Page:Popular Science Monthly Volume 1.djvu/300

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correspond to his bulk, for it exceeds the mass of the earth only three hundred times. So that, if the disk our astronomers see and measure actually represents the true globe of the planet, his substance must be, on the average, much less dense than that of the earth. In fact, while the earth's density is nearly six times as great as that of water, the density of Jupiter (thus judged) would exceed that of water by barely one-third. This vast globe rotates, in less than ten hours, on an axis nearly upright or square to the level in which the planet travels. This rapidity of rotation—so great that points on the planet's equator travel twenty seven times as fast as points on the terrestrial equator—results in a considerable flattening of the planet's globe; insomuch that the polar diameter is less than the equatorial by about a twelfth part, or by fully 7,000 miles. And it may be remarked in passing, that this circumstance—the fact, namely, that the poles of the planet are drawn in, as it were, 3,500 miles as compared with the equatorial regions, or 1,750 miles as compared with the mid-latitudes in either hemisphere—affords a striking illustration of the enormous amount of energy really represented by the rotation of Jupiter. It may also be added that the velocity with which points on Jupiter's equatorial zone are carried round, exceeds the corresponding velocity in the case of all the planets in the solar system, and is nearly six times greater than the equatorial velocity of the sun himself. It amounts, in fact, to about 7½ miles per second.

We do not propose to consider here at any length the system of satellites over which Jupiter bears sway; but this preliminary sketch would be incomplete without a few words on the subject. It is worthy of notice that, although our earth in some sort resembles the outer planets in being accompanied by a satellite, yet the relation which our moon bears to the earth is altogether different from that which the satellites of the outer planets bear to their respective primaries. Our moon is by no means a minute body by comparison with the earth, and compared with Mars or Mercury she may be regarded as having very respectable dimensions. We may, indeed, look upon the moon as a fifth member of the inner family of planets—a member inferior to the rest, doubtless, but still not so far inferior to Mercury as Mercury is inferior to the earth. In the case of the outer planets, however, and especially in Jupiter's case, moons hold an utterly subordinate position. Taking the accepted measurements, we find the largest of Jupiter's moons less than the 16,000th part of its primary as respects bulk, while its mass or weight is less than the 11,000th part of Jupiter's.[1]

  1. It is not uncommonly stated in our text-books of astronomy, that the density of Jupiter's moons is far less than Jupiter's density; and Lardner goes so far as to say that "the density of the matter composing these satellites is much smaller than that of any other body of the system whose density is known." But this is a mistake. All the satellites, save one, are of greater density than Jupiter, and that one—the innermost—is denser than Saturn, Uranus, or Neptune.