THE MOON
The Sun and Moon offer to our sight almost exactly the same apparent
diameters; to the eye, they look the same size. But as we know the Sun to
be 400 times as distant as the Moon, it is necessarily 400 times as large;
its surface must exceed that of the Moon by the square of 400, or 160,000;
its volume by the cube of 400, or 64,000,000. As the Sun is of low mean
density, its mass does not exceed that of the Moon in quite the same high
ratio; but it is equal in mass to
27,000,000 moons.
Compared with the Sun, the Moon is therefore an insignificant little
ball--a mere particle; but as a world for habitation it possesses some
advantages over the Sun. The first glance at it in a telescope is
sufficient to assure the observer that he is looking at a solid,
substantial globe. It is not only substantial, it is rugged; its surface
is broken up into mountains, hills, valleys, and plains; the mountains
stand out in sensible relief; it looks like a ball of solid silver boldly
embossed and chased.
So far all is to the good for the purpose of habitation. Wherever men
are, they must have a solid platform on which to stand; they must have a
stable terrene whereon their food may grow, and this the Moon could
supply. "The Earth's gloom of iron substance" is necessary for man here,
and the Moon appears to offer a like stability.
Another favourable condition is that we know that the Moon receives from
the Sun a sufficient supply of light and heat. Each square yard of its
surface receives, on the average, the same amount of light and heat that
would fall upon a square yard on the Earth that was presented towards the
Sun at the same inclination; and we know from our own experience that this
is sufficient for the maintenance of life.
And the Moon is near enough for us to subject her to a searching scrutiny.
Every part of the hemisphere turned toward us has been repeatedly
examined, measured, and photographed; to that extent our knowledge of its
topography is more complete than of the world on which we live. There are
no unexplored regions on our side of the Moon. The great photographs taken
in recent years at the observatories of Paris and of the University of
Chicago have shown thousands of "crater-pits," not more than a mile
across; and narrow lines on the Moon's surface have been detected with a
breadth less than one-tenth of this. An elevation on the Moon, if it rose
up abruptly from an open plain, would make its presence apparent by the
shadow which it would cast soon after sunrise or near sunset; in this way
an isolated building, if it were as large as the great pyramid of Ghizeh,
would also show itself, and all our great towns and cities would be
apparent as areas of indistinct mottling, though the details of the cities
would not be made out.
But if vegetation took the same forms on the Moon as on the Earth, and
passed through the same changes, we should have no difficulty in
perceiving the evidence of its presence. If we were transported to the
Moon and turned our eyes earthward, we should not need the assistance of
any telescope in order to detect terrestrial changes which would be
plainly connected with the seasonal changes of vegetation. The Earth would
present to us a disc four times the apparent diameter of the Moon, and on
that disc Canada would offer as great an area as the whole of the Moon
does to us. We could easily follow with the naked eye the change from the
glittering whiteness of the aspect of Canada when snow-covered in winter,
to the brown, green and gold which would succeed each other during the
brighter months of the year. And this type of change would alternate
between the northern and southern hemispheres, for the winter of Canada is
the summer of the Argentine, and conversely.
We ought, therefore, to have no difficulty in observing seasonal changes
on the Moon, if such take place. But nothing of the kind has ever been
remarked; no changes sufficiently pronounced for us to be sure of them are
ever witnessed. Here and there some slight mutations have been suspected,
nearly all accomplishing their cycle in the course of a lunar day; so that
it is difficult to separate them from changes purely apparent, brought
about by the change in the incidence of the illumination.
The difference in appearance of a given area on the Moon when viewed under
a low Sun and when the Sun is on the meridian is very striking. In the
first case everything is in the boldest relief; the shadows are long and
intensely black; the whole area under examination in the telescope seems
as if it might be handled. Under the high Sun, the contrasts are gone; the
scenery appears flat, many of the large conspicuous markings are only
recognized with difficulty. Thus the terse remark of Maedler, "The full
Moon knows no Maginus," has become a proverb amongst selenographers; yet
Maginus is a fine walled plain some eighty miles in diameter, and its
rampart attains a height in parts of 14,000 feet. Maginus lies near Tycho,
which has been well named "the lunar metropolis," for from it radiates the
principal system of bright streaks conspicuous on the full Moon. These
white streaks appear when the shadows have vanished or are growing short;
they are not seen under a low Sun.
The changes which appear to take place in the lunar formations owing to
the change in their illumination are much more striking and varied than
would be anticipated. But the question arises whether all the changes that
are associated with the progress of the lunar day can be ascribed to this
effect. Thus, Prof. W. H. Pickering writes concerning a well-known pair of
little craters of about nine miles in diameter, "known as Messier and
Messier A, situated side by side not far from the centre of the Mare
Fecunditatis. When the Sun rises first on them, the eastern one, A, is
triangular and larger than Messier, which latter is somewhat pear-shaped.
About three days after sunrise they both suddenly turn white, Messier
rapidly grows in size, soon surpasses A, and also becomes triangular in
shape. Six days after sunrise the craters are again nearly of the same
size, owing to the diminution of Messier. The shape of A has become
irregular, and differs in different lunations. At nine days after sunrise
the craters are exactly alike in size and shape, both now being
elliptical, with their major axes lying in a nearly N. and S. direction.
Just before sunset A is again the larger, being almost twice the size of
Messier."[12]
Some observers explain this cycle of changes as due merely to the peculiar
contour of the two objects, the change in the lighting during the lunar
day altering their apparent figures. Prof. W. H. Pickering, on the other
hand, while recognizing that some portion of the change of shape is
probably due to the contour of the ground, conceives that, in order to
explain the whole phenomenon, it is necessary to suppose that a white
layer of hoar frost is formed periodically round the two craters. It is
also alleged that whereas Maedler described the two craters as being
exactly alike eighty years ago, Messier A is now distinctly the larger;
but it is very doubtful whether Maedler's description can be trusted to
this degree of nicety. If it could, this would establish a permanent
change in the actual structure of the lunar surface at this point.
There are several other cases of the same order of ambiguity. The most
celebrated is Linne, a white spot about six miles in diameter on the Mare
Serentatis. This object appears to change in size during the progress of
the lunar day, and, as with Messier, some selenographers consider that it
has also suffered an actual permanent change in shape within the last
sixty or seventy years. Here again the evidence is not decisive; Neison is
by no means convinced that a change has taken place, yet does not think it
impossible that Linne may once have been a crater with steep walls which
have collapsed into its interior through the force of gravity.
Another type of suspected change is associated with the neighbourhood of
Aristarchus, the brightest formation on the Moon, so bright indeed that
Sir William Herschel, observing it when illuminated by earthshine in the
dark portion of the Moon, thought that he was watching a lunar volcano in
eruption. In 1897, on September 21, the late Major Molesworth noticed that
the crater was at that time under the rays of the setting Sun, and filled
with shadow, and the inner terraces, which should have been invisible,
were seen as faint, knotted, glimmering streaks under both the eastern and
western walls, and the central peak was also dimly discernible. He thought
this unusual lighting up of rocks on which the Sun had already set might
be due either to phosphorescence produced by long exposure to the Sun's
rays, or to inherent heat, or to reflected glare from the western rampart.
Still more important, both Major Molesworth and Mr. Walter Goodacre, each
on more than one occasion, observed what seemed to be a faint bluish mist
on the inner slope of the east wall, soon after sunrise, but this was
visible only for a short time. Other selenographers too, on rare
occasions, have made observations accordant with these, relating to
various regions on the Moon.
These, and a few other similar instances, are all that selenography has to
offer by way of evidence of actual lunar change. Of seeming change there
is abundance, but beyond that we have only cases for controversy, and one
of the most industrious of the present-day observers of the Moon, M.
Philip Fauth, declares that "as a student of the Moon for the last twenty
years, and as probably one of the few living investigators who have kept
in practical touch with the results of selenography, he is bound to
express his conviction that no eye has ever seen a physical change in the
plastic features of the Moon's surface."[13]
In this matter of change, then, the Earth and Moon stand in the greatest
contrast to each other. As we have seen, from the view-point of the Moon,
the appearance of the Earth would change so manifestly with the progress
of the seasons that no one could fail to remark the difference, even
though observing with the naked eye. But from the view-point of the Earth,
the Moon when examined by our most experienced observers, armed with our
most powerful telescopes, offers us only a few doubtful enigmatical
instances of possible change confined to small isolated localities; we see
no evidence that the "gloom of iron substance" below is ever concealed by
a veil of changing vegetation, or that "between the burning light and deep
vacuity" of the heavens above, the veil of the flying vapour has ever been
spread out. We see the Moon so clearly that we are assured it holds no
water to nourish plant life; we see it so clearly because there is no air
to carry the vapour that might dim our view.
Life is change, and a planet where there is no change, or where that
change is very small, can be no home for life. The "stability and
insensibility" are indeed required in the platform upon which life is to
appear, but there must be the presence of "the passion and the perishing,"
or life will be unable to find a home.
We infer the absence of water and air from the Moon not only from the
unchanging character of its features and the distinctness with which we
see them; we are able to make direct observations. Galileo, the first man
to observe the Moon to better advantage than with the naked eye, was not
long before he decided that the Moon contained no water, for though
Milton, in a well-known passage, makes Galileo discover
"Rivers or mountains on her spotty globe,"
Galileo himself wrote: "I do not believe that the body of the Moon is
composed of earth and water." The name of _maria_ was given to the great
grey plains of the Moon by Hevelius, but this was simply for convenience
of nomenclature, not because he actually believed them to be seas. One
observation is, in itself, sufficient to prove that the maria are not
water surfaces. The Moon's "terminator," that is to say, the line dividing
the part in sunlight from that in darkness, is clearly irregular when it
passes over the great plains; were they actually sea it would be a bright
line and perfectly smooth. The grey plains are therefore not expanses of
water now, nor were they in time past. It is obvious that in some remote
antiquity their surface was in a fluid condition, but it was the fluidity
of molten rock. This is seen by the way in which the maria have invaded,
breached, broken down, and submerged many of the circular formations on
their margins. Thus the Mare Humorum has swept away half the wall of the
rings, Hippalus and Doppelmayer, and far out in the open plain of the Mare
Nubium, great circles like Kies, and that immediately north of Flamsteed,
stand up in faint relief as of half-submerged rings. Clearly there was a
period after the age in which the great ring mountains and walled plains
came into existence, when an invasive flood attacked and partially
destroyed a large proportion of them. And the flood itself evidently
became more viscous and less fluid the further it spread from its original
centre of action, for the ridges and crumpling of the surface indicate
that the material found more and more difficulty in its flow.
We have evidence just as direct that there is no atmosphere. This is very
strikingly shown when the Moon, in its monthly progress among the stars,
passes before one of them and occults it. Such an occultation is
instantaneous, and is particularly impressive when either a disappearance
or a reappearance occurs at the defective limb; that is to say, at the
limb which is not illuminated by the Sun, and is therefore invisible. The
observer may have a bright star in the field of view, showing steadily in
a cloudless sky; there is not a hint of a weakening in its light; suddenly
it is gone. The first experience of such an observation is most
disconcerting; it is hardly less disconcerting to observe the reappearance
at the dark limb. One moment the field of view of the telescope is empty;
the next, without any sort of dawning, a bright star is shining steadily
in the void, and it almost seems to the observer as if an explosion had
taken place. If the Moon had an atmosphere extending upwards from its
surface in all directions and of any appreciable density, an occultation
would not be so exceedingly abrupt; and, in particular, if the occultation
were watched through a spectroscope, then, at the disappearance, the
spectrum of the star would not vanish as a whole, but the red end would go
first, and the rest of the spectrum would be swept out of sight
successively, from orange to the violet. This does not happen; the whole
spectrum goes out together, and it is clear that no appreciable atmosphere
can exist on the Moon. In actual observation so inappreciable is it that
its density at the Moon's surface is variously estimated as 1/300th of
that of the Earth by Neison, and as 1/10000th by W. H. Pickering. If the
Moon possessed an atmosphere bearing the same proportion to her total mass
as we find in the case of the Earth, she would have a density of
one-fortieth of our atmosphere at the sea level.
The Moon is at the same mean distance from the Sun as the Earth, and
therefore, surface for surface, receives from it on the average the same
amount of light and heat. But it makes a very different use of these
supplies. Bright as the Moon appears when seen at the full on some winter
night, it has really but a very low power of reflection, and is only
bright by contrast with the darkness of the midnight sky. If the full Moon
is seen in broad daylight, it is pale and ghost-like. Sir John Herschel
has put it on record that when in South Africa he often had the
opportunity of comparing the Moon with the face of Table Mountain, the Sun
shining full upon both, and the Moon appeared no brighter than the
weathered rock. The best determinations of the _albedo_ of the Moon, that
is to say, of its reflective power, give it as 0.17, so that only
one-sixth of the incident light is reflected, the other five-sixths being
absorbed. It is difficult to obtain a good determination of the Earth's
_albedo_, but the most probable estimate puts it as about 0.50, or three
times as great as that of the Moon. This high reflective power is partly
to be accounted for by the great extent of the terrestrial polar caps, but
chiefly by the clouds and dust layer always present in its atmosphere.
A larger proportion, therefore, of the solar rays are employed in heating
the soil of the Moon than in heating that of the Earth, and in this
connection the effect of an important difference between the two worlds
must be noted. The Earth rotates on its axis in 23 hours 56 minutes 4
seconds, the mean length of its rotation as referred to the Sun being 24
hours. The rotation of the Moon, on the other hand, takes 27 days 7 hours
43 minutes to accomplish, giving a mean rotation, as referred to the Sun,
of 29 days 12 hours 44 minutes. The lunar surface is therefore exposed
uninterruptedly to the solar scorching for very nearly fifteen of our days
at a time, and it is, in turn, exposed to the intense cold of outer space
for an equal period. As the surface absorbs heat so readily, it must
radiate it as quickly; hence radiation must go on with great rapidity
during the long lunar night. Lord Rosse and Prof. Very have both obtained
measures of the change in the lunar heat radiation during the progress of
a total eclipse of the Moon, with the result that the heat disappeared
almost completely, though not quite at the same time as the light. Prof.
Langley succeeded in obtaining from the Moon, far down in the long wave
lengths of the infra-red, a heat spectrum which was only partly due to
reflection from the Sun; part coming from the lunar soil itself, which,
having absorbed heat from the Sun, radiated it out again almost
immediately. In 1898, Prof. Very, following up Langley's line of work,
concluded that the temperature of the lunar soil must range through about
350 deg. Centigrade, considerably exceeding 100 deg. at the height of the lunar
day, and falling to about the temperature of liquid air during the lunar
night. So wide a range of temperature must be fatal to living organisms,
particularly when the range is repeated at short, regular intervals of
time. But this range of temperature comes directly from the length of the
Moon's rotation period; for the longer the day of the Moon, the higher the
temperature which may be attained in it; the longer the night, the greater
the cold which will in turn be experienced. We learn, therefore, that the
time of rotation of a planet is an important factor in its habitability.