Page:Popular Science Monthly Volume 4.djvu/52

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42
THE POPULAR SCIENCE MONTHLY.

of course, excepted) in the solar system. The reader is doubtless aware that the sun's mean density is almost exactly one-fourth of the earth's; Jupiter's is almost exactly the same as the sun's; but Saturn's is little more than half the sun's, being represented by the number 13 only, where 100 represents the earth's. Thus, instead of exceeding the earth nearly 700 times in mass, as he would if he were of the same density, he exceeds her but about 90 times. But this disproportion must still be regarded as enormous, especially when it is added that the combined mass of the four terrestrial planets amounts to little more than the forty-fourth part of Saturn's mass. The combined mass of Uranus and Neptune, though these are members of the family of major planets, falls short of one-third of Saturn's mass; yet, by comparison with Jupiter, whose mass exceeds his more than threefold, Saturn appears almost dwarfed. And it may be noted as a striking circumstance—one that is not sufficiently recognized in our astronomical treatises—that, while Jupiter's mass exceeds the combined mass of all the other planets (including Saturn) about two and a half times, Saturn exceeds all the remaining planets in mass about two and three-quarter times. So unequally is the material of the planetary system distributed.

Fig. 2.
PSM V04 D052 Saturn and his moons.jpg
Saturn and his Moons.

The mighty globe of Saturn rotates on its axis in about nine hours and a half, the most rapid rotation in the solar system so far as is yet known.

But what shall we say to indicate adequately the dimensions of that enormous ring-system which circles around Saturn? Here we have no unit of comparison, and scarcely any mode of presenting the facts except the mere statement of numerical relations. Thus the full span of the rings, measured across the centre of the planet, amounts to 167,000 miles; the full breadth of the ring-system amounts to 35,600 miles. But these numbers convey only imperfect ideas. Perhaps the best way of indicating the enormous extent of the ring-system is to mention that circumnavigation of the world by a ship sailing from England to New Zealand by the Cape of Good Hope, and from New Zealand to England by Cape Horn, would have to be repeated 21 times to give a distance equaling the outer circumference of