The principal planets are, therefore, themselves the
central bodies of subordinate systems. We seem to recognize in the fabric
of the universe the same process of arrangement so frequently exhibited in
the development of organic life, where we find in the manifold combinations
of groups of plants or animals the same typical form repeated in the
'subordinate classes'. The secondary planets or satellites are more
frequent in the external region of the planetary system, lying beyond the
intersecting orbits of the smaller planets or asteroids; in the inner region
none of the planets are attended by satellites, with the exception of the
Earth, whose moon is relatively of great magnitude, since its diameter is
equal to a fourth of that of the Earth, while the diameter of the largest of
all known secondary planets -- the sixth satellite of Saturn -- is probably
about one seventeenth, and the largest of Jupiter's moons, the third, only
about one twenty-sixth part that of the primary planet or central body. The
planets which are attended by the largest number of satellites are most
remote from the Sun,
p 96
and are at the same time the largest, most compressed at the poles, and the
least dense. According to the most recent measurements of M??dler, Uranus
has a greater planetary compression than any other of the planets, viz.
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