the most interesting of the series. Look at the crater ring Kepler, in
the midst of the _Oceanus Procellarum_. We have not before seen it in
the aspect which it now presents. Hitherto it has appeared only as a
bright point surrounded by a light patch covered with radiating streaks.
But now, with the late afternoon sunlight striking across it, its walls
are illuminated in such a manner that its very perfect ring shows very
clearly, about half of the interior lying in shadow, which serves to
give it a striking relief. If we suppose a time when the _Oceanus
Procellarum_ was a real ocean, and when Kepler was an active volcano
rising above its waters, its situation, far from all shores, would have
been not unlike that of the great volcano of Kilauea in the Hawaiian
Islands. In that case we might assume that the streaks around it
represent ancient lava flows, which spread far about over the bed of the
ocean. The same explanation would apply to the streaks and rays around
Copernicus, and half a dozen other similar ring mountains.
“You will also observe that the afternoon slant of the solar rays has
considerably changed the appearance of Aristarchus. Now for the first
time the crateriform shape of that most remarkable mountain has become
evident on account of the shadow in the interior. This shadow has almost
reached the central peak which is the brightest part of the entire
formation. You may be interested in the fact that the brilliance of the
central peak of Aristarchus is so great that it stands in an order by
itself, in what may be called the photometry of the moon’s surface. Ten
orders of relative brightness have been adopted to represent the various
reflective powers of different parts and spots of the moon. I copy them
from Mr. Elger’s list. They are as follows:
“0° = Black (example, the shadows of mountains).
“1° = Gray black (example, darkest places in the walled plains of
Grimaldi and Riccioli).
“2° = Dark gray (example, the floor of Endymion).
“3° = Medium gray (example, interior of Theophilus).
“4° = Yellowish gray (example, interior of Manilius).
“5° = Pure light gray (example, surface around Kepler).
“6° = Light whitish gray (example, walls of Macrobius).
“7° = Grayish white (example, Kepler).
“8° = Pure white (example, walls of Copernicus).
“9° = Glittering white (example, Proclus).
“10° = Dazzling white (sole example, the central peak of Aristarchus).”
“Really, I am greatly surprised by what you tell me,” said my friend. “I
would never have imagined that there were so many different neutral
tints on the moon.”
“You would be still more surprised,” I replied, “if I could present to
you a similar table of the different tints of color that have been
discovered there. But I am not aware that any scale of lunar colors has
been prepared. There are, however, various shades of brown, yellow, and
green. Most of them are found in the _mares_ and walled inclosures. Some
of them appear to be variable, and some are only to be detected under
particular illuminations.”
“Are not such colors an indication of something living there?”
“It may be so—an indication, for instance, of the existence of ‘lunar
grass,’ the mention of which so amused you a little while ago.”
“Oh, it was not the ‘grass’ that amused me, but your unexpected way of
introducing it. I _want_ to be convinced that there is grass there, and
a great many other things besides grass. But I am not yet satisfied
concerning that unique peak in Aristarchus. ‘Dazzling white’ you say is
its description in the scale of tints. That excites my curiosity
immensely. I think you have told me already that it cannot be snow, but
you have spoken of the possibility of crystals and of metal. Do you
know, I like the idea of ascribing the phenomenon to metal. It recalls
something that I read in childhood about the first discoverer of a
silver mine in Mexico. As I remember the story, an Aztec hunter, chasing
his game across a mountain, seized upon a bush to aid him, and the roots
giving way disclosed a glittering mass of silver. Why not let me imagine
that the peak of Aristarchus is composed of pure silver?”
“There is no harm in imagining that if you wish to do so. But then your
imagination, or rather your knowledge, should go a little farther and
recall the fact that silver does not remain dazzling bright when
exposed.”
“Ah, but you say there is no air, no water, no rains, no moisture on the
moon. Under such circumstances might not a metal remain bright?”
“It is possible, but I hardly think that it would. It is likely that
other corroding influences exist. A better explanation, I think, is
afforded by supposing that the reflecting surface is simply composed of
a rocky mineral, resembling in its power of reflection a mass of quartz
crystals or imbedded planes of mica. There is no absolute impossibility
involved in thinking that it may be simply white rock.”
“Why not say marble—a gigantic Carrara mountain on the moon?”
“I fear that that would involve a geological history for the lunar world
for which we have not sufficient warrant in observed facts. I prefer to
assume a volcanic origin for the phenomenon. Since you are so interested
in the mystery of Aristarchus I may add that a part of the floor and the
inner side of the ring are also extremely bright, but not quite so
bright as the central peak. That alone stands at the top of the scale.
Putting the peak at 10°, Mr. Elger finds that the other brilliant parts
of Aristarchus possess only 9½° of brightness. Yet the whole interior is
so glistening that when the sunlight falls vertically it almost
resembles the inside of a crystal cup, and details are hidden in the
glare.
“Now please look at the ‘Bay of Rainbows’ in the photograph before us.
Cape Laplace at its western end lies close to the terminator and appears
as a minute speck of light. The great bow-shaped shore is clearly
defined, the level surface within being very dark and the highlands
around it comparatively bright. The crater mountain Bianchini you will
recognize near the center of the bow. Several other similar crateriform
mountains are visible toward the north and east. In this light the
surface of the moon eastward from the North Pole appears as rough and
broken with craters and crater plains as we saw in the earlier pictures
that it is toward the west.
“Before directing our attention to photograph No. 20, let us return for
a moment to Aristarchus. When speaking of that formation a few minutes
ago I interrupted myself in order to give you the scale of tints on the
moon, which demonstrated the unique brilliance of the peak inclosed by
the ring. I intended to point out to you then the fact that in
photograph No. 19 we see, for the first time, not only the ring of
Aristarchus but its curious neighbor Herodotus. A light streak, which we
observed in an earlier picture, seems to connect the two. It is better,
however, to notice this now because in turning from No. 19 to No. 20 you
will perceive once more a change in the appearance of Aristarchus and
its neighborhood. In No. 20 Aristarchus is distinctly more conspicuous.
The night has advanced during almost exactly twenty-four hours, having
in the meantime swept across the entire length of the ‘Bay of Rainbows,’
which we now no longer see. If we had been using a telescope during that
interval we should have beheld a very interesting spectacle, for sunset
on the ‘Bay of Rainbows’ is quite as remarkable, although in a very
different way, as sunset on the Bay of Naples. The astronomer, seated
amid the lonely gloom of his observatory dome, and watching the change
of light and illumination on the surface of the moon, has many an hour
of solitary enjoyment of aspects of nature that are quite impossible on
the earth, and that frequently lure him into poetic meditations which
find no place in his notebook.”
“I am very glad to hear you say that. It enhances my opinion of the
astronomers, and convinces me that after all they are not so severely
scientific as they describe themselves.”
[Illustration:
NO. 20. AUGUST 17, 1903; MOON’S AGE 24.84 DAYS.
]
“If they were,” I replied, “or if all of them were, it would be a bad
augury for the future of their science. Do not think that in
occasionally seeking to restrain your imagination I wish to express
condemnation of what, after all, is the noblest of human faculties. But
again we are forgetting our principal business, which is with the facts.
Aristarchus, as I have said, has undergone another distinct change of
appearance from that which it showed before. The central peak is now
covered by the shadow of the eastern wall, but still the reflection from
the western wall alone is sufficient to make it the brightest spot on
the moon. Herodotus, on the other hand, has become indistinct and the
Harbinger Mountains are practically invisible, but we can detect the
existence of the enormous chasm or cañon, which I told you once issues
from the interior of Herodotus and goes winding nearly a hundred miles
over the floor of the _Oceanus Procellarum_.
“Notice, also, how clearly visible three or four relatively small
craters east of the ‘Bay of Rainbows’ have become, and how conspicuous
are several large walled plains on the northern ‘horn.’ The dark level
south of these formations and between them and the small craters has
also a name which I have not before mentioned. It is the _Sinus Roris_,
‘Gulf of Dew.’ It connects the _Mare Frigoris_ with the _Oceanus
Procellarum_. It is another legacy from your friend the imaginative
astronomer.”
“Then once more he receives my thanks for having done his best to make
the moon an ideal world. It is always painful to have one’s ideals
destroyed.”
“I hope that I have not been destroying any of yours.”
“No, but at least you have caused a change in my impressions about the
character of the moon. Henceforth there will be an element of terror as
well as of unexpected grandeur mingled with my thoughts of the ‘Queen of
Night.’”
“That element will not be diminished by what I am about to point out.
Look far over near the eastern border of the _Oceanus Procellarum_,
directly east of Aristarchus. There you will distinguish the outlines of
two or three vast submerged ring plains, which we may regard as relics
of that earlier lunar world, which preceded the outgush of lava that Mr.
Elger thinks covered the sea bottoms. Observe also the singular light
streak that runs from Kepler, now barely visible at the edge of night,
to a dark little crater, beyond which lies a bright point off the coast
of the ‘ocean.’ South of this there are other submerged ring plains, one
of which, named Letronne, has a high western wall, which forms in the
picture a sort of promontory projecting from the southern border of the
_Oceanus Procellarum_, almost directly north of Gassendi. The latter is
very clearly shown at the lower end of the _Mare Humorum_, the western
side of which is in shadow, while its whole surface has turned very
dark. On the southern horn of the crescent the ring plains, Schickard
and Schiller, are still prominent, and the northern and eastern edges of
the _Mare Humorum_ appear more ragged with mountains and crater rings
than before.”
“And have all these mountains and craters names?”
“Not all of them, but many more, perhaps, than you suppose. On the whole
visible surface of the moon about 500 objects, not including the ‘seas,’
have received names. It may surprise you to learn that the position of
the most important of these objects has been ascertained with an
accuracy which is still lacking in our determination of positions on the
earth. In other words our charts of the moon are more exact than those
of our own planet.”
“That does indeed surprise me. I should have thought that, living on the
earth, we could make very correct maps of it, while, as for the moon,
two or three hundred thousand miles away, it seems to me not so easy to
do that.”
“It is mainly because we are on the earth that we find such great
difficulty in making accurate maps of it. We cannot look at the earth as
a whole, but we have to crawl over its surface, making measurements as
we go, and afterwards translating those measurements into lines and
angles on paper. Thus we are still uncertain about the precise distance
between many important points on our globe, while for points on the moon
no corresponding uncertainty exists. The moon hangs before us in the
sky, with no clouds except those in our own atmosphere to obscure it,
and it is only necessary carefully to observe the position of particular
points, and with the proper instruments to measure their distance and
directions from one another. But even this is not a thing that can be
accomplished without much pains and much knowledge. The astronomer, no
matter what field he chooses, is necessarily a hard worker, and his
motto, above everything else, is accuracy. No one is more tempted than
he by the sublimity and the extraordinary character of the objects of
his study, to give rein to the imagination, and yet imagination is the
thing of all others from whose vagaries he must most carefully guard
himself. So you must not blame him too severely if he has not dotted the
shores of the moon with cities, and populated its plains with
industrious farmers.”
“If you will permit me to wander a little aside from our photographic
studies for a few minutes,” said my friend, “I should like to ask you
about two or three things concerning the moon which have long puzzled
me. From my earliest days, living the greater part of the time in the
country, I have heard that the moon exercises a decided influence over
the weather, and over the growth of vegetation. I have neighbors who
would never think of planting certain things except ‘in the New of the
moon’! Some will not cut timber except ‘in the Old of the Moon,’ as they
say that the sap is drawn up by the moon’s influence when she is
growing. Is there really any truth in all this?”
“Not the least. At any rate there is no scientific evidence whatever for
such statements, and no probability that they are based on facts. They
are the result of faulty observation, misled by coincidences. It is
_imaginable_ that the light of the moon might have some influence upon
vegetable growth if it were an original kind of light coming from the
moon herself. But moonlight is only reflected sunlight, and when we
examine it with the spectroscope we do not find that the rays of light
in visiting the moon and returning thence to the earth have had either
anything added to or anything taken away from them, except intensity.
The total amount of light reflected from the moon upon the earth is
estimated to be about 1/618000 of the total amount that comes to us from
the sun. Curiously enough the moon appears to reflect proportionally
more heat than light, the amount of lunar heat received by the earth
being reckoned at 1/185000 of the amount coming from the sun. The
popular idea that the moon affects the movement of sap in plants is
equally illusory.”
“But about the weather? I know people who believe that a change of the
moon from one phase to another brings about a change of weather. Is that
true?”
“Certainly it is not true. The moon is changing its apparent form all
the time. There is no sudden alteration at any phase. The popular
belief, however, has always been so firmly fixed that many
investigations have been made to ascertain whether there is, in reality,
any foundation for it. These investigations have shown that no
measurable effect of the kind exists.”
“And the Full Moon does not drive away clouds, as some assert?”
“Surely she does not. I will now tell you something that the persons who
plant and sow and cut timber according to the phases of the moon, and
who believe that she exercises a kind of magic control over the clouds,
probably have never heard of, although if they knew it they might use it
as an argument in favor of lunar influences. It is this: The alternate
approach and retreat of the moon with respect to the earth, as she
travels round her elliptical orbit, produce measurable, although slight,
disturbances of the magnetism of our planet. The distance of the moon
varies to the extent of about 30,000 miles. Now, if it could be shown
that these magnetic disturbances were reflected in the character of the
weather, then the supposed influence of the moon would be established.
But that has not been shown, and if it were shown it would still be
found that the phases of the moon had no relation to the fact, for the
moon may be at its greatest or its least distance from the earth, or at
any intermediate distance during any possible phase.”
“You will, perhaps, think me very persistent in asking foolish
questions, but there is one other on my mind that I should like to put,
now that we have gone so far. It is this: I have read, since the great
earthquakes at San Francisco and Valparaiso, and the great eruption of
Vesuvius in the same year, 1906, that the moon has an influence over
such things. Is this another unfounded popular superstition?”
“It is not a notion of _popular_ origin at all,” I replied. “It
originated rather from scientific considerations, and there may possibly
be a germ of truth in it, although it yet remains to be demonstrated,
and the evidence concerning it is confusingly contradictory. You will
recall, I trust, what has been said about the sun and the moon producing
tides in the oceans. We have also seen that before our globe had assumed
its present condition, while it was yet more or less plastic throughout
its whole mass, and before the birth of the moon, great tides were
produced in the body of the earth. The _tendency_ to the production of
such bodily tides still exists, and now that the moon has become a
near-by attendant of the earth, she acts more effectively in this regard
than does the sun. If the earth were still plastic the moon would
produce bodily tides in it. In other words the earth would be deformed
by the attraction of the moon. The question has arisen whether or not
the tendency to the production of such tides, now that the earth has
become rigid, may not disturb its crust sufficiently to induce
earthquakes and volcanic eruptions. Some students of the subject have
thought that they could detect evidence that this is the case. It has
frequently happened that such phenomena have occurred on a large scale,
at or very close to, the periods of New and of Full Moon. Those are the
times, as we saw when we were talking of the oceanic tides, when the sun
and the moon pull together. If all great eruptions and earthquakes
occurred at these conjunctions there would be little doubt of the
correctness of the theory. But, unfortunately for the clearness of our
conceptions, this is by no means the case. There have been many
earthquakes and volcanic outbursts when the sun and the moon were not
thus combining their tidal attractions. Thus the evidence is found to be
contradictory or inconsistent, and the question remains unsettled. It
is, however, a very interesting one, and the time will come, it is to be
hoped, when it will be answered decisively one way or the other.”
After this digression we returned to the study of the photographs.
“Photograph No. 20, which we have just been examining,” I said,
“represents the moon at the age of about twenty-four days and twenty
hours. The next, and the last of the series showing the moon in
progressive phases, is No. 21. Here the age of the moon is about
twenty-six days and twenty hours. It is the fast waning sickle of the
Old Moon which we behold. You perceive that it is relatively
uninteresting when compared with No. 20, because very little except the
eastern limb is illuminated. Nearly all the great circular and oval
formations and craters, and all the ‘seas,’ have passed into the lunar
night. Only the eastern verge of the _Oceanus Procellarum_ remains in
sight, dulling the brilliance of the inner curve of the sickle. The dark
walled plain above the center is Riccioli, and just below it appears
Hevel, a smaller, but yet large formation, with a low central mountain.
It is hardly worth our while to attempt to identify the other features
shown in the photograph. They include none that we have previously
studied. Yet this picture has an interest all its own because it is an
excellent representation of the moon at a time when she is so near to
the sun. Do not forget that, as I warned you when we began with the
crescent of the New Moon, in these photographs the moon appears reversed
top for bottom. Seen in the sky in the early morning this sickle would
have its rounded edge toward the left hand and directed more or less
downward, according to the position of the sun. A great deal of
confusion exists in the minds of well-educated people concerning the
position of the sickle of the New and the Old Moon. You have, of course,
heard of the classic instances in which artists have drawn the New Moon
with the concave side toward the sun! It is only necessary to remember
that a line drawn straight from the center of the convex side of the
sickle, whether it be the New Moon or the Old Moon, always extends
directly toward the place occupied by the sun.”
[Illustration:
NO. 21. AUGUST 19, 1903; MOON’S AGE 26.89 DAYS.
]
“There is,” said my friend, “an interesting old superstition
which I have often heard—I suppose it must of course be a
superstition—concerning ‘wet moons’ and ‘dry moons.’ As I recall it they
say that when the sickle of the New Moon appears nearly upright in the
sky that is a sign of dry weather, because the moon is then like an
overturned cup, but when the sickle has its ends turned upward that is a
sign of wet weather, because then the cup can hold water. I suppose that
these various positions of the moon actually occur, but I do not know
how they are brought about.”
“The supposed influence of the position of the New Moon on the weather,”
I replied, “is too gross a superstition to be worthy of any notice, but
the different attitudes of the sickle are interesting. They arise from
the changes in the position of the moon as seen from the earth with
respect to the direction of the sun, and these changes depend in turn on
the inclination of the moon’s path in the sky to the plane of the
earth’s equator as well as to the plane of the ecliptic or the earth’s
orbit. The ecliptic has an inclination of about 23½° to the plane of the
equator, and the moon’s orbit is inclined a little over 5° to the
ecliptic. The moon may, in consequence, appear more than 28° above or
below the equator. But since, as I told you in the beginning, the orbit
of the moon itself turns slowly about in space, the distance of the moon
above or below the equator is not constant. It may be only a little more
than 18°. In consequence of these changes of relative position the
situation of the horns of the crescent moon varies. But you need never
be in doubt as to what position they will occupy at any time if you will
simply remember that a straight line drawn from the point of one horn to
that of the other must always form a right angle with the direction of
the sun.
[Illustration:
Diagram Showing Why the Winter Moon Runs High.
]
“There is another very interesting fact about the position of the moon
in the sky which we should not neglect to notice. Did you ever observe
the superior brilliancy of the light of the Full Moon in winter? It is
one of the compensations that nature offers us. Since the Full Moon is
necessarily situated opposite to the point occupied by the sun, and
since the sun is far south of the equator in midwinter, it follows that
at the same season the Full Moon appears high above the equator in the
northern hemisphere. You will, perhaps, permit me to show you a diagram
intended to explain this phenomenon.
You observe that the sun being south of the equator, in the direction
indicated by the dotted line, the Full Moon is correspondingly situated
north of the equator, and must necessarily appear high in the sky at
midnight, when the sun is at its lowest declination. This is the reason
why the winter Full Moons are so brilliant, making the snow-clad hills
gleam with a splendor that sometimes dazzles the eyes of the beholders.
In the Arctic regions the long winter night, when the sun does not rise
for months, is periodically brightened by the presence of the Full Moon.
Just the opposite condition of affairs exists in summer. Then the sun
being north of the equator the Full Moon is south of it, and ‘runs low,’
appearing in high latitudes to skim along the southern horizon.”
“Thank you, and now I will ask you one more question,” said my friend.
“I have often heard of the ‘Harvest Moon’ and the ‘Hunter’s Moon.’ Will
you not kindly explain what is meant by these terms and when the
‘Harvest Moon’ can be seen? There is a poetic suggestiveness in the name
that appeals to me.”