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.
[Illustration: 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.