that some of these floating hoops
are tilted with regard to the others, so that one half of a hoop rises
out of the water and the other half consequently sinks beneath the
surface. This indeed is the actual case with regard to the planetary
orbits. They do not by any means lie all exactly in the same plane. Each
one of them is tilted, or _inclined_, a little with respect to the plane
of the earth's orbit, which astronomers, for convenience, regard as the
_level_ of the solar system. This tilting, or "inclination," is, in the
larger planets, greatest for the orbit of Mercury, least for that of
Uranus. Mercury's orbit is inclined to that of the earth at an angle of
about 7 deg., that of Venus at a little over 3 deg., that of Saturn 2-1/2
deg.; while in those of Mars, Neptune, and Jupiter the inclination is less
than 2 deg. But greater than any of these is the inclination of the orbit
of the tiny planet Eros, viz. nearly 11 deg.

The systems of satellites revolving around their respective planets
being, as we have already pointed out, mere miniature editions of the
solar system, the considerations so far detailed, which regulate the
behaviour of the planets in their relations to the sun, will of
necessity apply to the satellites very closely. In one respect, however,
a system of satellites differs materially from a system of planets. The
central body around which planets are in motion is self-luminous,
whereas the planetary body around which a satellite revolves is not.
True, planets shine, and shine very brightly too; as, for instance,
Venus and Jupiter. But they do not give forth any light of their own, as
the sun does; they merely reflect the sunlight which they receive from
him. Putting this one fact aside, the analogy between the planetary
system and a satellite system is remarkable. The satellites are
spherical in form, and differ markedly in size; they rotate, so far as
we know, upon their axes in varying times; they revolve around their
governing planets in orbits, not circular, but elliptic; and these
orbits, furthermore, do not of necessity lie in the same plane. Last of
all the satellites revolve around their primaries at rates which are
directly comparable with those at which the planets revolve around the
sun, the rule in fact holding good that the nearer a satellite is to its
primary the faster it revolves.

[3] As there seems to be much difference of opinion concerning the
diameters of Uranus and Neptune,