THE PLACE OF ORIGIN OF THE MOON—THE VOL- 
CANIC PROBLEM 
WILLIAM H. PICKERING 
In 1879 Professor George H. Darwin propounded the view that 
the Moon formerly formed a part of the Earth. That it was origi- 
nally much nearer to the Earth than it is at present, and is now 
slowly receding from us, was clearly shown by his equations. After 
considerable discussion, his conclusions have been accepted by the 
great majority of astronomers, although many of the geologists do 
not view them with favor. Assuming the correctness of his hypoth- 
esis, it will be of interest to determine, first, if possible, from what 
part of the Earth the Moon originated, and, second, to follow out 
our conclusions on this point and see to what results they may lead. 
_ When the separation took place, it has been shown that the com- 
bined planet was not very much larger than is the Earth at present. 
It must therefore have been mostly in the solid or liquid condition. 
If in the latter state, it is obvious that no indication of the Moon’s 
former place could be found at the present time. Very few astron- 
omers or geologists today, however, believe that the Earth ever was 
completely liquid. It has probably always been partly solid, partly 
liquid, and partly gaseous. It is composed of such diverse materials, 
and these are exposed at different points throughout its volume to 
such diverse pressures, that, unless we assume it to have condensed 
from a highly ineandescent nebula, which is unlikely, we should 
scarcely expect it ever to have presented a uniform liquid surface. 
The surface was probably hot, but how hot we have no means of 
knowing. Beneath the surface, however, where radiation was 
impossible, much higher temperatures were found, as is still the case 
and in what follows we shall assume that the interior was practically 
liquid, or was ready to become actually so where relieved of the 
pressure due to the gravity of the outer layers; that is, where the 
centrifugal force became sufficiently high, as in the equatorial regions. 
Precisely how the Earth came into its present form, whether by 
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