arts very appreciably from a circle, the
greatest distance from the sun being 5.45, while the least distance is
about 4.95, the earth's distance from the sun being taken as unity. In
the most favourable circumstances for seeing Jupiter at opposition, it
must still be about four times as far from the earth as the earth is
from the sun. This great globe will also illustrate the law that the
more distant a planet is, the slower is the velocity with which its
orbital motion is accomplished. While the earth passes over eighteen
miles each second, Jupiter only accomplishes eight miles. Thus for a
twofold reason the time occupied by an exterior planet in completing a
revolution is greater than the period of the earth. Not only has the
outer planet to complete a longer course than the earth, but the speed
is less; it thus happens that Jupiter requires 4,332.6 days, or about
fifty days less than twelve years, to make a circuit of the heavens.

The mean diameter of the great planet is about 87,000 miles. We say the
_mean_ diameter, because there is a conspicuous difference in the case
of Jupiter between his equatorial and his polar diameters. We have
already seen that there is a similar difference in the case of the
earth, where we find the polar diameter to be shorter than the
equatorial; but the inequality of these two dimensions is very much
larger in Jupiter than in the earth. The equatorial diameter of Jupiter
is 89,600 miles, while the polar is not more than 84,400 miles. The
ellipticity of Jupiter indicated by these figures is sufficiently marked
to be obvious without any refined measures. Around the shortest diameter
the planet spins with what must be considered an enormous velocity when
we reflect on the size of the globe. Each rotation is completed in about
9 hrs. 55 mins.

We may naturally contrast the period of rotation of Jupiter with the
much slower rotation of our earth in twenty-four hours. The difference
becomes much more striking if we consider the relative speeds at which
an object on the equator of the earth and on that of Jupiter actually
moves. As the diameter of Jupiter is nearly eleven times that of the
earth, it will follow that the speed of the equator on Jupiter must be
about twenty-seven times as great as that on the earth. It is no doubt
to this high velocity of rotation that we must ascribe the extraordinary
ellipticity of Jupiter; the rapid rotation causes a great centrifugal
force, and this bulges