might be expected to appear. We could not expect to see depressions any
more than elevations, since both would alike be projected on the same
imaginary disk. But we may recognise such depressions, from the manner
in which the light of the sun shines into them. When we hold a china
tea-cup at some distance from a candle, in the night, the candle being
elevated but little above the level of the top of the cup, a luminous
crescent will be formed on the side of the cup opposite to the candle,
while the side next to the candle will be covered by a deep shadow. As
we gradually elevate the candle, the crescent enlarges and travels down
the side of the cup, until finally the whole interior becomes
illuminated. We observe similar appearances in the moon, which we
recognise as deep depressions. They are those circular spots near the
terminator before spoken of, which look like bubbles of oil floating on
water. They are nothing else than circular craters or deep valleys. When
they are so situated that the light of the sun is just beginning to
shine into them, you may see, as in the tea-cup, a luminous crescent
around the side furthest from the sun, while a deep black shadow is cast
on the side next to the sun. As the cavity is turned more and more
towards the light, the crescent enlarges, until at length the whole
interior is illuminated. If the tea-cup be placed on a table, and a
candle be held at some distance from it, nearly on a level with the top,
but a little above it, the cup itself will cast a shadow on the table,
like any other elevated object. In like manner, many of these circular
spots on the moon cast deep shadows behind them, indicating that the
tops of the craters are elevated far above the general level of the
moon. The regularity of some of these circular spots is very remarkable.
The circle, in some instances, appears as well formed as could be
described by a pair of compasses, while in the centre there not
unfrequently is seen a conical mountain casting its pointed shadow on
the bottom of the crater. I hope you will enjoy repeated opportunities
to view the moon through a telescope. Allow me to recommend to you, not
to rest satisfied with a hasty or even with a single view, but to verify
the preceding remarks by repeated and careful inspection of the lunar
disk, at different ages of the moon.
The various places on the moon's disk have received appropriate names.
The dusky regions being formerly supposed to be seas, were named
accordingly; and other remarkable places have each two names, one
derived from some well-known spot on the earth, and the other from some
distinguished personage. Thus, the same bright spot on the surface of
the moon is called _Mount Sinai_ or _Tycho_, and another, _Mount Etna_
or _Copernicus_. The names of individuals, however, are more used than
the others. The diagram, Fig. 36, (see page 159,) represents rudely, the
telescopic appearance of the full moon. The reality is far more
beautiful. A few of the most remarkable points have the following names
corresponding to the numbers and letters on the map.
1. Tycho, 6. Eratosthenes,
2. Kepler, 7. Plato,
3. Copernicus, 8. Archimedes,
4. Aristarchus, 9. Eudoxus,
5. Helicon, 10. Aristotle.
A. Mare Humorum, _Sea of Humors_,
B. Mare Nubium, _Sea of Clouds_,
C. Mare Imbrium, _Sea of Rains_,
D. Mare Nectaris, _Sea of Nectar_,
E. Mare Tranquillitatis, _Sea of Tranquillity_,
F. Mare Serenitatis, _Sea of Serenity_,
G. Mare Fecunditatis, _Sea of Plenty_,
H. Mare Crisium, _Crisian Sea_.
The heights of the lunar mountains, and the depths of the valleys, can
be estimated with a considerable degree of accuracy. Some of the
mountains are as high as five miles, and the valleys, in some instances,
are four miles deep. Hence it is inferred, that the surface of the moon
is more broken and irregular than that of the earth, its mountains being
higher and its valleys deeper, in proportion to its magnitude, than
those of the earth.
The varieties of surface in the moon, as seen by the aid of large
telescopes, have been well described by Dr. Dick, in his 'Celestial
Scenery,' and I cannot give you a better idea of them, than to add a few
extracts from his work. The lunar mountains in general exhibit an
arrangement and an aspect very different from the mountain scenery of
our globe. They may be arranged under the four following varieties:
First, _insulated mountains_, which rise from plains nearly level,
shaped like a sugar loaf, which may be supposed to present an appearance
somewhat similar to Mount Etna, or the Peak of Teneriffe. The shadows
of these mountains, in certain phases of the moon, are as distinctly
perceived as the shadow of an upright staff, when placed opposite to the
sun; and these heights can be calculated from the length of their
shadows. Some of these mountains being elevated in the midst of
extensive plains, would present to a spectator on their summits
magnificent views of the surrounding regions.
Secondly, _mountain ranges_, extending in length two or three hundred
miles. These ranges bear a distant resemblance to our Alps, Apennines,
and Andes; but they are much less in extent. Some of them appear very
rugged and precipitous; and the highest ranges are in some places more
than four miles in perpendicular altitude. In some instances, they are
nearly in a straight line from northeast to southwest, as in the range
called the _Apennines_; in other cases, they assume the form of a
semicircle, or crescent.
Thirdly, _circular ranges_, which appear on almost every part of the
moon's surface, particularly in its southern regions. This is one grand
peculiarity of the lunar ranges, to which we have nothing similar on the
earth. A plain, and sometimes a large cavity, is surrounded with a
circular ridge of mountains, which encompasses it like a mighty rampart.
These annular ridges and plains are of all dimensions, from a mile to
forty or fifty miles in diameter, and are to be seen in great numbers
over every region of the moon's surface; they are most conspicuous,
however, near the upper and lower limbs, about the time of the half
moon.
The mountains which form these circular ridges are of different
elevations, from one fifth of a mile to three miles and a half, and
their shadows cover one half of the plain at the base. These plains are
sometimes on a level with the general surface of the moon, and in other
cases they are sunk a mile or more below the level of the ground which
surrounds the exterior circle of the mountains.
Fourthly, _central mountains_, or those which are placed in the middle
of circular plains. In many of the plains and cavities surrounded by
circular ranges of mountains there stands a single insulated mountain,
which rises from the centre of the plain, and whose shadow sometimes
extends, in the form of a pyramid, half across the plain to the opposite
ridges. These central mountains are generally from half a mile to a mile
and a half in perpendicular altitude. In some instances, they have two,
and sometimes three, different tops, whose shadows can be easily
distinguished from each other. Sometimes they are situated towards one
side of the plain, or cavity; but in the great majority of instances
their position is nearly or exactly central. The lengths of their bases
vary from five to about fifteen or sixteen miles.
The _lunar caverns_ form a very peculiar and prominent feature of the
moon's surface, and are to be seen throughout almost every region, but
are most numerous in the southwest part of the moon. Nearly a hundred of
them, great and small, may be distinguished in that quarter. They are
all nearly of a circular shape, and appear like a very shallow egg-cup.
The smaller cavities appear, within, almost like a hollow cone, with the
sides tapering towards the centre; but the larger ones have, for the
most part, flat bottoms, from the centre of which there frequently rises
a small, steep, conical hill, which gives them a resemblance to the
circular ridges and central mountains before described. In some
instances, their margins are level with the general surface of the moon;
but, in most cases, they are encircled with a high annular ridge of
mountains, marked with lofty peaks. Some of the larger of these cavities
contain smaller cavities of the same kind and form, particularly in
their sides. The mountainous ridges which surround these cavities
reflect the greatest quantity of light; and hence that region of the
moon in which they abound appears brighter than any other. From their
lying in every possible direction, they appear, at and near the time of
full moon, like a number of brilliant streaks, or radiations. These
radiations appear to converge towards a large brilliant spot,
surrounded by a faint shade, near the lower part of the moon, which is
named Tycho,--a spot easily distinguished even by a small telescope. The
spots named Kepler and Copernicus are each composed of a central spot
with luminous radiations.[8]
The broken surface and apparent geological structure of the moon has
suggested the opinion, that the moon has been subject to powerful
_volcanic_ action. This opinion receives support from certain actual
appearances of volcanic fires, which have at different times been
observed. In a total eclipse of the sun, the moon comes directly between
us and that luminary, and presents her dark side towards us under
circumstances very favorable for observation. At such times, several
astronomers, at different periods, have noticed bright spots, which they
took to be volcanoes. It must evidently require a large fire to be
visible at all, at such a distance; and even a burning spark, or point
but just visible in a large telescope, might be in fact a volcano raging
like Etna or Vesuvius. Still, as fires might be supposed to exist in the
moon from different causes, we should require some marks peculiar to
volcanic fires, to assure us that such was their origin in a given case.
Dr. Herschel examined this point with great attention, and with better
means of observation than any of his predecessors enjoyed, and fully
embraced the opinion that what he saw were volcanoes. In April, 1787, he
records his observations as follows: "I perceive three volcanoes in
different places in the dark part of the moon. Two of them are already
nearly extinct, or otherwise in a state of going to break out; the third
shows an eruption of fire or luminous matter." On the next night, he
says: "The volcano burns with greater violence than last night; its
diameter cannot be less than three seconds; and hence the shining or
burning matter must be above three miles in diameter. The appearance
resembles a small piece of burning charcoal, when it is covered with a
very thin coat of white ashes; and it has a degree of brightness about
as strong as that with which such a coal would be seen to glow in faint
daylight." That these were really volcanic fires, he considered further
evident from the fact, that where a fire, supposed to have been
volcanic, had been burning, there was seen, after its extinction, an
accumulation of matter, such as would arise from the production of a
great quantity of lava, sufficient to form a mountain.
It is probable that the moon has an _atmosphere_, although it is
difficult to obtain perfectly satisfactory evidence of its existence;
for granting the existence of an atmosphere bearing the same proportion
to that planet as our atmosphere bears to the earth, its dimensions and
its density would be so small, that we could detect its presence only by
the most refined observations. As our twilight is owing to the agency of
our atmosphere, so, could we discern any appearance of twilight in the
moon, we should regard that fact as indicating that she is surrounded by
an atmosphere. Or, when the moon covers the sun in a solar eclipse,
could we see around her circumference a faint luminous ring, indicating
that the sunlight shone through an aerial medium, we might likewise
infer the existence of such a medium. Such a faint ring of light has
sometimes, as is supposed, been observed. Schroeter, a German
astronomer, distinguished for the acuteness of his vision and his powers
of observation in general, was very confident of having obtained, from
different sources, clear evidence of a lunar atmosphere. He concluded,
that the inferior or more dense part of the moon's atmosphere is not
more than fifteen hundred feet high, and that the entire height, at
least to the limit where it would be too rare to produce any of the
phenomena which are relied on as proofs of its existence, is not more
than a mile.
It has been a question, much agitated among astronomers, whether there
is _water_ in the moon. Analogy strongly inclines us to reply in the
affirmative. But the analogy between the earth and the moon, as derived
from all the particulars in which we can compare the two bodies, is too
feeble to warrant such a conclusion, and we must have recourse to other
evidence, before we can decide the point. In the first place, then,
there is no positive evidence in favor of the existence of water in the
moon. Those extensive level regions, before spoken of, and denominated
seas in the geography of this planet, have no other signs of being
water, except that they are level and dark. But both these particulars
would characterize an earthly plain, like the deserts of Arabia and
Africa. In the second place, were those dark regions composed of water,
the terminator would be entirely smooth where it passed over these
oceans or seas. It is indeed indented by few inequalities, compared with
those which it exhibits where it passes over the mountainous regions;
but still, the inequalities are too considerable to permit the
conclusion, that these level spots are such perfect levels as water
would form. They do not appear to be more perfect levels than many plain
countries on the globe. The deep caverns, moreover, seen in those dusky
spots which were supposed to be seas, are unfavorable to the supposition
that those regions are covered by water. In the third place, the face of
the moon, when illuminated by the sun and not obscured by the state of
our own atmosphere, is always serene, and therefore free from clouds.
Clouds are objects of great extent; they frequently intercept light,
like solid bodies; and did they exist about the moon, we should
certainly see them, and should lose sight of certain parts of the lunar
disk which they covered. But neither position is true; we neither see
any clouds about the moon, with our best telescopes, nor do we, by the
intervention of clouds, ever lose sight of any portion of the moon when
our own atmosphere is clear. But the want of clouds in the lunar
atmosphere almost necessarily implies the absence of water in the moon.
This planet is at the same distance from the sun as our own, and has, in
this respect, an equal opportunity to feel the influence of his rays.
Its days are also twenty-seven times as long as ours, a circumstance
which would augment the solar heat. When the pressure of the atmosphere
is diminished on the surface of water, its tendency to pass into the
state of vapor is increased. Were the whole pressure of the atmosphere
removed from the surface of a lake, in a Summer's day, when the
temperature was no higher than seventy-two degrees, the water would
begin to boil. Now it is well ascertained, that if there be any
atmosphere about the moon, it is much lighter than ours, and presses on
the surface of that body with a proportionally small force. This
circumstance, therefore, would conspire with the other causes mentioned,
to convert all the water of the moon into vapor, if we could suppose it
to have existed at any given time.
But those, who are anxious to furnish the moon and other planets with
all the accommodations which they find in our own, have a subterfuge in
readiness, to which they invariably resort in all cases like the
foregoing. "There may be," say they, "some means, unknown to us,
provided for retaining water on the surface of the moon, and for
preventing its being wasted by evaporation: perhaps it remains unaltered
in quantity, imparting to the lunar regions perpetual verdure and
fertility." To this I reply, that the bare possibility of a thing is but
slight evidence of its reality; nor is such a condition possible, except
by miracle. If they grant that the laws of Nature are the same in the
moon as in the earth, then, according to the foregoing reasoning, there
cannot be water in the moon; but if they say that the laws of Nature are
not the same there as here, then we cannot reason at all respecting
them. One who resorts to a subterfuge of this kind ruins his own cause.
He argues the existence of water in the moon, from the analogy of that
planet to this. But if the laws of Nature are not the same there as
here, what becomes of his analogy? A liquid substance which would not
evaporate by such a degree of solar heat as falls on the moon, which
would not evaporate the faster, in consequence of the diminished
atmospheric pressure which prevails there, could not be water, for it
would not have the properties of water, and things are known by their
properties. Whenever we desert the cardinal principle of the Newtonian
philosophy,--that the laws of Nature are uniform throughout all her
realms,--we wander in a labyrinth; all analogies are made void; all
physical reasonings cease; and imaginary possibilities or direct
miracles take the place of legitimate natural causes.
On the supposition that the moon is inhabited, the question has often
been raised, whether we may hope that our telescopes will ever be so
much improved, and our other means of observation so much augmented,
that we shall be able to discover either the lunar inhabitants or any of
their works.
The improbability of our ever identifying _artificial structures_ in the
moon may be inferred from the fact, that a space a mile in diameter is
the least space that could be distinctly seen. Extensive works of art,
as large cities, or the clearing up of large tracts of country for
settlement or tillage, might indeed afford some varieties of surface;
but they would be merely varieties of light and shade, and the
individual objects that occasioned them would probably never be
recognised by their distinctive characters. Thus, a building equal to
the great pyramid of Egypt, which covers a space less than the fifth of
a mile in diameter, would not be distinguished by its figure; indeed, it
would be a mere point. Still less is it probable that we shall ever
discover any inhabitants in the moon. Were we to view the moon with a
telescope that magnifies ten thousand times, it would bring the moon
apparently ten thousand times nearer, and present it to the eye like a
body twenty-four miles off. But even this is a distance too great for us
to see the works of man with distinctness. Moreover, from the nature of
the telescope itself, we can never hope to apply a magnifying power so
high as that here supposed. As I explained to you, when speaking of the
telescope, whenever we increase the magnifying power of this instrument
we diminish its field of view, so that with very high magnifiers we can
see nothing but a point, such as a fixed star. We at the same time,
also, magnify the vapors and smoke of the atmosphere, and all the
imperfections of the medium, which greatly obscures the object, and
prevents our seeing it distinctly. Hence it is generally most
satisfactory to view the moon with low powers, which afford a large
field of view and give a clear light. With Clark's telescope, belonging
to Yale College, we seldom gain any thing by applying to the moon a
higher power than one hundred and eighty, although the instrument admits
of magnifiers as high as four hundred and fifty.
Some writers, however, suppose that possibly we may trace indications of
lunar inhabitants in their works, and that they may in like manner
recognise the existence of the inhabitants of our planet. An author, who
has reflected much on subjects of this kind, reasons as follows: "A
navigator who approaches within a certain distance of a small island,
although he perceives no human being upon it, can judge with certainty
that it is inhabited, if he perceives human habitations, villages,
corn-fields, or other traces of cultivation. In like manner, if we could
perceive changes or operations in the moon, which could be traced to the
agency of intelligent beings, we should then obtain satisfactory
evidence that such beings exist on that planet; and it is thought
possible that such operations may be traced. A telescope which magnifies
twelve hundred times will enable us to perceive, as a visible point on
the surface of the moon, an object whose diameter is only about three
hundred feet. Such an object is not larger than many of our public
edifices; and therefore, were any such edifices rearing in the moon, or
were a town or city extending its boundaries, or were operations of this
description carrying on, in a district where no such edifices had
previously been erected, such objects and operations might probably be
detected by a minute inspection. Were a multitude of living creatures
moving from place to place, in a body, or were they even encamping in an
extensive plain, like a large army, or like a tribe of Arabs in the
desert, and afterwards removing, it is possible such changes might be
traced by the difference of shade or color, which such movements would
produce. In order to detect such minute objects and operations, it would
be requisite that the surface of the moon should be distributed among at
least a hundred astronomers, each having a spot or two allotted to him,
as the object of his more particular investigation, and that the
observations be continued for a period of at least thirty or forty
years, during which time certain changes would probably be perceived,
arising either from physical causes, or from the operations of living
agents."[9]
FOOTNOTE:
[8] Dick's 'Celestial Scenery,' Chapter IV
LETTER XVI.
THE MOON.--PHASES.--HARVEST MOON.--LIBRATIONS.
"First to the neighboring Moon this mighty key
Of nature he applied. Behold! it turned
The secret wards, it opened wide the course
And various aspects of the queen of night:
Whether she wanes into a scanty orb,
Or, waxing broad, with her pale shadowy light,
In a soft deluge overflows the sky."--_Thomson's Elegy._
LET us now inquire into the revolutions of the moon around the earth,
and the various changes she undergoes every month, called her _phases_,
which depend on the different positions she assumes, with respect to the
earth and the sun, in the course of her revolution.
The moon revolves about the earth from west to east. Her apparent orbit,
as traced out on the face of the sky, is a great circle; but this fact
would not certainly prove that the orbit is really a circle, since, if
it were an ellipse, or even a more irregular curve, the projection of
it on the face of the sky would be a circle, as explained to you before.
(See page 148.) The moon is comparatively so near to the earth, that her
apparent movements are very rapid, so that, by attentively watching her
progress in a clear night, we may see her move from star to star,
changing her place perceptibly, every few hours. The interval during
which she goes through the entire circuit of the heavens, from any star
until she comes round to the same star again, is called a _sidereal
month_, and consists of about twenty-seven and one fourth days. The time
which intervenes between one new moon and another is called a _synodical
month_, and consists of nearly twenty-nine and a half days. A new moon
occurs when the sun and moon meet in the same part of the heavens; but
the sun as well as the moon is apparently travelling eastward, and
nearly at the rate of one degree a day, and consequently, during the
twenty-seven days while the moon has been going round the earth, the sun
has been going forward about the same number of degrees in the same
direction. Hence, when the moon comes round to the part of the heavens
where she passed the sun last, she does not find him there, but must go
on more than two days, before she comes up with him again.
The moon does not pursue precisely the same track around the earth as
the sun does, in his apparent annual motion, though she never deviates
far from that track. The inclination of her orbit to the ecliptic is
only about five degrees, and of course the moon is never seen further
from the ecliptic than about that distance, and she is commonly much
nearer to the ecliptic than five degrees. We may therefore see nearly
what is the situation of the ecliptic in our evening sky at any
particular time of year, just by watching the path which the moon
pursues, from night to night, from new to full moon.
The two points where the moon's orbit crosses the ecliptic are called
her _nodes_. They are the intersections of the lunar and solar orbits,
as the equinoxes are the intersections of the equinoctial and ecliptic,
and, like the latter, are one hundred and eighty degrees apart.