the Earth, the lunar craters differ from them in many particulars,
showing that volcanic forces acting on different globes may produce
widely different results. For example, the floors of terrestrial craters
are usually situated at considerable elevations above the general
surface, while those of the lunar craters are generally much depressed,
the height of their walls being only about one-half the depth of their
cavities. Again, while on the Earth the mass of the volcanic cones far
exceeds the capacity of their openings, on the Moon it is not rare to
see the capacity of the crater cavities exceeding the mass of the
surrounding walls. On the Earth, the volcanic cones and mouths are
comparatively regular and smooth, and are generally due to the
accumulation of the ashes and the debris of all kinds which are ejected
from the volcanic mouths. On the Moon, very few craters show this
character, and for the most part their walls have a very different
structure, being irregular, very rugged, and composed of a succession of
concentric ridges, rising at many points to great elevations, and
forming peaks of stupendous height. Again, many of the larger
terrestrial craters have their interior occupied by a central cone, or
several such cones, having a volcanic mouth on their summits; on the
Moon such central cones are very rare. Although many of the large lunar
craters have their interior occupied by central masses which have been
often compared to the central cones of our great volcanoes, yet these
objects have a very different character and origin. For the most part,
they are mountainous masses of different forms--having very rarely any
craters on them--and seem to have resulted from the crowding and lifting
up of the crater floor by the phenomena of subsidence, of which these
craters show abundant signs. Besides, the terrestrial craters are
characterized by large and important lava streams, while on the Moon the
traces of such phenomena are quite rare, and when they are shown, they
generally differ from those of the Earth by their numerous and
complicated ramifications, and also by the fact that many of these lava
streamlets take their origin at a considerable distance from the crater
slopes, and are grooved and depressed as if the burning liquids which
are supposed to have produced them had subsequently disappeared, by
evaporation or otherwise, leaving the furrow empty.
The dark spots of the Moon, when viewed through a telescope, exhibit a
totally different character, and show that they belong to a different
formation from that of the brighter portions. These darker tracts do not
seem to have had a direct volcanic origin like the latter, but rather
appear to have resulted from the solidification of semi-fluid materials,
which have overflowed vast areas at different times. The surface of this
system is comparatively smooth and uniform, only some small craters and
low ridges being seen upon it. The level and dark appearance of these
areas led the ancient astronomers to the belief that they were produced
by a liquid strongly absorbing the rays of light, and were seas like our
seas. Accordingly, these dark surfaces were called _Maria_, or Seas, a
name which it is convenient to retain, although it is well known to have
originated in an error. The so-called seas of the Moon are evidently
large flat surfaces similar to the deserts, steppes, pampas, and
prairies of the Earth in general appearance. The great plains of the
Moon are at a lower level than that of the other formation, and that
which first attracts the observer's attention is the fact that they are
surrounded almost on all sides by an irregular line of abrupt cliffs and
mountain chains, showing phenomena of dislocation. This character of
dislocation, which is general, and is visible everywhere upon the
contours of the plains, seems to indicate that phenomena of subsidence,
either slow or rapid, have occurred on the Moon; while, at the same
time, the sunken surfaces were overflowed by a semi-fluid liquid, which
solidified afterwards. The evidences of subsidence and overflowing
become unmistakable when we observe that, along the borders of the gray
plains, numerous craters are more or less embedded in the gray
formation, only parts of the summit of their walls remaining visible, to
attest that once large craters existed there. The farther from the
border of the plain the vestiges of these craters are observed, the
deeper they are embedded in the gray formation. That phenomena of
subsidence have occurred on a grand scale on the Moon, is further
indicated by the fact that the singular systems of fractures called
clefts and rifts generally follow closely the outside border of the gray
plains, often forming parallel lines of dislocation and fractures. In
the interior regions of the gray formation, these fractures are
comparatively rare.
The gray, lava-like formation is obviously of later origin than the
mountainous system to which belong the embedded craters above described.
Its comparatively recent origin might also be inferred from the
smallness of its craters and its low ridges. The few large craters
observed on this formation evidently belong to the earlier system.
The color of this system of gray plains is far from being uniform. In
general appearance it is of a bluish gray, but when observed
attentively, large areas appear tinted with a dusky olive-green, while
others are slightly tinged with yellow. Some patches appear brownish,
and even purplish. A remarkable example of the first case is seen on the
surface, which encloses within a large parallelogram the two conspicuous
craters, Aristarchus and Herodotus. This surface evidently belongs to a
different system from that of the Oceanus Procellarum surrounding it,
as, besides its color, which totally differs from that of the gray
formation, its surface shows the rugged structure of the volcanic
formation.
When the Moon is full, some very curious white, luminous streaks are
seen radiating from different centres, which, for the most part, are
important craters, occupied by interior mountains. The great crater
Tycho is the centre of the most imposing of the systems of white
streaks. Some of the diverging rays of this great centre extend to a
distance equal to one-quarter of the Moon's circumference, or about
1,700 miles. The true nature of these luminous streaks is unknown, but
it seems certain that they have their origin in the crater from which
they diverge. They do not form any relief on the surface, and are seen
going up over the mountains and steep walls of the crater, as well as
down the ravines and on the floors of craters.
The Moon seems to be deprived of an atmosphere; or, if it has any, it
must be so excessively rare that its density is less than of the density
of the Earth's atmosphere, since delicate tests afforded by the
occultation of stars have failed to reveal its presence. Although no
atmosphere of any consequence exists on the Moon, yet phenomena which I
have observed seem to indicate the occasional presence there of vapors
of some sort. On several occasions, I have seen a purplish light over
some parts of the Moon, which prevented well-known objects being as
distinctly seen as they were at other times, causing them to appear as
if seen through a fog. One of the most striking of these observations
was made on January 4th, 1873, on the crater Kant and its vicinity,
which then appeared as if seen through luminous purplish vapors. On one
occasion, the great crater Godin, which was entirely involved in the
shadow of its western wall, appeared illuminated in its interior by a
faint purplish light, which enabled me to recognize the structure of
this interior. The phenomenon could not be attributed in this case to
reflection, since the Sun, then just rising on the western wall of the
crater, had not yet grazed the eastern wall, which was invisible. It is
not impossible that a very rare atmosphere composed of such vapors
exists in the lower parts of the Moon.
If the Moon has no air, and no liquids of any sort, it seems impossible
that its surface can maintain any form of life, either vegetable or
animal, analogous to those on the Earth. In fact, nothing indicating
life has been detected on the Moon--our satellite looking like a barren,
lifeless desert. If life is to be found there at all, it must be of a
very elementary nature. Aside from the want of air and water to sustain
it, the climatic conditions of our satellite are very unfavorable for
the development of life. The nights and days of the Moon are each equal
to nearly fifteen of our days and nights. For fifteen consecutive
terrestrial days the Sun's light is absent from one hemisphere of the
Moon; while for the same number of days the Sun pours down on the other
hemisphere its light and heat, the effects of which are not in any way
mitigated by an atmosphere. During the long lunar nights the temperature
must at least fall to that of our polar regions, while during its long
days it must be far above that of our tropical zone. It has been
calculated that during the lunar nights the temperature descends to 23°
below zero, while during the days it rises to 468°, or 256° above the
boiling point.
It has been a question among astronomers whether changes are still
taking place at the surface of the Moon. Aside from the fact that
change, not constancy, is the law of nature, it does not seem doubtful
that changes occur on the Moon, especially in view of the powerful
influences of contraction and dilatation to which its materials are
submitted by its severe alternations of temperature. From the distance
at which we view our satellite, we cannot expect, of course, to be able
to see changes, unless they are produced on a large scale. Theoretically
speaking, the largest telescopes ever constructed ought to show us the
Moon as it would appear to the naked eye from a distance of 40 miles;
but in practice it is very different. The difficulty is in the fact
that, while we magnify the surface of a telescopic image, we are unable
to increase its light; so that, practically, in magnifying an object, we
weaken its light proportionally to the magnifying power employed. The
light of the Moon, especially near the terminator, where we almost
always make our observations, is not sufficiently bright to bear a very
high magnifying power, and only moderate ones can be applied to its
study. What we gain by enlarging an object, we more than lose by the
weakening of its light. Besides, a high magnifying power, by increasing
the disturbances generally present in our atmosphere, renders the
telescopic image unsteady and very indistinct. On the whole, the largest
telescopes now in existence do not show us our satellite better than if
we could see it with the naked eye from a distance of 300 miles or more.
At such a distance only considerable changes would be visible.
Notwithstanding these difficulties, it is believed that changes have
been detected in Linné, Marius, Messier, and several other craters. An
observation of mine seems to indicate that changes have recently taken
place in the great crater Eudoxus. On February 20th, 1877, between 9h.
30m. and 10h. 30m., I observed a straight, narrow wall crossing this
crater from east to west, a little to the south of its centre. This wall
had a considerable elevation, as was proved by the shadow it cast on its
northern side. Towards its western end this wall appeared as a brilliant
thread of light on the black shadow cast by the western wall of the
crater. The first time I had occasion to observe this crater again,
after this observation, was a year later, on February 17th, 1878; no
traces of the wall were then detected. Many times since I have tried to
find this narrow wall again, when the Moon presented the same phase and
the same illumination, but always with negative results. It seems
probable that this structure has crumbled down, yet it is very singular
that so prominent a feature should not have been noticed before.
[PLATE VI.--MARE HUMORUM.
From a study made in 1875]
The "Mare Humorum," or sea of moisture, as it is called, which is
represented on Plate VI., is one of the smaller gray lunar plains. Its
diameter, which is very nearly the same in all directions, is about 270
miles, the total area of this plain being about 50,000 square miles. It
is one of the most distinct plains of the Moon, and is easily seen with
the naked eye on the left-hand side of the disk. The floor of the plain
is, like that of the other gray plains, traversed by several systems of
very extended but low hills and ridges, while small craters are
disseminated upon its surface. The color of this formation is of a dusky
greenish gray along the border, while in the interior it is of a lighter
shade, and is of brownish olivaceous tint. This plain, which is
surrounded by high clefts and rifts, well illustrates the phenomena of
dislocation and subsidence. The double-ringed crater Vitello, whose
walls rise from 4,000 to 5,000 feet in height, is seen in the upper
left-hand corner of the gray plain. Close to Vitello, at the east, is
the large broken ring-plain Lee, and farther east, and a little below,
is a similarly broken crater called Doppelmayer. Both of these open
craters have mountainous masses and peaks on their floor, which is on a
level with that of the Mare Humorum. A little below, and to the left of
these objects, is seen a deeply embedded oval crater, whose walls barely
rise above the level of the plain. On the right-hand side of the great
plain, is a long _fault_, with a system of fracture running along its
border. On this right-hand side, may be seen a part of the line of the
terminator, which separates the light from the darkness. Towards the
lower right-hand corner, is the great ring-plain Gassendi, 55 miles in
diameter, with its system of fractures and its central mountains, which
rise from 3,000 to 4,000 feet above its floor. This crater slopes
southward towards the plain, showing the subsidence to which it has been
submitted. While the northern portion of the wall of this crater rises
to 10,000 feet, that on the plain is only 500 feet high, and is even
wholly demolished at one place where the floor of the crater is in
direct communication with the plain. In the lower part of the _mare_,
and a little to the west of the middle line, is found the crater
Agatharchides, which shows below its north wall the marks of rills
impressed by a flood of lava, which once issued from the side of the
crater. On the left-hand side of the plain, is seen the half-demolished
crater Hippalus, resembling a large bay, which has its interior strewn
with peaks and mountains. On this same side can be seen one of the most
important systems of clefts and fractures visible on the Moon, these
clefts varying in length from 150 to 200 miles.
ECLIPSES OF THE MOON
PLATE VII
Since the Moon is not a self-luminous body, but shines by the light
which it borrows from the Sun, it follows that when the Sun's light is
prevented from reaching its surface, our satellite becomes obscured. The
Earth, like all opaque bodies exposed to sunlight, casts a shadow in
space, the direction of which is always opposite to the Sun's place. The
form of the Earth's shadow is that of a long, sharply-pointed cone,
which has our globe for its base. Its length, varying with the distance
of the Earth from the Sun, is, on an average, 855,000 miles, or 108
times the terrestrial diameter. This conical shadow of the Earth,
divided longitudinally by the plane of the ecliptic, lies half above and
half below that plane, on which the summit of the shadow describes a
whole circumference in the course of a year. If the Moon's orbit were
not inclined to the ecliptic, our satellite would pass at every Full
Moon directly through the Earth's shadow; but, owing to that
inclination, it usually passes above or below the shadow. Twice,
however, during each of its revolutions, it must cross the plane of the
ecliptic, the points of its orbit where this happens being called nodes.
Accordingly, if it is near a node at the time of Full Moon, it will
enter the shadow of the Earth, and become either partly or wholly
obscured, according to the distance of its centre from the plane of the
ecliptic. The partial or total obscuration of the Moon's disk thus
produced constitutes a partial or total eclipse of the Moon. The
essential conditions for an eclipse of the Moon are, therefore, that our
satellite must not only be full, but must also be at or very near one of
its nodes.
Although inferior in importance to the eclipses of the Sun, the eclipses
of the Moon are, nevertheless, very interesting and remarkable
phenomena, which never fail to produce a deep impression on the mind of
the observer, inasmuch as they give him a clear insight into the silent
motions of the planetary bodies.
At the mean distance of the Moon from the Earth, the diameter of the
conical shadow cast in space by our globe is more than twice as large as
that of our satellite. But, besides this pure dark shadow of the Earth,
its cone is enveloped by a partial shadow called "Penumbra," which is
produced by the Sun's light being partially, but not wholly, cut off by
our globe.
While the Moon is passing into the penumbra, a slight reduction of the
light of that part of the disk which has entered it, is noticeable. As
the progress of the Moon continues, the reduction becomes more
remarkable, giving the impression that rare and invisible vapors are
passing over our satellite. Some time after, a small dark-indentation,
marking the instant of first contact, appears on the eastern or
left-hand border of the Moon, which is always the first to encounter the
Earth's shadow, since our satellite is moving from west to east. The
dark indentation slowly and gradually enlarges with the onward progress
of the Moon into the Earth's shadow, while the luminous surface of its
disk diminishes in the same proportion. The form of the Earth's shadow
on the Moon's disk clearly indicates the rotundity of our globe by its
circular outline. Little by little the dark segment covers the Moon's
disk, and its crescent, at last reduced to a mere thread of light,
disappears at the moment of the second contact. With this the phase of
totality begins, our satellite being then completely involved in the
Earth's shadow.
The Moon remains so eclipsed for a period of time which varies with its
distance from the Earth, and with the point of its orbit where it
crosses the conical shadow. When it passes through the middle of this
shadow, while its distance from our globe is the least, the total phase
of an eclipse of the Moon may last nearly two hours. The left-hand
border of our satellite having gone first into the Earth's shadow, is
also the first to emerge, and, at the moment of doing so, it receives
the Sun's light, and totality ends with the third contact. The lunar
crescent gradually increases in breadth after its exit from the shadow,
and finally the Moon recovers its fully illuminated disk as before, at
the moment its western border leaves the Earth's shadow. Soon after, it
passes out of the penumbra, and the eclipse is over. In total eclipses,
the interval of time from the first to last contact may last 5h. 30m,
but it is usually shorter.
Soon after the beginning of an eclipse, the dark segment produced by the
Earth's shadow on the Moon's disk generally appears of a dark grayish
opaque color, but with the progress of the phenomenon, this dark tint is
changed into a dull reddish color, which, gradually increasing, attains
its greatest intensity when the eclipse is total. At that moment the
color of the Moon is of a dusky, reddish, coppery hue, and the general
features of the Moon's surface are visible as darker and lighter tints
of the same color. It sometimes happens, however, that our satellite
does not exhibit this peculiar coppery tint, but appears either blackish
or bluish, in which case it is hardly distinguishable from the sky.
It is very rare for the Moon to disappear completely during totality,
and even when involved in the deepest part of the Earth's shadow, our
satellite usually remains visible to the naked eye, or, at least, to the
telescope. This phenomenon is to be attributed to the fact that the
portion of the solar rays which traverse the lower strata of our
atmosphere are strongly refracted, and bend inward in such a manner that
they fall on the Moon, and sufficiently illuminate its surface to make
it visible. The reddish color observed is caused by the absorption of
the blue rays of light by the vapors which ordinarily-saturate the lower
regions of our atmosphere, leaving only red rays to reach the Moon's
surface. Of course, these phenomena are liable to vary with every
eclipse, and depend almost exclusively on the meteorological conditions
of our atmosphere.
In some cases the phase of totality lasts longer than it should,
according to calculation. This can be attributed to the fact that the
Earth is enveloped in a dense atmosphere, in which opaque clouds of
considerable extent are often forming at great elevations. Such strata
of clouds, in intercepting the Sun's light, would have, of course, the
effect of increasing the diameter of the Earth's shadow, in a direction
corresponding to the place they occupy, and, if the Moon were moving in
this direction, would increase the phase of total obscuration.
The eclipses of the Moon, like those of the Sun, as shown above, have a
cycle of 18 years, 11 days and 7 hours, and recur after this period of
time in nearly the same order. They can, therefore, be approximately
predicted by adding 18y. 11d. 7h. to the date of the eclipses which have
occurred during the preceding period. During this cycle 70 eclipses will
occur--41 being eclipses of the Sun and 29 eclipses of the Moon. At no
time can there ever be more than seven eclipses in a year, and there are
never less than two. When there are only two eclipses in a year, they
are both eclipses of the Sun.
Although the number of solar eclipses occurring at some point or other
of the Earth's surface is greater than that of the eclipses of the Moon,
yet at any single terrestrial station the eclipses of the Moon are the
more frequent. While an eclipse of the Sun is only visible on a narrow
belt, which is but a very small fraction of the hemisphere then
illuminated by the Sun, an eclipse of the Moon is visible from all the
points of the Earth which have the Moon above their horizon at the time.
Furthermore, an eclipse of the Sun is not visible at one time over the
whole length of its narrow tract, but moves gradually from one end of it
to the other; while, on the contrary, an eclipse of the Moon begins and
ends at the very same instant for all places from which it can be seen,
but, of course, not at the same local time, which varies with the
longitude of the place.
[PLATE VII.--PARTIAL ECLIPSE OF THE MOON.
Observed October 24, 1874]
The partial eclipse of the Moon, represented on Plate VII., shows quite
plainly the configuration of our satellite as seen with the naked eye
during the eclipse, with its bright and dark spots, and its radiating
streaks. This eclipse was observed on October 24th, 1874.
THE PLANETS
Around the Sun circulate a number of celestial bodies, which are called
"_Planets_." The planets are opaque bodies, and appear luminous because
their surfaces reflect the light they receive from the Sun.
The planets are situated at various distances from the Sun, and revolve
around this body in widely different periods of time, which are,
however, constant for each planet, so far as ascertained, and doubtless
are so in the other cases.
The ideal line traced in space by a planet in going around the Sun, is
called _the orbit_ of the planet; while the period of time employed by a
planet to travel over its entire orbit and return to its starting point,
is called _the sidereal revolution_, _or year_ of the planet. The
dimensions of the orbits of the different planets necessarily vary with
the distance of these bodies from the Sun, as does also the length of
their sidereal revolution.
The distance of a planet from the Sun does not remain constant, but is
subject to variations, which in certain cases are quite large. These
variations result from the fact that the planetary orbits are not
perfect circles having the Sun for centre, but curves called
"_Ellipses_," which have two centres, or foci, one of which is always
occupied by the Sun. This is in accordance with Kepler's first law.