revolution about the sun, is somewhat less than twelve of our years
(11.86 years). His axis is very nearly upright to the plane of his
orbit, so that, as upon Venus, there is practically no variation of
seasons. Gigantic though he is in dimensions, Jupiter is the swiftest of
all the planets in axial rotation. While the earth requires twenty-four
hours to make a complete turn, Jupiter takes less than ten hours (nine
hours fifty-five minutes), and a point on his equator moves, in
consequence of axial rotation, between 27,000 and 28,000 miles in an
hour.

The density of the mighty planet is slight, only about one quarter of
the mean density of the earth and virtually the same as that of the sun.
This fact at once calls attention to a contrast between Jupiter and our
globe that is even more significant than their immense difference in
size. The force of gravity upon Jupiter's surface is more than two and a
half times greater than upon the earth's surface (more accurately 2.65
times), so that a hundred-pound weight removed from the planet on which
we live to Jupiter would there weigh 265 pounds, and an average man,
similarly transported, would be oppressed with a weight of at least 400
pounds. But, as a result of the rapid rotation of the great planet, and
the ellipticity of its figure, the unfortunate visitor could find a
perceptible relief from his troublesome weight by seeking the planet's
equator, where the centrifugal tendency would remove about twenty pounds
from every one hundred as compared with his weight at the poles.

If we could go to the moon, or to Mercury, Venus, or Mars, we may be
certain that upon reaching any of those globes we should find ourselves
upon a solid surface, probably composed of rock not unlike the rocky
crust of the earth; but with Jupiter the case would evidently be very
different. As already remarked, the mean density of that planet is only