JUPITER, THE GREATEST OF KNOWN WORLDS

When we are thinking of worlds, and trying to exalt the imagination with them, it is well to turn to Jupiter, for there is a planet worth pondering upon!  A world thirteen hundred times as voluminous as the earth is a phenomenon calculated to make us feel somewhat as the inhabitant of a rural village does when his amazed vision ranges across the million roofs of a metropolis.  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.

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.