du Syst. du Monde', p. 396, 414.
It has been proposed to consider the telescopic planets, Vesta, Juno, Ceres,
and Pallas, with their more closely intersecting, inclined, and eccentric
orbits, as a zone of separation, or as a middle group in space; and if this
view be adopted, we shall discover that the interior planetary group
(consisting of Mercury, Venus, the Earth, and Mars) presents several very
striking contrasts* when compared with the exterior group, comprising
Jupiter, Saturn, and Uranus.
[footnote] *Littrow, 'Astronomie', 1825, bd.xi., 107. M??dler, 'Astron.',
1841, ?ยค 212. Laplace, 'Exp. du Syst. du Monde', p. 210.
The planets nearest the Sun, and consequently included in the inner group,
are of more moderate size, denser, rotate more slowly and with nearly equal
velocity (their periods of revolution being almost all about 24 hours), are
less compressed at the poles, and with the exception of one, are without
satellites. The exterior planets, which are further removed from the Sun,
are very considerably larger, have a density five times less, more than
twice as great a velocity in the period of their rotation round their axes,
are more compressed at the poles, and if six satellites may be ascribed to
Uranus, have a quantitative preponderance in the number of their attendant
moons, which is as seventeen to one.
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