If, instead of absolute distances, we take the semi-diameters of the
principal planets, we shall find that even the first or nearest of the moons
of Jupiter (which is 26,000 miles further removed from the center of that
planet than our moon is from that of the Earth) is only six semi-diameters
of Jupiter from its center, while our moon is removed from us fully 60 1/3d
semi-diameters of the Earth.
In the subordinate systems of satellites, we find that the same laws of
gravitation which regulate the revolutions of the principal planets round
the Sun likewise govern the mutual relations existing between these planets
among one another and with reference to their attendant satellites. The
twelve moons of Saturn, Jupiter, and the Earth all most like the primary
planets from west to east, and in elliptic orbits, deviating
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but little from circles. It is only in the case of one moon, and perhaps in
that of the first and innermost of the satellites of Saturn (0.068), that we
discover an eccentricity greater than that of Jupiter; according to the very
exact observations of Bessel, the eccentricity of the sixth of Saturn's
satellites (0.029) exceeds that of the Earth. On the extremest limits of
the planetary system, where, at a distance nineteen times greater than that
of our Earth, the centripetal force of the Sun is greatly diminished, the
satellites of Uranus (which most striking contrasts from the facts observed
with regard to other secondary planets.
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