twenty-seven days, the lunar day ought to be twenty-seven days long,
instead of twenty-nine. There is, however, a solar, as well as a
sidereal, day at the moon, as on the earth; and the solar day at the
moon is longer than the sidereal day, for the same reason as on the
earth. During the solar day the moon must make both a _synodical
rotation_ and a _synodical revolution_. This will be evident from
Fig. 125, in which is shown the path of the moon during one complete
lunation. _E_, _E'_, _E''_, etc., are the successive positions of
the earth; and 1, 2, 3, 4, 5, the successive positions of the moon.
The small arrows indicate the direction of the moon's rotation. The
moon is full at 1 and 5. At 1, _A_, at the centre of the moon's
disk, will have the sun, which lies in the direction _AS_, upon the
meridian. Before _A_ will again have the sun on the meridian, the
moon must have made a synodical revolution; and, as will be seen by
the dotted lines, she must have made more than a complete rotation.
The rotation which brings the point _A_ into the same relation to
the earth and sun is called a _synodical_ rotation.
It will also be evident from this diagram that the moon must make a
synodical rotation during a synodical revolution, in order always to
present the same side to the earth.
110. _The Earth as seen from the Moon._--To an observer on the moon, the
earth would be an immense moon, going through the same phases that the
moon does to us; but, instead of rising and setting, it would only
oscillate to and fro through a few degrees. On the other side of the
moon it would never be seen at all. The peculiarities of the moon's
motions which cause the librations, and make a spot on the moon's disk
seem to an observer on the earth to oscillate to and fro, would cause
the earth as a whole to appear to a lunar observer to oscillate to and
fro in the heavens in a similar manner.
It is a well-known fact, that, at the time of new moon, the dark part of
the moon's surface is partially illumined, so that it becomes visible to
the naked eye. This must be due to the light reflected to the moon from
the earth. Since at new moon the moon is between the earth and sun, it
follows, that, when it is new moon at the earth, it must be _full earth_
at the moon: hence, while the bright crescent is enjoying full sunlight,
the dark part of its surface is enjoying the light of the full _earth_.
Fig. 126 represents the full earth as seen from the moon.
[Illustration: Fig. 126.]
The Atmosphere of the Moon.
111. _The Moon has no Appreciable Atmosphere._--There are several
reasons for believing that the moon has little or no atmosphere.
(1) Had the moon an atmosphere, it would be indicated at the time of a
solar eclipse, when the moon passes over the disk of the sun. If the
atmosphere were of any considerable density, it would absorb a part of
the sun's rays, so as to produce a dusky border in front of the moon's
disk, as shown in Fig. 127. In reality no such dusky border is ever
seen; but the limb of the moon appears sharp, and clearly defined, as in
Fig. 128.
[Illustration: Fig. 127.]
[Illustration: Fig. 128.]
If the atmosphere were not dense enough to produce this dusky border,
its refraction would be sufficient to distort the delicate cusps of the
sun's crescent in the manner shown at the top of Fig. 125; but no such
distortion is ever observed. The cusps always appear clear and sharp, as
shown at the bottom of the figure: hence it would seem that there can be
no atmosphere of appreciable density at the moon.
(2) The absence of an atmosphere from the moon is also shown by the
absence of twilight and of diffused daylight.
Upon the earth, twilight continues until the sun is eighteen degrees
below the horizon; that is, day and night are separated by a belt twelve
hundred miles in breadth, in which the transition from light to darkness
is gradual. We have seen (66) that this twilight results from the
refraction and reflection of light by our atmosphere; and, if the moon
had an atmosphere, we should notice a similar gradual transition from
the bright to the dark portions of her surface. Such, however, is not
the case. The boundary between the light and darkness, though irregular,
is sharply defined. Close to this boundary the unillumined portion of
the moon appears just as dark as at any distance from it.
The shadows on the moon are also pitchy black, without a trace of
diffused daylight.
[Illustration: Fig. 129.]
(3) The absence of an atmosphere is also proved by the absence of
refraction when the moon passes between us and the stars. Let _AB_
(Fig. 129) represent the disk of the moon, and _CD_ an atmosphere
supposed to surround it. Let _SAE_ represent a straight line from
the earth, touching the moon at _A_, and let _S_ be a star situated
in the direction of this line. If the moon had no atmosphere, this
star would appear to touch the edge of the moon at _A_; but, if the
moon had an atmosphere, a star behind the edge of the moon, at _S'_,
would be visible at the earth; for the ray _S'A_ would be bent by
the atmosphere into the direction _AE'_. So, also, on the opposite
side of the moon, a star might be seen at the earth, although really
behind the edge of the moon: hence, if the moon had an atmosphere,
the time during which a star would be concealed by the moon would be
less than if it had no atmosphere, and the amount of this effect
must be proportional to the density of the atmosphere.
The moon, in her orbital course across the heavens, is continually
passing before, or _occulting_, some of the stars that so thickly
stud her apparent path; and when we see a star thus pass behind the
lunar disk on one side, and come out again on the other side, we are
virtually observing the setting and rising of that star upon the
moon. The moon's apparent diameter has been measured over and over
again, and is known with great accuracy; the rate of her motion
across the sky is also known with perfect accuracy: hence it is easy
to calculate how long the moon will take to travel across a part of
the sky exactly equal in length to her own diameter. Supposing,
then, that we observe a star pass behind the moon, and out again, it
is clear, that, if there is no atmosphere, the interval of time
during which it remains occulted ought to be exactly equal to the
computed time which the moon would take to pass over the star. If,
however, from the existence of a lunar atmosphere, the star
disappears too late, and re-appears too soon, as we have seen it
would, these two intervals will not agree; the computed time will be
greater than the observed time, and the difference will represent
the amount of refraction the star's light has sustained or suffered,
and hence the extent of atmosphere it has had to pass through.
Comparisons of these two intervals of time have been repeatedly
made, the most extensive being executed under the direction of the
Astronomer Royal of England, several years ago, and based upon no
less than two hundred and ninety-six occultation observations. In
this determination the measured or telescopic diameter of the moon
was compared with the diameter deduced from the occultations; and it
was found that the telescopic diameter was greater than the
occultation diameter by two seconds of angular measurement, or by
about a thousandth part of the whole diameter of the moon. This
discrepancy is probably due, in part at least, to _irradiation_
(91), which augments the apparent size of the moon, as seen in the
telescope as well as with the naked eye; but, if the whole two
seconds were caused by atmospheric refraction, this would imply a
horizontal refraction of one second, which is only one
two-thousandth of the earth's horizontal refraction. It is possible
that an atmosphere competent to produce this refraction would not
make itself visible in any other way.
But an atmosphere two thousand times rarer than our air can scarcely
be regarded as an atmosphere at all. The contents of an air-pump
receiver can seldom be rarefied to a greater extent than to about a
thousandth of the density of air at the earth's surface; and the
lunar atmosphere, if it exists at all, is thus proved to be twice as
attenuated as what we commonly call a vacuum.
The Surface of the Moon.
[Illustration: Fig. 130.]
112. _Dusky Patches on the Disk of the Moon._--With the naked eye, large
dusky patches are seen on the moon, in which popular fancy has detected
a resemblance to a human face. With a telescope of low power, these dark
patches appear as smooth as water, and they were once supposed to be
seas. This theory was the origin of the name _mare_ (Latin for _sea_),
which is still applied to the larger of these plains; but, if there were
water on the surface of the moon, it could not fail to manifest its
presence by its vapor, which would form an appreciable atmosphere.
Moreover, with a high telescopic power, these plains present a more or
less uneven surface; and, as the elevations and depressions are found to
be permanent, they cannot, of course, belong to the surface of water.
The chief of these plains are shown in Fig. 130. They are _Mare
Crisium_, _Mare Foecunditatis_, _Mare Nectaris_, _Mare
Tranquillitatis_, _Mare Serenitatis_, _Mare Imbrium_, _Mare
Frigoris_, and _Oceanus Procellarum_. All these plains can easily be
recognized on the surface of the full moon with the unaided eye.
113. _The Terminator of the Moon._--The terminator of the moon is the
line which separates the bright and dark portions of its disk. When
viewed with a telescope of even moderate power, the terminator is seen
to be very irregular and uneven. Many bright points are seen just
outside of the terminator in the dark portion of the disk, while all
along in the neighborhood of the terminator are bright patches and dense
shadows. These appearances are shown in Figs. 131 and 132, which
represent the moon near the first and last quarters. They indicate that
the surface of the moon is very rough and uneven.
[Illustration: Fig. 131.]
[Illustration: Fig. 132.]
As it is always either sunrise or sunset along the terminator, the
bright spots outside of it are clearly the tops of mountains, which
catch the rays of the sun while their bases are in the shade. The bright
patches in the neighborhood of the terminator are the sides of hills and
mountains which are receiving the full light of the sun, while the dense
shadows near by are cast by these elevations.
114. _Height of the Lunar Mountains._--There are two methods of finding
the height of lunar mountains:--
(1) We may measure the length of the shadows, and then calculate the
height of the mountains that would cast such shadows with the sun at the
required height above the horizon.
The length of a shadow may be obtained by the following method: the
longitudinal wire of the micrometer (19) is adjusted so as to pass
through the shadow whose length is to be measured, and the
transverse wires are placed one at each end of the shadow, as shown
in Fig. 133. The micrometer screw is then turned till the wires are
brought together, so as to ascertain the length of the arc between
them. We may then form the proportion: the number of seconds in the
semi-diameter of the moon is to the number of seconds in the length
of the shadow, as the length of the moon's radius in miles to the
length of the shadow in miles.
[Illustration: Fig. 133.]
The height of the sun above the horizon is ascertained by measuring
the angular distance of the mountain from the terminator.
(2) We may measure the distance of a bright point from the terminator,
and then construct a right-angled triangle, as shown in Fig. 134. A
solution of this triangle will enable us to ascertain the height of the
mountain whose top is just catching the level rays of the sun.
[Illustration: Fig. 134.]
_B_ is the centre of the moon, _M_ the top of the mountain, and
_SAM_ a ray of sunlight which just grazes the terminator at _A_, and
then strikes the top of the mountain at _M_. The triangle _BAM_ is
right-angled at _A_. _BA_ is the radius of the moon, and _AM_ is
known by measurement; _BM_, the hypothenuse, may then be found by
computation. _BM_ is evidently equal to the radius of the moon
_plus_ the height of the mountain.
By one or the other of these methods, the heights of the lunar mountains
have been found with a great degree of accuracy. It is claimed that the
heights of the lunar mountains are more accurately known than those of
the mountains on the earth. Compared with the size of the moon, lunar
mountains attain a greater height than those on the earth.
115. _General Aspect of the Lunar Surface._--A cursory examination of
the moon with a low power is sufficient to show the prevalence of
crater-like inequalities and the general tendency to _circular_ shape
which is apparent in nearly all the surface markings; for even the large
"seas" and the smaller patches of the same character repeat in their
outlines the round form of the craters. It is along the terminator that
we see these crater-like spots to the best advantage; as it is there
that the rising or setting sun casts long shadows over the lunar
landscape, and brings elevations into bold relief. They vary greatly in
size; some being so large as to bear a sensible proportion to the moon's
diameter, while the smallest are so minute as to need the most powerful
telescopes and the finest conditions of atmosphere to perceive them.
[Illustration: Fig. 135.]
The prevalence of ring-shaped mountains and plains willbe evident from
Fig. 135, which is from a photograph of a model of the moon constructed
by Nasmyth.
This same feature is nearly as marked in Figs. 131 and 132, which are
copies of Rutherfurd's photographs of the moon.
116. _Lunar Craters._--The smaller saucer-shaped formations on the
surface of the moon are called _craters_. They are of all sizes, from a
mile to a hundred and fifty miles in diameter; and they are supposed to
be of volcanic origin. A high telescopic power shows that these craters
vary remarkably, not only in size, but also in structure and
arrangement. Some are considerably elevated above the surrounding
surface, others are basins hollowed out of that surface, and with low
surrounding ramparts; some are like walled plains, while the majority
have their lowest depression considerably below the surrounding surface;
some are isolated upon the plains, others are thickly crowded together,
overlapping and intruding upon each other; some have elevated peaks or
cones in their centres, and some are without these central cones, while
others, again, contain several minute craters instead; some have their
ramparts whole and perfect, others have them broken or deformed, and
many have them divided into terraces, especially on their inner sides.
A typical lunar crater is shown in Fig. 136.
[Illustration: Fig. 136.]
It is not generally believed that any active volcanoes exist on the moon
at the present time, though some observers have thought they discerned
indications of such volcanoes.
[Illustration: Fig. 137.]
117. _Copernicus._--This is one of the grandest of lunar craters (Fig.
137). Although its diameter (forty-six miles) is exceeded by others,
yet, taken as a whole, it forms one of the most impressive and
interesting objects of its class. Its situation, near the centre of the
lunar disk, renders all its wonderful details conspicuous, as well as
those of objects immediately surrounding it. Its vast rampart rises to
upwards of twelve thousand feet above the level of the plateau, nearly
in the centre of which stands a magnificent group of cones, three of
which attain a height of more than twenty-four hundred feet.
Many ridges, or spurs, may be observed leading away from the outer banks
of the great rampart. Around the crater, extending to a distance of more
than a hundred miles on every side, there is a complex network of bright
streaks, which diverge in all directions. These streaks do not appear in
the figure, nor are they seen upon the moon, except at and near the full
phase. They show conspicuously, however, by their united lustre on the
full moon.
This crater is seen just to the south-west of the large dusky plain in
the upper part of Fig. 132. This plain is _Mare Imbrium_, and the
mountain-chain seen a little to the right of Copernicus is named the
_Apennines_. Copernicus is also seen in Fig. 135, a little to the left
of the same range.
Under circumstances specially favorable, myriads of comparatively minute
but perfectly formed craters may be observed for more than seventy miles
on all sides around Copernicus. The district on the south-east side is
specially rich in these thickly scattered craters, which we have reason
to suppose stand over or upon the bright streaks.
118. _Dark Chasms._--Dark cracks, or chasms, have been observed on
various parts of the moon's surface. They sometimes occur singly, and
sometimes in groups. They are often seen to radiate from some central
cone, and they appear to be of volcanic origin. They have been called
_canals_ and _rills_.
[Illustration: Fig. 138.]
One of the most remarkable groups of these chasms is that to the west of
the crater named _Triesneker_. The crater and the chasms are shown in
Fig. 138. Several of these great cracks obviously diverge from a small
crater near the west bank of the great one, and they subdivide as they
extend from the apparent point of divergence, while they are crossed by
others. These cracks, or chasms, are nearly a mile broad at the widest
part, and, after extending full a hundred miles, taper away till they
become invisible.
[Illustration: Fig. 139.]
119. _Mountain-Ranges._--There are comparatively few mountain-ranges on
the moon. The three most conspicuous are those which partially enclose
Mare Imbrium; namely, the _Apennines_ on the south, and the _Caucasus_
and the _Alps_ on the east and north-east. The Apennines are the most
extended of these, having a length of about four hundred and fifty
miles. They rise gradually, from a comparatively level surface towards
the south-west, in the form of innumerable small elevations, which
increase in number and height towards the north-east, where they
culminate in a range of peaks whose altitude and rugged aspect must form
one of the most terribly grand and romantic scenes which imagination can
conceive. The north-east face of the range terminates abruptly in an
almost vertical precipice; while over the plain beneath, intensely black
spire-like shadows are cast, some of which at sunrise extend full ninety
miles, till they lose themselves in the general shading due to the
curvature of the lunar surface. Many of the peaks rise to heights of
from eighteen thousand to twenty thousand feet above the plain at their
north-east base (Fig. 139).
[Illustration: Fig. 140.]
Fig. 140 represents an ideal lunar landscape near the base of such a
lunar range. Owing to the absence of an atmosphere, the stars will be
visible in full daylight.
[Illustration: Fig. 141.]
120. _The Valley of the Alps._--The range of the _Alps_ is shown in Fig.
141. The great crater at the north end of this range is named _Plato_.
It is seventy miles in diameter.
The most remarkable feature of the Alps is the valley near the centre of
the range. It is more than seventy-five miles long, and about six miles
wide at the broadest part. When examined under favorable circumstances,
with a high magnifying power, it is seen to be a vast flat-bottomed
valley, bordered by gigantic mountains, some of which attain heights of
ten thousand feet or more.
[Illustration: Fig. 142.]
121. _Isolated Peaks._--There are comparatively few isolated peaks to be
found on the surface of the moon. One of the most remarkable of these is
that known as _Pico_, and shown in Fig. 142. Its height exceeds eight
thousand feet, and it is about three times as long at the base as it is
broad. The summit is cleft into three peaks, as is shown by the
three-peaked shadow it casts on the plain.
122. _Bright Rays._--About the time of full moon, with a telescope of
moderate power, a number of bright lines may be seen radiating from
several of the lunar craters, extending often to the distance of
hundreds of miles. These streaks do not arise from any perceptible
difference of level of the surface, they have no very definite outline,
and they do not present any sloping sides to catch more sunlight, and
thus shine brighter, than the general surface. Indeed, one great
peculiarity of them is, that they come out most forcibly when the sun is
shining perpendicularly upon them: hence they are best seen when the
moon is at full, and they are not visible at all at those regions upon
which the sun is rising or setting. They are not diverted by elevations
in their path, but traverse in their course craters, mountains, and
plains alike, giving a slight additional brightness to all objects over
which they pass, but producing no other effect upon them. "They look as
if, after the whole surface of the moon had assumed its final
configuration, a vast brush charged with a whitish pigment had been
drawn over the globe in straight lines, radiating from a central point,
leaving its trail upon every thing it touched, but obscuring nothing."
[Illustration: Fig. 143.]
The three most conspicuous craters from which these lines radiate are
_Tycho_, _Copernicus_, and _Kepler_. Tycho is seen at the bottom of
Figs. 143 and 130. Kepler is a little to the left of Copernicus in the
same figures.
It has been thought that these bright streaks are chasms which have been
filled with molten lava, which, on cooling, would afford a smooth
reflecting surface on the top.
123. _Tycho._--This crater is fifty-four miles in diameter, and about
sixteen thousand feet deep, from the highest ridge of the rampart to the
surface of the plateau, whence rises a central cone five thousand feet
high. It is one of the most conspicuous of all the lunar craters; not so
much on account of its dimensions as from its being the centre from
whence diverge those remarkable bright streaks, many of which may be
traced over a thousand miles of the moon's surface (Fig. 143). Tycho
appears to be an instance of a vast disruptive action which rent the
solid crust of the moon into radiating fissures, which were subsequently
filled with molten matter, whose superior luminosity marks the course of
the cracks in all directions from the crater as their common centre. So
numerous are these bright streaks when examined by the aid of the
telescope, and they give to this region of the moon's surface such
increased luminosity, that, when viewed as a whole, the locality can be
distinctly seen at full moon by the unassisted eye, as a bright patch of
light on the southern portion of the disk.
III. INFERIOR AND SUPERIOR PLANETS.
Inferior Planets.
124. _The Inferior Planets._--The _inferior planets_ are those which lie
between the earth and the sun, and whose orbits are included by that of
the earth. They are _Mercury_ and _Venus_.
[Illustration: Fig. 144.]
125. _Aspects of an Inferior Planet._--The four chief _aspects_ of an
inferior planet as seen from the earth are shown in Fig. 144, in which
_S_ represents the sun, _P_ the planet, and _E_ the earth.
When the planet is between the earth and the sun, as at _P_, it is said
to be in _inferior conjunction_.
When it is in the same direction as the sun, but beyond it, as at _P''_,
it is said to be in _superior conjunction_.
When the planet is at such a point in its orbit that a line drawn from
the earth to it would be tangent to the orbit, as at _P'_ and _P'''_, it
is said to be at its _greatest elongation_.
[Illustration: Fig. 145.]
126. _Apparent Motion of an Inferior Planet._--When the planet is at
_P_, if it could be seen at all, it would appear in the heavens at _A_.
As it moves from _P_ to _P'_, it will appear to move in the heavens from
_A_ to _B_. Then, as it moves from _P'_ to _P''_, it will appear to move
back again from _B_ to _A_. While it moves from _P''_ to _P'''_, it will
appear to move from _A_ to _C_; and, while moving from _P'''_ to _P_, it
will appear to move back again from _C_ to _A_. Thus the planet will
appear to oscillate to and fro across the sun from _B_ to _C_, never
getting farther from the sun than _B_ on the west, or _C_ on the east:
hence, when at these points, it is said to be at its _greatest western_
and _eastern elongations_. This oscillating motion of an inferior planet
across the sun, combined with the sun's motion among the stars, causes
the planet to describe a path among the stars similar to that shown in
Fig. 145.
[Illustration: Fig. 146.]