The Ways of the Planets

by Martha Evans Martin

XIII

JUPITER

One never feels so impressed with the power of the sun as when one contemplates it in relation to Jupiter. Great Jupiter, he may well be called, nearly five hundred million miles out in space, almost a sun himself, the center of a system containing bodies larger than the sun’s nearest planet, Mercury; and yet just Jupiter, one of the planets, held firmly in leash like the others by the sun’s overwhelming force of gravity, forever compelled to revolve about that parent body with the rest of its offspring, to stay at home within the bounds of the sun’s domain, to keep within certain limits in his own orbit, forced to hasten on when he comes nearest the power that controls him, and unable to keep up the same rate of speed when he is farther away. One may well wonder at the immensity beyond comprehension of the stars, among which our sun is but a very small one, when one considers how even this small one can thus swing huge Jupiter about. For Jupiter is, after the sun itself, the mammoth member of our system. In volume he is larger than all the other planets put together, and in mass he is more than double as large as the combined mass of all the others. He is about equal to the sun in density, and about one-fourth as dense as the earth.

There is less difference in size between Jupiter and the sun than there is between Jupiter and the earth. His diameter is eleven times greater than that of the earth. The sun’s diameter is only ten times greater than Jupiter’s. His surface is one hundred and sixteen times that of the earth; the sun’s own surface is only a hundred times larger than his. Jupiter weighs more than three hundred times as much as the earth; the sun weighs only six times more than Jupiter. At the equator his diameter is about ninety thousand miles; but, as the planet is much flattened at the poles, the diameter from pole to pole is only a little more than eighty-four thousand miles. This flattening is due to the very rapid spinning of the planet on its axis, a motion that will always cause a plastic body to bulge at the equator, and thus flatten at the poles.

JUPITER, THE MAMMOTH MEMBER OF THE SOLAR FAMILY—LARGER THAN ALL THE OTHER PLANETS PUT TOGETHER

This photograph shows the flattening at the poles and also the belts encircling the planet. It was photographed at the Yerkes Observatory.

The force of gravity on Jupiter is about two and one-half times greater than on the earth. A fairy-like figure weighing here only a hundred pounds would be held to the surface of Jupiter with a force equal to two hundred and sixty pounds. This tremendous power makes Jupiter the greatest disturbing body among all the planets. He gives Saturn a mighty pull when the two planets come near each other; he draws some of the little asteroids five or six degrees out of their course when it carries them into the field of his influence; and there are as many as thirty comets that have become permanent members of the solar system, because through his great power of attraction he has made them captive.

Jupiter is so much farther from the sun than we are that his orbit is about five times larger than that of the earth. In consequence also of his greater distance from the sun, he moves much more slowly than the earth. His average velocity is about eight miles a second. It requires more than four thousand days, or nearly twelve of our years, for him to make one revolution around the sun, and he thus consumes more than ten thousand of his own days. He travels through about one sign of the zodiac each year, and is thus not very difficult to keep trace of, since the signs and the constellations of the zodiac so nearly coincide. His synodic period, or the period from one opposition to another, is a fraction less than three hundred and ninety-nine days, or about one year and a little more than a month. His daily motion in the skies is almost too small for us to detect it without observation for more than a day. It is in one day about equal to one-sixth of the apparent diameter of the moon; but in a month he has moved a distance about half as great as that between the two pointers in the Big Dipper, as can be easily seen by comparison with the stars near him.