The sixth
of these satellites is probably not much smaller than Mars, while our moon
has a diameter which does not amount to more than half that of the latter
planet. With respect to volume, the two outer, the sixth and seventh of
Saturn's satellites, approach the nearest to the third and brightest of
Jupiter's moons. The two innermost of these satellites belong perhaps,
together with the remote moons of Uranus to the smallest cosmical bodies of
our solar system, being only made visible under favorable circumstances by
the most powerful instruments. They were first discovered by the forty-foot
telescope of William Herschel in 1789, and were seen again by John Herschel
at the Cape of Good Hope, by Vico at Rome, and by Lamont at Munich.
Determinations of the 'true' diameter of satellites, made by the measurement
of the apparent size of their small disks, are subjected to many optical
difficulties; but numerical astronomy, whose task it is to predetermine by
calculation the motions of the heavenly bodies as they will appear when
viewed from the Earth, is directed almost
p 97
exclusively to motion and mass, and but little to volume. The absolute
distance of a satellite from its central body is greatest in the case of the
outermost or seventh satellite of Saturn, its distance from the body round
which it revolves amounting to more than two millions of miles, or ten times
as great a distance as that of our moon from the Earth.
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