THE MOON AS A WORLD: DAY AND NIGHT UPON ITS SURFACE.
A wide interest, if not a deep one, attaches to the general question as
to the existence of living beings, or at least the possibility of
organic existence, on planetary bodies other than our own. The question
has been examined in all ages, by the lights of the science peculiar to
each. With every important accession to our astronomical knowledge it
has been re-raised: every considerable discovery has given rise to some
new step or phase in the discussion, and in this way there has grown up
a somewhat extensive literature exclusively relating to mundane
plurality. It will readily be understood that the moon, from its
proximity to the earth, has from the first received a large, perhaps the
largest, share of attention from wanderers into this field of
speculation: and we might add greatly to the bulk of this volume by
merely reviewing some of the more curious and, in their way, instructive
conjectures specially relating to the moon as a world—to imaginary
journeys towards her, and to the beings conjectured to dwell upon and
within her. This, however, we feel there is no occasion to do, for it is
our purpose merely to point out the two or three almost conclusive
arguments against the possibility of any life, animal or vegetable,
having existence on our satellite.
We well know what are the requisite conditions of life on the earth; and
we can go no further for grounds of inference; for if we were to start
by assuming forms of life capable of existence under conditions widely
and essentially different from those pertaining to our planet, there
would be no need for discussing our subject further: we could revel in
conjectures, without a thought as to their extravagance. The only
legitimate phase of the question we can entertain is this:—can there be
on the moon any kind of living things analogous to any kind of living
things upon the earth? And this question, we think, admits only of a
negative answer. The lowest forms of vitality cannot exist without air,
moisture, and a moderate range of temperature. It may be true, as recent
experiments seem to show, that organic germs will retain their vitality
without either of the first, and with exposure to intense cold and to a
considerable degree of heat; and it is conceivable that the mere germs
of life may be present on the moon.[16] But this is not the case with
living organisms themselves. We have, in Chapter V., specially devoted
to the subject, cited the evidence from which we know that there can be
at the most, no more air on the moon than is left in the receiver of an
air-pump after the ordinary process of exhaustion. And with regard to
moisture, it could not exist in any but the vaporous state, and we know
that no appreciable amount of vapour can be discovered by any
observation (and some of them are crucial enough) that we are capable of
making. We may suppose it just within the verge of possibility that some
low forms of vegetation might exist upon the moon with a paucity of air
and moisture such as would be beyond even our most severe powers of
detection: but granting even this, we are met by the temperature
difficulty; for it is inconceivable that any plant-life could survive
exposure first to a degree of cold vastly surpassing that of our arctic
regions, and then in a short time (14 days) to a degree of heat capable
of melting the more fusible metals—the total range being equal, as we
have elsewhere shown, to perhaps 600 or 700 degrees of our thermometric
scale.
The higher forms of vegetation could not reasonably be expected to exist
under conditions which the lower forms could not survive. And as regards
the possibility of the existence of animal life in any form or condition
on the lunar surface, the reasons we have adduced in reference to the
non-existence of vegetable life bear still more strongly against the
possibility of the existence of the former. We know of no animal that
could live in what may be considered a vacuum and under such thermal
conditions as we have indicated.
[Illustration: PLATE XXI.
NORMAL LUNAR CRATER.]
As to man, aëronautic experience teaches us that human life is
endangered when the atmosphere is still sufficiently dense to support 12
inches of mercury in the barometer tube; what then would be his
condition in a medium only sufficiently dense to sustain one-tenth of an
inch of the barometric column? We have evidence from the most delicate
tests that no atmosphere or vapour approaching even this degree of
attenuation exists around the moon’s surface.
Taking all these adverse conditions into consideration we are in every
respect justified in concluding that there is no possibility of animal
or vegetable life existing on the moon, and that our satellite must
therefore be regarded as a barren world.
* * * * * * * *
After this disquisition upon lunar uninhabitability it may appear
somewhat inconsistent for us to attempt a description of the scenery of
the moon and some other effects that would be visible to a spectator,
and of which he would be otherwise sensible, during a day and a night
upon her surface. But we can offer the sufficient apology that an
imaginary sojourn of one complete lunar day and night upon the moon
affords an opportunity of marshalling before our readers some phenomena
that are proper to be noticed in a work of this character, and that have
necessarily been passed over in the series of chapters on consecutive
and special points that have gone before. It may be urged that, in
depicting the moon from such a standpoint as that now to be taken, we
are describing scenes that never have been such in the literal sense of
the word, since no eye has ever beheld them. Still we have this
justification—that we are invoking the conception of things that
actually exist; and that we are not, like some imaginary voyagers to the
moon, indulging in mere flights of fancy. Although it is impossible for
a habitant of this earth fully to realise existence upon the moon, it is
yet possible, indeed almost inevitable, for a thoughtful
telescopist—watching the moon night after night, observing the sun rise
upon a lunar scene, and noting the course of effects that follow till it
sets—it is almost inevitable, we say, for such an observer to identify
himself so far with the object of his scrutiny, as sometimes to become
in thought a lunar being. Seated in silence and in solitude at a
powerful telescope, abstracted from terrestrial influences, and gazing
upon the revealed details of some strikingly characteristic region of
the moon, it requires but a small effort of the imagination to suppose
one’s self actually upon the lunar globe, viewing some distant landscape
thereupon; and under these circumstances there is an irresistible
tendency in the mind to pass beyond the actually _visible_, and to fill
in with what it knows must exist those accessory features and phenomena
that are only hidden from us by distance and by our peculiar point of
view. Where the material eye is baffled, the clairvoyance of reason and
analogy comes to its aid.
Let us then endeavour to realize the strange consequences which the
position and conditions of the moon produce upon the aspect of a lunar
landscape in the course of a lunar day and night.
The moon’s day is a long one. From the time that the sun rises upon a
scene[17] till it sets, a period of 304 hours elapses, and of course
double this interval passes between one sunrise and the next. The
consequences of this slow march of the sun begin to show themselves from
the instant that he rises above the lunar horizon. Dawn, as we have it
on earth, can have no counterpart upon the moon. No atmosphere is there
to reflect the solar beams while the luminary is yet out of actual
sight, and only the glimmer of the zodiacal light heralds the approach
of day. From the black horizon the sun suddenly darts his bright
untempered beams upon the mountain tops, crowning them with dazzling
brilliance while their flanks and valleys are yet in utter darkness.
There is no blending of the night into day. And yet there is a growth of
illumination that in its early stages may be called a twilight, and
which is caused by the slow rise of the sun. Upon the earth, in central
latitudes, the average time occupied by the sun in rising, from the
first glint of his upper edge till the whole disc is in sight, is but
two minutes and a quarter. Upon the moon, however, this time is extended
to a few minutes short of an hour, and, therefore, during the first few
minutes a dim light will be shed by the small visible chord of the solar
disc, and this will give a proportionately modified degree of
illumination upon the prominent portion of the landscape, and impart to
it something of the weird aspect which so strikes an observer of a total
solar eclipse on earth when the scene is lit by the thin crescent of the
re-appearing sun. This impaired illumination constitutes the only dawn
that a lunar spectator could behold. And it must be of short duration;
for when, in the course of half an hour, the solar disc has risen half
into view the lighting would no doubt appear nearly as bright to the eye
as when the entire disc of the sun is above the horizon. In this lunar
sunrise, however, there is none of that gilding and glowing which makes
the phenomenon on earth so gorgeous. Those crimson sky-tints with which
we are familiar are due to the absorption of certain of the polychromous
rays of light by our atmosphere. The blue and violet components of the
solar beams are intercepted by our envelope of vapour, and only the red
portions are free to pass; while on the moon, as there is no atmosphere,
this selective absorption does not occur. If it did, an observer gazing
from the earth upon the regions of the moon upon which the sun is just
rising would see the surface tinted with rosy light. This, however, is
not the case; the faintest lunar features just catching the sun are seen
simply under white light diluted to a low degree of brightness. Only
upon rare occasions is the lunar scenery suffused with coloured
illumination, and these are when, as we shall presently have to
describe, the solar rays reach the moon after traversing the earth’s
atmosphere during an eclipse of the sun.
This atmosphere of ours is the most influential element in beautifying
our terrestrial scenery, and the absence of such an appendage from the
moon is the great modifying cause that affects lunar scenery as compared
with that of the earth. We are accustomed to the sun with its dazzling
brightness—overpowering though it be—subdued and softened by our
vaporous screen. Upon the moon there is no such modification. The sun’s
intrinsic brilliancy is undiminished, its apparent distance is
shortened, and it gleams out in fierce splendour only to be realised,
and then imperfectly, by the conception of a gigantic electric light a
few feet from the eye. And the brightness is rendered the more striking
by the blackness of the surrounding sky. Since there is no atmosphere
there can be no sky-light, for there is nothing above the lunar world to
diffuse the solar beams; not a trace of that moisture which even in our
tropical skies scatters some of the sun’s light and gives a certain
degree of opacity or blueness, deep though it be, to the heavens by day.
Upon the moon, with no light-diffusing vapour, the sky must be as dark
or even darker than that with which we are familiar upon the finest of
moonless nights. And this blackness prevails in the full blaze of the
lunar noon-day sun. If the eye (upon the moon) could bear to gaze upon
the solar orb (which would be less possible than upon earth) or could it
be screened from the direct beams, as doubtless it could by intervening
objects, it would perceive the nebulous and other appendages which we
know as the corona, the zodiacal light, and the red solar protuberances:
or if these appendages could not be viewed with the sun above the
horizon they would certainly be seen in glorious perfection when the
luminary was about to rise or immediately after it had set.
And, notwithstanding the sun’s presence, the planets and stars would be
seen to shine more brilliantly than we see them on the clearest of
nights; the constellations would have the same configurations, though
they would be differently situated with respect to the celestial pole
about which they would appear to turn, for the axis of rotation of the
moon is directed towards a point in the constellation Draco. The stars
would never twinkle or change colour as they appear to us to do, for
scintillation or twinkling is a phenomenon of atmospheric origin, and
they would retain their full brightness, down even to the horizon, since
there would be no haze to diminish their light. The planets, and the
brighter stars at least, would be seen even when they were situated very
near to the sun. The planet Mercury, so seldom detected by terrestrial
gazers, would be almost constantly in view during the lunar day,
manifesting his close attendance on the central luminary by making only
short excursions of about two (lunar) days’ length, first on one side
and then on the other. Venus would be nearly as continuously visible,
though her wanderings would be more extensive on either side. The
zodiacal light also, which in our English latitude and climate is but
rarely seen and in more favourable climes appears only when the sun
itself is hidden beneath the horizon, would upon the moon be seen as a
constant accompaniment to the luminary throughout his daily course
across the lunar sky. The other planets would appear generally as they
do to us on earth, but, never being lost in daylight, their courses
among the stars could be traced with scarcely any interruption.
One planet, however, that adorns the sky of the lunar hemisphere which
is turned towards us deserves special mention from the conspicuous and
highly interesting appearance it must present. We allude to the earth.
To nearly one-half of the moon (that which we never see) this imposing
object can never be visible; but to the half that faces us the
terrestrial planet must appear almost fixed in the sky. A lunar
spectator in (what is to us) the centre of the disc, or about the region
north of the lunar mountains Ptolemy and Hipparchus, would have the
earth in his zenith. From regions upon the moon a little out of what is
to us the centre, a spectator would see the earth a little declining
from the zenith, and this declination would increase as the regions
corresponding to the (to us) apparent edge of the moon were approached,
till at the actual edge it would be seen only upon the horizon. From the
phenomena of libration (explained in Chap. VI.) the earth would appear
from nearly all parts of the lunar hemisphere to which it is visible at
all to describe a small circle in the sky. To an observer, however, upon
the (to us) marginal regions of the lunar globe, it would appear only
during a portion of the lunar day—being visible in fact only in that
part of its small circular path which happened to lie above the
observer’s horizon: in some regions only a portion of the terrestrial
disc would make its brief appearance. From the lunar hemisphere beyond
this marginal line the earth can never be seen at all.
The lunar spectator whose situation enabled him to view the earth would
see it as a moon; and a glorious moon indeed it must be. Its diameter
would be four times as great as that of the moon itself as seen by us,
and the area of its full disc 13 times as great. It would be seen to
pass through its phases, just as does our satellite, once in a lunar day
or a terrestrial month, and during that cycle of phases, since 29 of our
days would be occupied by it, the axial rotation would bring all the
features of its surface configuration into view so many times in
succession. But the greatest beauty of this noble moon would be seen
during the lunar night, in considering which we shall again allude to
it; for when it is full-moon to the earth it is new-earth to the moon.
At lunar midnight this globe of ours is fully illuminated; as morning
nears, the earth-moon wanes, its disc slowly passing through the gibbous
phases until at sunrise it would be just half-illuminated. During the
long forenoon it assumes a crescent which narrows and narrows till at
midday the sun is in line with the earth and the latter is invisible,
save perhaps by a thin line of light marking its upper or lower edge,
accordingly as the sun is apparently above or below it. In the lunar
afternoon an illuminated crescent appears upon the opposite side of the
terrestrial globe, and this widens and widens till it becomes a half
disc by lunar sunset and a full disc by lunar midnight.
The sun in his daily course passes at various distances, sometimes above
and sometimes below, the nearly stationary earth. Obviously it will at
times pass actually behind it, and then the lunar spectator would behold
the sublime spectacle of a total solar eclipse, and that under
circumstances which render the phenomenon far more imposing than its
counterpart can appear from the earth; for whereas, when we see the moon
eclipse the sun, the nearly similar (apparent) diameters of the two
bodies renders the duration of totality extremely short—at most 7
minutes—a lunar spectator, the earth appearing to him four times the
diameter of the sun, and he and the earth being relatively stationary,
would enjoy a view of the totality extending over several hours. During
the passage of the solar disc behind that of the earth, a beautiful
succession of luminous phenomena would be observed to follow from the
refractions and dispersions which the sunbeams would suffer in passing
tangentially through those parts of our atmospheric envelope which lie
in their course; those, for instance, on the margin of the earth, as
seen from the moon. As the sun passed behind the earth, the latter would
be encircled upon the in-going side with a beautiful line of golden
light, deepening in places to glowing crimson, due to the absorption,
already spoken of, of all but the red and orange rays of the sun’s light
by the vapours of our atmosphere. As the eclipse proceeded and totality
came on, this ruddy glow would extend itself nearly, if not all, around
the black earth, and so bright would it be, that the whole lunar
landscape covered by the earth’s shadow would be illuminated with faint
crimson light,[18] save, perhaps, in some parts of the far distance,
upon which the earth had not yet cast its shadow, or off which the
shadow had passed. Although the crimson light would preponderate, it
would not appear bright and red alike all around the earth’s periphery.
The circle of light would be, in fact, _the ring of twilight_ round our
globe, and it would only appear red in those places where the atmosphere
chanced to be in that condition favourable for producing what on earth
we know as red sunset and sunrise. We know that the sun, even in clear
sky, does not always set and rise with the beautiful red glow, which may
be determined by merely local causes, and will therefore vary in
different parts of the earth. Now a lunar spectator watching the sun
eclipsed by the earth, would see, during totality and at a _coup d’œil_,
every point around our world upon which the sun is setting on one side
and rising upon the other. To every part of the earth around what is
then the margin, as seen from the moon, the sun is upon the horizon,
shining through a great thickness of atmosphere, reddening it, and being
reddened by it wherever the vaporous conditions conduce to that
colouration. And at all parts where these conditions obtain, the lunar
eclipse-observer would see the ring of light around the black
earth-globe brilliantly crimsoned; at other parts it would have other
shades of red and yellow, and the whole effect would be to make the
grand earth-ball, hanging in the lunar sky, like a dark sphere in a
circle of glittering gold and rubies.
During the early stages of the eclipse, this chaplet of
brilliant-coloured lights would be brightest upon the side of the
_disappearing_ sun; at the time of central eclipse the radiance
(supposing the sun to pass centrally behind the earth) would be equally
distributed, and during the later stages it would preponderate upon the
side of the _reappearing_ sun. We have endeavoured to give a pictorial
realization of this phenomenon and of the effect of the eclipse upon the
lunar landscape, but such a picture cannot but fall very, very far short
of the reality. (See Plate XXII.)
And now for a time let us turn attention from the lunar sky to the
scenery of the lunar landscape. Let us, in imagination, take our stand
high upon the eastern side of the rampart of one of the great craters.
Height, it must be remarked, is more essential on the moon to command
extent of view than upon the earth, for on account of the comparative
smallness of the lunar sphere the dip of the horizon is very rapid. Such
height, however, would be attained without great exercise of muscular
power, since equal amounts of climbing energy would, from the smallness
of lunar gravity, take a man six times as high on the moon as on the
earth. Let us choose, for instance, the hill-side of Copernicus. The day
begins by a sudden transition. The faint looming of objects under the
united illumination of the half-full earth and the zodiacal light is the
lunar precursor of day-break. Suddenly the highest mountain peaks
receive the direct rays of a portion of the sun’s disc as it emerges
from below the horizon. The brilliant lighting of these summits serves
but to increase, by contrast, the prevailing darkness, for they seem to
float like islands of light in a sea of gloom. At a rate of motion
twenty-eight times slower than we are accustomed to, the light tardily
creeps down the mountain-sides, and in the course of about twelve hours
the whole of the circular rampart of the great crater below us, and
towards the east, shines out in brilliant light, unsoftened by a trace
of mountain-mist. But on the opposite side, looking into the crater,
nothing but blackness is to be seen. As hour succeeds hour, the sunbeams
reach peak after peak of the circular rampart in slow succession, till
at length the circle is complete and the vast crater-rim, 50 miles in
diameter, glistens like a silver-margined abyss of darkness. By-and-by,
in the centre, appears a group of bright peaks or bosses. These are the
now illuminated summits of the central cones, and the development of the
great mountain cluster they form henceforth becomes an imposing feature
of the scene. From our high standpoint, and looking backwards to the
sunny side of our cosmorama, we glance over a vast region of the wildest
volcanic desolation. Craters from five miles diameter downwards crowd
together in countless numbers, so that the surface, as far as the eye
can reach, looks veritably frothed over with them. Nearer the base of
the rampart on which we stand, extensive mountain chains run to north
and to south, casting long shadows towards us; and away to southward run
several great chasms a mile wide and of appalling blackness and depth.
Nearer still, almost beneath us, crag rises on crag and precipice upon
precipice, mingled with craters and yawning pits, towering pinnacles of
rock and piles of scoria and volcanic _débris_. But we behold no sign of
existing or vestige of past organic life. No heaths or mosses soften the
sharp edges and hard surfaces: no tints of cryptogamous or lichenous
vegetation give a complexion of life to the hard fire-worn countenance
of the scene. The whole landscape, as far as the eye can reach, is a
realization of a fearful dream of desolation and lifelessness—not a
dream of death, for that implies evidence of preexisting life, but a
vision of a world upon which the light of life has never dawned.
[Illustration: PLATE XXII.
ASPECT OF AN ECLIPSE OF THE SUN BY THE EARTH, AS IT WOULD APPEAR AS
SEEN FROM THE MOON.]
Looking again, after some hours’ interval, into the great crateral
amphitheatre, we see that the rays of the morning sun have crept down
the distant side of the rampart, opposite to that on which we stand, and
lighted up its vast landslipped terraces into a series of seeming
hill-circles with all the rude and rugged features of a terrestrial
mountain view, and none of the beauties save those of desolate grandeur.
The plateau of the crater is half in shadow 10,000 feet below, with its
grand group of cones, now fully in sight, rising from its centre.
Although these last are twenty miles away and the base of the opposite
rampart fully double that distance, we have no means of judging their
remoteness, for in the absence of an atmosphere there can be no aërial
perspective, and distant objects appear as brilliant and distinct as
those which are close to the observer. Not the brightness only, but the
various colours also of the distant objects are preserved in their full
intensity; for colour we may fairly assume there must be. Mineral
chlorates and sublimates will give vivid tints to certain parts of the
landscape surface, and there must be all the more sombre colours which
are common to mineral matters that have been subjected to fiery
influence. All these tints will shine and glow with their greater or
less intrinsic lustres, since they have not been deteriorated by
atmospheric agencies, and far and near they will appear clear alike,
since there is no aërial medium to veil them or tarnish their pristine
brightness.
In the lunar landscape, in the line of sight, there are no means of
estimating distances; only from an eminence, where the intervening
ground can be seen, is it possible to realize _magnitude_ in a lunar
cosmorama and comprehend the dimensions of the objects it includes.
And with no air there can be no diffusion of light. As a consequence, no
illumination reaches those parts of the scene which do not receive the
direct solar rays, save the feeble amount reflected from contiguous
illuminated objects, and a small quantity shed by the crescent earth.
The shadows have an awful blackness. As we stand upon our chosen point
of observation, we see on the lighted side of the rampart almost
dazzling brightness, while beneath us, on the side away from the sun,
there is a region many miles in area impenetrable to the sight, for
there is no object within it receiving sufficient light to render it
discernible; and all around us, far and near, there is the violent
contrast between intense brightness of insolated parts and deep gloom of
those in equally intense shadow. The black though starlit sky helps the
violence of this contrast, for the bright mountains in the distance
around us stand forth upon a background formed by the darkness of
interplanetary space. The visible effects of these conditions must be in
every sense unearthly and truly terrible. The hard, harsh glowing light
and pitchy shadows; the absence of all the conditions that give
tenderness to an earthly landscape; the black noonday sky, with the
glaring sun ghastly in its brightness; the entire absence of vestiges of
any life save that of the long since expired volcanoes—all these
conspire to make up a scene of dreary, desolate grandeur that is
scarcely conceivable by an earthly habitant, and that the description we
have attempted but insufficiently pourtrays.
A legitimate extension of the imagination leads us to impressions of
lunar conditions upon other senses than that of sight, to which we have
hitherto confined our fancy. We are met at the outset with a difficulty
in this extension; for it is impossible to conceive the sensations which
the absence of an atmosphere would produce upon the most important of
our bodily functions. If we would attempt the task we must conjure up
feelings of suffocation, of which the thoughts are, however, too
horrible to be dwelt upon; we must therefore maintain the delusion that
we can exist without air, and attempt to realize some of the less
discomforting effects of the absence of this medium. Most notable among
these are the untempered heat of the direct solar rays, and the
influence thereof upon the surface material upon which we suppose
ourselves to stand. During a period of over three hundred hours the sun
pours down his beams with unmitigated ferocity upon a soil never
sheltered by a cloud or cooled by a shower, till that soil is heated, as
we have shown, to a temperature equal nearly to that of melting lead;
and this scorching influence is felt by everything upon which the sun
shines on the lunar globe. But while regions directly insolated are thus
heated, those parts turned from the sun would remain intensely cold, and
that scorching in sunshine and freezing in shade with which mountaineers
on the earth are familiar would be experienced in a terribly exaggerated
degree. Among the consequences, already alluded to, of the alternations
of temperature to which the moon’s crust is thus exposed, are doubtless
more or less considerable expansions and contractions of the surface
material, and we may conceive that a cracking and crumbling of the more
brittle constituents would ensue, together with a grating of contiguous
but disconnected masses, and an occasional dislocation of them. We refer
again to these phenomena to remark that if an atmospheric medium existed
they would be attended with noisy manifestations. There are abundant
causes for grating and crackling sounds, and such are the only sources
of noise upon the moon, where there is no life to raise a hum, no wind
to murmur, no ocean to boom and foam, and no brook to plash. Yet even
these crust-cracking commotions, though they might be felt by the
vibrations of the ground, would not manifest themselves audibly, for
without air there can be no communication between the grating or
cracking body and the nerves of hearing. Dead silence reigns on the
moon: a thousand cannons might be fired and a thousand drums beaten upon
that airless world, but no sound could come from them: lips might quiver
and tongues essay to speak, but no action of theirs could break the
utter silence of the lunar scene.
At a rate twenty-eight times slower than upon earth, the shadows shorten
till the sun attains his meridian height, and then, from the tropical
region upon which we have in imagination stood, nothing is to be seen on
any side, save towards the black sky, but dazzling light. The relief of
afternoon shadow comes but tardily, and the darkness drags its slow
length along the valleys and creeps sluggishly up the mountain-sides
till, in a hundred hours or more, the time of sunset approaches. This
phenomenon is but daybreak reversed, and is unaccompanied by any of the
gorgeous sky tints that make the kindred event so enrapturing on earth.
The sun declines towards the dark horizon without losing one jot of its
brilliancy, and darts the full intensity of its heat upon all it shines
on to the last. Its disc touches the horizon, and in half an hour dips
half-way beneath it, its intrinsic brightness and colour remaining
unchanged. The brief interval of twilight occurs, as in the morning,
when only a small chord of the disc is visible, and the long shadows now
sharpen as the area of light that casts them decreases. For a while the
zodiacal light vies with the earth-moon high in the heavens in
illuminating the scene; but in a few hours this solar appendage passes
out of view, and our world becomes the queen of the lunar night.
At this sunset time the earth, nearly in the zenith of us, will be at
its half-illuminated phase, and even then it will shed more light than
we receive upon the brightest of moonlight nights. As the night
proceeds, the earth-phase will increase through the gibbous stages until
at midnight it will be “full,” and our orb will be seen in its entire
beauty. It will perform at least one of its twenty-four-hourly rotations
during the time that it appears quite full, and the whole of its surface
features will in that time pass before the lunar spectator’s eye. At
times the northern pole will be turned towards our view, at times the
southern; and its polar ice-caps will appear as bright white spots,
marking its axis of rotation. If our lunar sojourn were prolonged we
should observe the northern ice-cap creep downwards to lower latitudes
(during our winter) and retreat again (during our summer); and this
variation would be perceptible in a less degree at the southern pole, on
account of the watery area surrounding it. The seas would appear (so far
as can be inferred) of pale blue-green tint; the continents
parti-coloured: and the tinted spots would vary with the changing
terrestrial seasons, as these are indicated by the positions and
magnitudes of the polar ice-caps. The permanent markings would be ever
undergoing apparent modification by the variations of the white
cloud-belts that encircle the terrestrial sphere. Of the nature of these
variations meteorological science is not as yet in a position to speak:
it would indeed be vastly to the benefit of that science if a view of
the distribution of clouds and vapours over the earth’s surface, as
comprehensive as that we are imagining, could really be obtained.
It might happen at “full-earth,” that a black spot with a fainter
penumbral fringe would appear on one side of the illuminated disc and
pass somewhat rapidly across it. This would occur when the moon passed
exactly between the sun and the earth, and the shadow of the moon was
cast upon the terrestrial disc. We need hardly say that these
shadow-transits would occur upon those astronomically important
occasions when an eclipse of the sun is beheld from the earth.
The other features of the sky during the long lunar night would not
differ greatly from those to which we alluded in speaking of its day
aspects. The stars would be the more brightly visible, from the greater
power of the eye-pupil to open in the absence of the glaring sun, and on
this account the milky-way would be very conspicuous and the brighter
nebulæ would come into view. The constellations would mark the night by
their positions, or the hours might be told off (in periods of
twenty-four each) by the successive reappearances of surface features on
certain parts of the terrestrial disc. The planets in opposition to the
sun would now be seen, and a comet might appear to vary the monotony of
the long lunar night. But a meteor would never flash across the sky,
though dark meteoric particles and masses would continually bombard the
lunar surface, sometimes singly, sometimes in showers. And these would
fall with a compound force due to their initial velocity added to that
of the moon’s attraction. As there is no atmosphere to consume the
meteors by frictional heat or break by its resistance the velocity of
their descent, they must strike the moon with a force to which that of a
cannon-ball striking a target is feeble indeed. A position on the moon
would be an unenviable stand-point from this cause alone.
The lunar landscape by night needs little description: it would be lit
by the earth-moon sufficiently to allow salient features, even at a
distance, to be easily made out, for its moon (_i.e._ the earth) has
thirteen times the light-reflecting area that our’s has. But the night
illumination will change in intensity, since the earth-moon varies from
half-full to full, and again to half-full, between sunset and the next
sunrise. The direction of the light, and hence the positions of the
shadows, will scarcely alter on account of the apparent fixity of the
earth in the lunar sky. A slight degree of warmth might possibly be felt
with the reflected earth-light; but it would be insufficient to mollify
the intensity of the prevailing cold. The heat accumulated by the ground
during the three hundred hours’ sunshine radiates rapidly into space,
there being no atmospheric coat to retain it, and a cooling process
ensues that goes on till, all warmth having rapidly departed, the
previously parched soil assumes a temperature approaching that of
celestial space itself, and which has been, as we have stated, estimated
at about 200° below the Fahrenheit zero. If moisture existed upon the
moon, its night-side would be bound in a grip of frost to which our
Arctic regions would be comparatively tropical. But since there is no
water, the aspect of the lunar scenery remains unmodified by effects of
changing temperature.
Such, then, are the most prominent effects that would manifest
themselves to the visual and other senses of a being transported to the
moon. The picture is not on the whole a pleasant one, but it is
instructive; and our rendering of it, imperfect though it be, may serve
to suggest other inferences that cannot but add to the interest which
always attaches to the contemplation of natural scenes and phenomena
from points of view different from those which we ordinarily occupy.
[Illustration: PLATE XXIII.
GROUP of LUNAR MOUNTAINS. ideal lunar landscape.]