Jupiter accomplishes one rotation in a little less than ten hours; but,
curiously enough, all parts of the planet do not rotate in the same
length of time. A day at the equator is nine hours and fifty minutes in
length. In some of the higher latitudes it is nine hours and fifty-five
minutes, and this notwithstanding the equator is so much larger in
circumference than any other part and any one point on it has farther
to go in a revolution. As many as eight different rates of rotation
have been observed; and even in the same zones some parts seem to lag
behind others, taking a little more time to complete the rotation than
other parts surrounding them. This is another indication that Jupiter
is not a solid body. The surface features are none of them permanent,
though some of them remain practically the same for years. It is
through this occasional stability of them that it has been possible to
mark the planet’s time of rotation.
In the matter of seasons Jupiter has very little variety. The axis of
the planet is inclined but little more than three degrees to its orbit,
so that whatever amount of heat the sun’s radiance affords must be
very nearly uniform during the entire Jovian year. Its distance, too,
is at all times so great that there would be no appreciable change in
temperature between its perihelion and aphelion positions.
There is every indication that Jupiter has an extraordinarily dense
and deep atmosphere. It has been sometimes estimated to be as much
as a thousand miles in depth, and the spectroscope shows it to be
heavily laden with vapor. But beyond these very general facts not much
is definitely known about it. It is certain, though, that it is very
different from our atmosphere. The spectroscope shows in it elements,
or compounds of elements, which are not familiar to us. The enormous
gravitative power of Jupiter would enable him to hold gases rarer than
the earth, or the smaller planets like the earth, ever acquired. A
molecule of gas would have to move more rapidly than thirty-seven miles
a second to escape from Jupiter. The earth, as we have seen, cannot
hold any gases moving faster than seven miles a second. So there are
many gases which may forever remain in Jupiter’s atmosphere and yet
have never had a place in ours.