Year: 1876

ScAL w AhE IANOhAlE LUNAR STUDIES.


There are two lunar spots which the
selenOgrapher regards with special favor,
because of the evidences they seem to
give of change. One is q crater living
on the so-called Sea of Serenity. which
some popular lunar observers regard as
the left eye of the man in the moon.
Here there was once a deep crater nearly
seven miles across, a very distinct and
obvious feature, even with the small
telescope (less than four inches in aper-
ture), used by Beer and Madler in
forming their celebrated chart. But
ten years ago, the skillful astronomer
Schmidt, a selenographer of seleno-
graphers-who has, in fact, given the
best energies of his life to moon-gazing
found this crater missing. When he
announced the fact to the scientific
world, other astronomers, armed with
very powerful instruments, looked for
the crater which had been so clearly
seen with Madler's small telescope; but.
though they found crater, it was moth
ing like the crater described by Madler.
The present crater is scarcely two miles
in diameter, and only Just visible with
powerful telescopes; all around it there
is shallow depression, occupying a re-
gion about as large as the whole crater
had been before. 1t seems impossible
to doubt that a great change has taken
place here, and the question arises
whether the change has been produced
by volcanic activity or otherwise. Sir
John Herschel pronounced somewhat
0nnfidnnf1xy in o9
hypothesis. "The most plausible con-
lecture" said he, ''as to the cause of
this disappearance seems to be the fill
ing up of the crater from beneath by an
effusion of viscous lava, which, over
flowing the rim on all sides, may have
so flowed down the outside slope as to
efface its ruggedness, and convert it
into gradual declivity, casting no
strong shadows" But how tremendous
the volcanic energy required to fill with
lava crater nearly seven miles in
diameter, and more than half mile
deep! The volcanic hypothesis seems
on this account utterly incredible; for
if such energy resided in the moons in
terror we should find her whole suface
continually changing. Far more proba-
BILE seems the idea that the wall of this
crater has simply fallen in, scattering
its fragments over what had been the
floor Of the crater. The forces at work
on the moon are quite competent to
throw down steep crater walls like those
which seem formerly to have girt about
this deep cavity. Under the tremen-
dous and long-lasting heat of the lunar
midday sun, the rock substance of the
moons surface must expand, while dur
ing the intense cold of the lunar night
a corresponding contradiction must
take place. Under the influence of this
alternate expansion and contraction the
strongest of the lunar crater walls must
be tending to their downfall. Their
substance must be gradually crumbling
away. From time to time large masses
must topple over, and occasionally long
ranges of crater-wall must be brought
to the ground. 1t seems conceivable
enough, certainly far more probable
than any other Interpretation which has
been offered, that the crater-wall first
missed by Schmidt was destroyed in this
way. The other favorite religion of
selenographers is a much larger one
the great walled plain called Plato, and
by the older astronomers the Greater
Black Lake, sixty miles in diameter,
and surrounded by mountains, some of
which rise nearly $500 yards above the
level of the floor.