364 The American Geologist. December, i904. 
elude with reasonable safety, that what in this respect is true of 
all the planets fromi Mercury to Uranus and which also holds 
good as to hundreds of minor planets, will, no doubt, hold 
good also as to Neptune. 
§15 HOW THE MOONS HAVE COME TO THE PLANETS. 
It appears clearly froni what we have already said, that he 
relation with regard to age in which one planet stands to an- 
other, cannot be determined by the planet's distance from the 
Sun, since their origin is independent of that body. As we 
have shown, further, that no heavenly body appears origin- 
ally with a satellite, the moon has, consequently, been develop- 
ed out of a nebula different from that of the Earth, and has 
at some time revolved around the Sun. The moons of the 
other planets have commenced their existence in the same 
manner, that is, as free bodies circling around the same center 
of force. But how, then, has it happened that these little 
bodies have been captured by the larger ones, and lost their 
original orbits? 
In §12 it was pointed out that the excentricity of the plane- 
tary orbits depends on the size of the planets and their dis- 
tances from the Sun. 
We have mentioned that Mercury's diameter is about 3,- 
200 miles, its orbital excentricity somewhat more than seven 
million miles, and its distance from the Sun 36 million miles ; 
also that its aphelion distance from the Svm is about 15 mil- 
lion miles greater than its perihelion distance. From t:iis it 
follows, that the Moon, which has a diameter of 2,160 miles, 
would, if it had Mercury's average distance from the Sun, 
and in obedience to the law of excentricity. have a greater or- 
bital excentricity than Mercury. We understand, thus, that if 
the Moon had an orbit of about double the distance of ^lor- 
cury's from the Sun, it would have an excentricity of rather 
more than 12 million miles or thereabout, and it would reach 
about 25 million miles farther from the Sun at aphelion iflian 
at perihelion. But now the question is this: Was the dis,- 
tance of the Moon somewhere between 70 and 80 million 
miles? 
Inasmuch as it was not captured by Venus, it must have 
occupied a place so far outside of this planet, that it did not 
