direction is freckled with crater pits, which resemble the
impressions made by raindrops in soft sand. But the smallest of these
pits is larger than the greatest volcanic crater on the earth.
“The _Oceanus Procellarum_ is beautifully illuminated in this picture.
In several places, particularly north of the _Mare Humorum_, parts of
_submerged_ rings are visible. These are great curiosities, and we shall
see more of them elsewhere. Some selenographers believe that they are
the remains of an earlier world in the moon, which was buried by a
tremendous upheaval and outrush of molten material from the interior.
You will remember, perhaps, that I spoke of a catastrophe of that kind
when pointing out the half-buried ring of Fracastorius at the southern
end of the _Mare Nectaris_.”
“Did that catastrophe occur after the formation of the huge lunar
volcanoes?”
“It is difficult to say just when it occurred, but the appearances
generally favor the view that it was subsequent to the great volcanic
age. It is the opinion of Mr. Elger, whom I have once or twice mentioned
as an English observer who has devoted special attention to the study of
the moon’s surface, that the _mares_, as we now see them, do not
represent the original beds of the lunar oceans. These beds, which,
according to this view, were at first deeper, have been covered up, at
least over a great part of their areas, by the outrush of molten lava.
If they were ever filled with water it was very likely prior to that
occurrence. But you must remember that all this is speculation, very
interesting, it is true, but based upon insufficient data to enable us
to be sure of our conclusions. I shall show you later that some recent
students of lunar phenomena have formed the opinion that there is a
strong argument to be drawn from geological analogies in favor of the
view that the lunar _mares_, practically in the state in which we see
them, have been true sea beds.