Jupiter is the first of the outer and
greater planets, the major, or Jovian, group. His mean diameter is
86,500 miles, and his average girth more than 270,000 miles. An
inhabitant of Jupiter, in making a trip around his planet, along any
great circle of the sphere, would have to travel more than 30,000 miles
farther than the distance between the earth and the moon. The polar
compression of Jupiter, owing to his rapid rotation, amounts in the
aggregate to more than 5,000 miles, the equatorial diameter being 88,200
miles and the polar diameter 83,000 miles.
Jupiter's mean distance from the sun is 483,000,000 miles, and the
eccentricity of his orbit is sufficient to make this distance variable
to the extent of 21,000,000 miles; but, in view of his great average
distance, the consequent variation in the amount of solar light and heat
received by the planet is not of serious importance.
When he is in opposition to the sun as seen from the earth Jupiter's
mean distance from us is about 390,000,000 miles. His year, or period of
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.
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