The Moon possesses neither Sea nor Atmosphere of appreciable
extent. Still, as a negative, in such case, is relative only to
the capabilities of the instruments employed, the search for the
indications of a lunar atmosphere has been renewed with fresh
augmentation of telescopic power. Of such indications, the most
delicate, perhaps, are those afforded by the occultation of a planet
by the moon. The occultation of Jupiter, which took place on January
2, 1857, was observed with this reference, and is said to have
exhibited no _hesitation_, or change of form or brightness, such as
would be produced by the refraction or absorption of an atmosphere. As
respects the sea, if water existed on the moon’s surface, the sun’s
light reflected from it should be completely polarised at a certain
elongation of the moon from the sun; and no traces of such light have
been observed.
MM. Baer and Maedler conclude that the moon is not entirely without
an atmosphere, but, owing to the smallness of her mass, she is
incapacitated from holding an extensive covering of gas; and they add,
“it is possible that this weak envelope may sometimes, through local
causes, in some measure dim or condense itself.” But if any atmosphere
exists on our satellite, it must be, as Laplace says, more attenuated
than what is termed a vacuum in an air-pump.
Mr. Hopkins thinks that if there be any lunar atmosphere, it must
be very rare in comparison with the terrestrial atmosphere, and
inappreciable to the kind of observation by which it has been tested;
yet the absence of any refraction of the light of the stars during
occultation is a very refined test. Mr. Nasmyth observes that “the
sudden disappearance of the stars behind the moon, without any change
or diminution of her brilliancy, is one of the most beautiful phenomena
that can be witnessed.”
Sir John Herschel observes: The fact of the moon turning always the
same face towards the earth is, in all probability, the result of an
elongation of its figure in the direction of a line joining the centres
of both the bodies, acting conjointly with a non-coincidence of its
centre of gravity with its centre of symmetry.
If to this we add the supposition that the substance of the moon is
not homogeneous, and that some considerable preponderance of weight
is placed excentrically in it, it will be easily apprehended that the
portion of its surface nearer to that heavier portion of its solid
content, under all the circumstances of the moon’s rotation, will
permanently occupy the situation most remote from the earth.
In what regards its assumption of a definite level, air obeys
precisely the same hydrostatical laws as water. The lunar
atmosphere would rest upon the lunar ocean, and form in its basin a
lake of air, whose upper portions at an altitude such as we are now
contemplating would be of excessive tenuity, especially should the
provision of air be less abundant in proportion than our own. It by
no means follows, then, from the absence of visible indications of
water or air on this side of the moon, that the other is equally
destitute of them, and equally unfitted for maintaining animal or
vegetable life. Some slight approach to such a state of things
actually obtains on the earth itself. Nearly all the land is
collected in one of its hemispheres, and much the larger portion
of the sea in the opposite. There is evidently an excess of heavy
material vertically beneath the middle of the Pacific; while not
very remote from the point of the globe diametrically opposite
rises the great table-land of India and the Himalaya chain, on the
summits of which the air has not more than a third of the density
it has on the sea-level, and from which animated existence is for
ever excluded.--_Herschel’s Outlines_, 5th edit.