trouble you with another diagram, or perhaps with two.”
“Oh, I shall not mind that at all. I have grown used to diagrams as well
as to the nomenclature of the moon.”
“Well, if my diagrams conduct your thoughts to things as interesting as
many that lie concealed behind the prosaic names on the moon I shall be
content. To begin, then, I must remind you that in her monthly journey
around the earth the moon moves from west toward east in her orbit, and
thus she gets a little over 12° farther east every twenty-four hours, as
reckoned from the position of the sun. The earth turning on its axis in
the same direction causes the moon to appear to rise in the east and set
in the west once every twenty-four hours. But in consequence of the
constant eastward motion of the moon she rises at a later hour every
night. Here is a graphic representation of what I mean:
“The earth is turning on its axis in the direction represented by the
arrows, and simultaneously the moon is moving in its orbit in the same
direction, as is shown by the large arrow. Suppose that some night the
moon is seen rising at a particular hour from the point A on the earth.
Then, the following night, when the observer has again arrived at A,
with the rotation of the earth, the moon will have advanced from M^1 to
M^2, and will not be seen rising until the point occupied by the
observer has arrived at B. This retardation of the hour of moonrise is
variable on account of changes in the position of the moon, arising from
the inclination of her orbit to the plane of the equator, and from the
inequalities of her motion, to which I have before referred. On the
average it amounts to fifty-one minutes daily. It varies also with the
distance of the observer from the equator, the variation being greater
in high latitudes. In the latitude of New York the retardation of
moonrise may be as great as an hour and a quarter, or as little as
twenty-three minutes.
[Illustration:
Diagram Showing Why Moon Rises Later Every Night.
]
“Now it is upon this variation that the phenomenon of the ‘Harvest’ and
the ‘Hunter’s Moon’ depends. If I had a celestial globe here I could
show you that at the time of the Autumn Equinox, September 22d, when the
sun crosses the equator moving southward, the apparent path of the moon
in the sky intersects the eastern horizon at a comparatively small
inclination. In other words the moon at that time instead of rising
steeply from the horizon rises on a long slope almost parallel with the
horizon. The consequence is that for several evenings in succession the
Full Moon near the time of the Autumn Equinox may be seen rising just
after sunset at almost the same hour. Look at this second diagram and
you will see why this is so.
[Illustration:
Diagram Illustrating the Harvest Moon.
]
“The little circles M show the moon at several successive positions in
her orbit, just twenty-four hours apart. You perceive that in
consequence of the slight inclination to the eastern horizon the sinking
of the latter caused by the earth’s rotation will bring the moon into
view night after night at almost the same hour. In fact, in high
northern latitudes like those of Norway and Sweden the moon’s path at
this time of the year may actually coincide with the horizon, so that
for several evenings she will rise at exactly the same hour. The name
‘Harvest Moon’ explains itself, since it always occurs at the time of
the autumn harvests and the vintage, and seems to supplement the fading
daylight for the benefit of late laborers in the fields. The ‘Harvest
Moon’ does not occur every year at precisely the same date. It is very
rare that Full Moon happens to fall just on September 22d. It usually
either precedes or follows that date. The ‘Harvest Moon’ is the Full
Moon which occurs nearest to the Autumn Equinox, either before or after.
The ‘Hunter’s Moon’ is the first Full Moon which follows the ‘Harvest
Moon.’ Like the former it rises for several successive evenings near the
same hour, but this phenomenon is less marked in the case of the
‘Hunter’s Moon,’ because it is farther from the Equinox.”
“Thank you, again,” said my friend. “I shall never henceforth look at
the moon without thinking of circles, straight lines, and arrows as well
as of ‘ring mountains’ and ‘seas.’”
“Then you are making good progress toward science,” I replied. “One last
look, now, at the photograph of the Old Moon’s sickle, and then we had
better postpone our examination of the large photographs, showing
certain particularly interesting districts on the moon, until to-morrow
morning. There is here another interesting point for artists to note.
The convex side of the sickle of the Old Moon, or the New Moon, is
always an arc of a circle, but the concave side is never circular
although it is often thus represented. The concave side, neglecting its
irregularities arising from the differences of level and of brilliancy
of the lunar surface, is elliptical in outline, that is to say, it is a
semicircle viewed obliquely.”
“Whatever its geometry may be,” replied my friend, “it is certainly very
beautiful. Good night, and I shall demand to see those large photographs
before the sun is very high to-morrow.”
------------------------------------------------------------------------
IV
GREAT SCENES ON THE MOON
------------------------------------------------------------------------
IV
GREAT SCENES ON THE MOON
MY friend did not leave me in doubt on the following morning as to the
genuineness of her interest in her new studies. The shadows of the trees
in the park were yet as long drawn out as the silhouettes of lunar peaks
at sunrise, when we resumed our place under the elm, and, at her
request, I opened once more my portfolio.
“The series of photographs that we are now about to examine,” I began,
“are on so large a scale that only a selected part of the moon is seen
in each of them. But within the restricted limits of these pictures the
amount of detail shown is truly astonishing, far more indeed than can be
found on the most elaborate lunar charts. These photographs were made by
Mr. Ritchey with the great 40-inch telescope of the Yerkes Observatory.
Many more besides those that we are going to look at were taken by him,
but I have selected, where choice was difficult, six which seemed to me
to be of special interest. We shall begin with one which covers the
larger part of the _Mare Nubium_, in the southeastern quarter of the
moon. You certainly must remember the _Mare Nubium_, for it forms the
head of the ‘dark woman’ whom you discovered in the moon last evening,
and if you will hold this photograph at arm’s length you will see that
her face is unmistakably stamped upon it.”
“I am greatly flattered,” she replied, “that you should remember my
discovery so well. I begin to feel hopeful that it may yet find a place
in the books.”
“It certainly is as deserving of such a place as many things that get
into books. You ought to find a suitable name for this woman in the
moon.”
“If I believed myself capable of rivaling the man who christened the
‘Marsh of a Dream,’ I should surely try my hand at lunar nomenclature,
but I fear that I should fall too far short of the ideal he has set up,
and so I shall leave her nameless.”
“Permit me then to continue to call her the ‘dark woman’ whenever a
reference to her may seem useful in fixing the localities that we shall
talk about in this photograph. The most striking object shown in the
picture is the great ring mountain Bullialdus which forms an
extraordinary ornament on the top of the ‘dark woman’s’ ear. This
photograph was taken when the line of sunrise ran just along the border
between the _Mare Nubium_ and the _Oceanus Procellarum_. The _Mare
Humorum_ is yet buried in night beyond the upper right-hand edge of the
picture, but some of its bordering mountains and craters have been
touched by the morning sunbeams. You will observe that a little more
than half of the interior of Bullialdus—which, by the way, I did not
mention by name when we were studying the series of phase photographs—is
yet filled with shadow, but its double-headed central peak rises clear
and bright in the sunlight. The shadow of this central mountain can be
seen projecting toward the east over the floor. The east wall, which is
distinctly terraced, lies in full sunshine, and the light streaming over
the lofty crest of the western wall touches the floor on its eastern
half. The steep outer slopes that lead up to the western rampart, and
the deep parallel ravines cut near the crest are clearly shown. The
distance across the ring from the summit of the wall on one side to that
on the other is 38 miles. The depth of the depression is 8,000 feet
below the crest of the walls, but the latter rise only 4,000 feet above
the level of the _Mare Nubium_ outside, so that Bullialdus is an
excellent example of the characteristic form of the lunar volcano, which
I tried to illustrate for you last evening. The central mountain is
3,000 feet high. East of the south point of the ring a shadow shows the
existence of a profound cleft in the wall, while a little west of south
appears a smaller crater ring very black with shadow, except on its
eastern side. If we stood on the _Mare Nubium_ and looked toward
Bullialdus and its neighbor from a distance of 25 or 30 miles they would
resemble a double, flat-topped mountain, with its serrated crests
connected by a high neck. The summit of one of the little peaks shown in
the photograph in the plain just west of Bullialdus would form an
excellent point of observation. Still farther south stands another
crater ring most of whose interior is also, at present, filled with
shadow. East of this, and a little farther south, is still a third ring
of similar aspect, from which a curious range of hills runs southward.
Returning to Bullialdus you will notice the radiating lines of hills
that surround it, and particularly a more lofty and broken range which
runs eastward.”
[Illustration:
BULLIALDUS AND THE _Mare Nubium_.
]
“Bullialdus verily frightens me!” exclaimed my friend. “What an
unearthly look it has! The longer I regard it the stronger becomes the
indescribable impression that it produces. I begin to understand now
what you meant when you promised to find a history in the moon. Truly
there never can have been such another history. I almost feel that I do
not care whether the moon ever had inhabitants or not. Its own story is
more fascinating than that of any puny race of beings, passing their
ephemeral lives upon its wonderful surface, could possibly be.”
“I am glad,” I replied, “that you have begun to enter into the spirit of
those who long and carefully study the earth’s satellite. You see now,
that it is not necessary to the astronomer to find evidences either of
former or of present life upon the moon in order to stimulate his zeal.
For him, as you have yourself intimated, the relics of its past history,
which this little world in the sky exhibits so abundantly, are of higher
interest than any story of human empire, for they have an incomparably
vaster theme. But to lighten our labor a little, let me once more refer
to the ‘dark woman,’ whose features, like the outlines of a
constellation, serve for points of reference. I began by remarking that
Bullialdus seems to be placed just over her ear. Observe now that, taken
together with its immediate surroundings, the great crater ring forms a
kind of barbaric ear-ornament of most extraordinary form and richness of
detail. The line of hills east of Bullialdus, of which I spoke a few
minutes ago, connects the ring with a tumbled mass of mountains on the
border of the _Mare Humorum_. These mountains run northward, or downward
in the picture, for a distance of perhaps 150 miles, and then turn
abruptly westward for a like distance; after which, in the form of a
broken chain, constituting the eastern walls of a row of half-submerged
ring plains, they change direction once more and run southward in the
_Mare Nubium_. The whole system bears some resemblance to a gigantic
buckle.”
“What is that curious object below Bullialdus which resembles an
old-fashioned gold earring?”
“I was about to speak of that. It is a ring plain named Lubiniesky,
about 23 miles in diameter with a wall a thousand feet in height, except
in the direction of Bullialdus where it is broken down. The interior is
very flat, and it forms a fine example of the half-submerged lunar
volcanoes which abound in this hemisphere. It may have had a central
mountain like Bullialdus, but if so it has been completely buried under
the influx of molten lava or whatever it was that covered this part of
the moon. The perfect form of Bullialdus in all its details when
compared with the mere outline that remains of Lubiniesky indicates that
the former probably burst forth after the inundation of liquid rock that
drowned the latter. Thus we have in these two neighboring formations two
chapters of lunar history which, like the monuments of Egypt, tell the
story of widely separated epochs. The row of still more completely
submerged crater rings westward from Lubiniesky and Bullialdus show by
their condition that the depth of the lava flood was probably greater in
their vicinity than it was farther eastward.
“Now look southward from Bullialdus, at a distance about twice as great
as that of Lubiniesky and you will see another partially submerged ring,
with a more serrated crest. The name of this is Kies. It is remarkable
for the lofty mountain spur which sets off from its southern wall, and
also for the fact that one of the bright streaks from Tycho—one of a
parallel pair that I pointed out to you last evening—traverses its flat
floor and continues on, broadening as it goes, to a deep crater ring
which we have already noticed, southeast of Bullialdus.
“South of Kies, at the edge of the _Mare Nubium_, is a lofty mountain
range whose summits and slopes are very bright in the sunrise. At one
point a great pass breaks through these mountains, leading to a sort of
bay shut in on all sides by precipices and the walls of gigantic crater
rings. The large crater ring at the eastern corner of this bay is
Capuanus. The smaller ring on its western side with a conspicuous crater
on its eastern wall is Cichus. Notice the fine shadow that Cichus casts,
whose pointed edge is evidently due to the little crater on the wall.
That ‘little’ crater is six miles across! The twin rings apparently
terminating the mountain mass northeast of the bay are Mercator and
Capuanus. Between these and Kies you perceive two short ranges of small
mountains and then a kind of round swelling of the surface of the plain
resembling a great mound. These formations are rare on the moon. They
look like bubbles raised by imprisoned gases. The United States
Geological Survey has discovered something similar in form, but
infinitely inferior in magnitude, in the great mud bubbles that rise to
the surface of the Gulf of Mexico off the mouth of the Mississippi
River. But I do not mean to aver that the two phenomena are similar in
origin.
“Near the southern shore of the _Mare Nubium_ appears a long, dark line
which starts at the edge of a crater ring, crosses the southern arm of
the ‘sea,’ evidently penetrates the bordering mountains, and reappears
traversing the dark bay near its northern edge, cleaving both walls of a
small crater ring in its way.
“I should weary you, perhaps, with too much detail if I undertook to
identify all of the prominent objects in this photograph. Returning to
the southern shore of the _Mare Nubium_, I shall simply call your
attention to the very large ring plain with terraced walls and a peak a
little east of its center. This is Pitatus. An enormous ravine breaks
through its eastern side and connects it with a smaller ring from which
the dark line already mentioned starts. This dark line represents one of
the most remarkable clefts on the moon. It looks as though the crust had
been split asunder there over a distance of at least 150 miles. It bears
some resemblance to the great cañon near Aristarchus and Herodotus,
except that the latter is very tortuous and this is nearly straight.”
“Have I not heard of something similar in connection with the California
earthquake in 1906?” asked my friend.
“No doubt you are thinking of the great ‘fault’ which geologists have
discovered off the Pacific coast of North America. There is perhaps some
resemblance between these phenomena. Pitatus, I may add, is 58 miles in
diameter. You will observe how its southern wall has apparently been
broken down by the deluge of lava which buried so many smaller rings in
the _Mare Nubium_. If you will now turn your attention to the left-hand
side of the photograph, somewhat above the center, you will perceive a
very strange object, the so-called ‘Straight Wall.’ It lies just west of
a large conical crater pit which has a much smaller pit near its western
edge. You might easily mistake the ‘Straight Wall’ for an accidental
mark in the photograph. It is not absolutely straight, and near its
southern end it makes a slight turn eastward and terminates in a
curious, branched mountain, whose most conspicuous part is
crescent-shaped. The wall is about 65 miles in length and 500 feet in
height. It is as perpendicular on its east face as the Palisades on the
Hudson. It is not a ridge of hills at all, but a place where the level
of the ground suddenly falls away. Approaching it from the west you
would probably be unaware of its existence until you stood upon its
verge. The dark line that we see in the photograph is the shadow cast by
the wall upon the lower plain. In the lunar afternoon the appearance is
changed, and the face of the cliff is seen bright with sunlight. This
curious object has attracted the attention of students of the moon for
generations, and many speculations were formerly indulged in concerning
its possible artificial origin. It has sometimes been called the ‘Lunar
Railroad.’ Manifestly, whatever else it may be, it is not artificial.
The closest analogy perhaps is with what we were speaking of a little
while ago, a geological fault, that is to say, a line in the crust of
the planet where the rocky strata have been broken across and one side
has dropped to a lower level.
“The crater pit in the _Mare Nubium_, east of the ‘Straight Wall,’ is
named Birt, and its twin, 75 miles farther east, is Nicollet. Look now
at the hooked nose of your ‘dark woman.’ The huge wart upon it is a
crater plain named Lassell. Between the lower end of the ‘Straight Wall’
and Lassell, and over the bridge of the ‘nose,’ a wedge-shaped mountain
runs out into the _mare_. This is called the Promontorium Ænarium, and
must have formed a magnificent outlook if ever a real ocean flowed at
the foot of its cliffs. The ring with a crater on its wall below Lassell
is Davy. You will note some very somber regions scattered over this part
of the _Mare Nubium_. One of them forms the ‘dark woman’s’ eye, and just
over it, like an eyebrow, is a curving range of hillocks, including some
little craters. On the ‘cheek’—I am still utilizing the ‘dark woman’ as
a kind of signboard—at the base of the ‘chin,’ appears a partly double
range of large ring plains. The greatest of these, at the bottom, is
named Fra Mauro, and you will notice within it a curious speckling of
small craters. Adjoining Fra Mauro on the south are two intersecting
rings, Barry being the name of the western and Bonpland that of the
eastern one. The partially submerged ring is nameless, as far as I know,
while the upper or southern member of the group, with a broad valley
shut in between broken mountain walls opening out of its northern side,
is Guerike. There is only one other object, on the extreme lower
right-hand corner of the picture, to which I will ask your attention. It
is a singular range of mountains thrown into a great loop at its
northern end, and known as the Riphæan Mountains.”
“It seems to me,” said my friend, putting her elbow on the table, and
leaning her head a little wearily on her hand, “that there is a great
sameness in these lunar scenes—always crater rings with or without
central mountains, always peaks and ridges and chasms and black shadows.
Truly variety is lacking.”
“But what could you expect?” I replied. “Is it not enough to stimulate
your curiosity that you are looking intimately into the details of a
foreign world? When you go to Europe you see there mountains, plains,
rivers, lakes, cities, people, absolutely identical in their main
features with what you see in America. But you find them endlessly
interesting because of their comparatively slight differences from
similar things with which you are familiar, because of the great age of
many of the objects to which your attention is directed, because of the
long course of history which they represent, and principally, perhaps,
because you are aware of the sensation of being far from home. It ought
to be the same for you here on the moon. These things that we are
looking upon belong to a globe suspended in space 239,000 miles from the
earth. If the features of our globe are practically the same everywhere,
differing only in the arrangement of their details, you should not be
surprised at finding that nature does not vary from her rule of
uniformity on the moon.
“In the next photograph of the series,” I continued, “we have a
marvelous specimen of the lunar landscapes. It is perhaps the most
rugged region on the moon. It includes two objects of supreme interest,
Tycho, the ‘Metropolitan Crater,’ and Clavius, the most remarkable of
the ring plains. You will no doubt recognize Tycho at a glance. It is
near the center of the picture. Like the last photograph this one
represents an early morning scene. The western wall of Tycho throws a
broad, irregular crescent of shadow into the cavernous interior, but all
of the eastern, northern, and southern sides of the wall are illuminated
on their inner faces. The central mountain group is emphasized by its
black shadow. A little close inspection reveals the existence of the
complicated system of terraces by which the walls drop from greater to
lesser heights until the deep sunken floor is reached. The diameter of
Tycho is 54 miles, and it is at least 17,000 feet deep, measured from
the summits of the peaks that tower on both the eastern and the western
sides of its wall. The vast system of bright streaks radiating from
Tycho is not seen here, the time when the photograph was made being too
near the sunrise on this part of the moon. The dish-shaped plains
crowded around Tycho form a remarkable feature of this part of the lunar
surface. It would be useless to mention them all by name, and I shall
ask your attention only to some of the principal ones.”
“Thank you for being so considerate,” said my friend, smiling. “I am
sure that I should forget the names as fast as you mentioned them.”
[Illustration:
TYCHO, CLAVIUS, AND THEIR SURROUNDINGS.
]