permanent. Cassini's great spot vanished and reappeared nine times
between 1665 and 1713. It was close to the northern margin of the
southern belt. Herschel supposed the belts to be the body of the
planet, and the lighter parts to be clouds confined to certain
latitudes.

In 1665 Cassini observed transits of the four satellites, and also saw
their shadows on the planet, and worked out a lunar theory for
Jupiter. Mathematical astronomers have taken great interest in the
perturbations of the satellites, because their relative periods
introduce peculiar effects. Airy, in his delightful book,
_Gravitation_, has reduced these investigations to simple
geometrical explanations.

In 1707 and 1713 Miraldi noticed that the fourth satellite varies much
in brightness. W. Herschel found this variation to depend upon its
position in its orbit, and concluded that in the positions of
feebleness it is always presenting to us a portion of its surface,
which does not well reflect the sun's light; proving that it always
turns the same face to Jupiter, as is the case with our moon. This
fact had also been established for Saturn's fifth satellite, and may
be true for all satellites.

In 1826 Struve measured the diameters of the four satellites, and
found them to be 2,429, 2,180, 3,561, and 3,046 miles.

In modern times much interest has been taken in watching a rival to
Cassini's famous spot. The "great red spot" was first observed by
Niesten, Pritchett, and Tempel, in 1878, as a rosy cloud attached to a
whitish zone beneath the dark southern equatorial band, shaped like