Of all these bodies Jupiter is by far the greatest; he is, indeed,
greater than all the other planets rolled into one. The relative
insignificance of the earth when compared with Jupiter is well
illustrated by the fact that if we took 1200 globes each as big as our
earth, and made them into a single globe, it would only be as large as
the greatest of the planets. A view of the comparative sizes of the
earth and Jupiter is shown in Fig. 66.
[Illustration: FIG. 66.--Jupiter and the Earth compared.]
Fig. 67 shows a picture of Jupiter as seen through the telescope.
First, you will notice that the outline of the planet’s shape is not
circular, for it is plain that the vertical diameter in this picture
is shorter than the horizontal one; in fact, Jupiter is flattened at
the Poles and bulges out at the equator, so that a section through the
Poles is an ellipse. Jupiter is turning round rapidly on his axis, and
this will account for the protuberance. We find that the planet has
assumed almost the same form as if it were actually a liquid. This
we can illustrate by a globe of oil which is poised in a mixture of
spirits of wine and water so carefully adjusted that the oil has no
tendency to rise or fall. As we make the globe of oil rotate, which we
can easily do by passing a spindle through it, we see that it bulges
out in the form that Jupiter as well as other planets have taken.
[Illustration: FIG. 67.--The Clouds of Jupiter.]
On the picture of the planets you will see shaded bands. These are
constantly changing their aspect, and for a double reason. In the first
place, they change because Jupiter is rotating so quickly that in
five hours the whole side of the planet which is towards us has been
carried out of sight. In another five hours the original side of the
globe will be back again, for the entire rotation occupies about 10
hours, or, more precisely, 9 hours 55 minutes 21 seconds.
But these bands are themselves not permanent objects. They have no more
permanence than the clouds over our own sky. Sometimes Jupiter’s clouds
are more strongly marked than on other occasions. Sometimes, indeed,
they are hardly to be seen at all. It is from this we learn that those
markings which we see when we look at the great planet are merely the
masses of cloud which surround and obscure whatever may constitute his
interior.
There is a circumstance which demonstrates that Jupiter must be an
object exceedingly different from the earth, though both bodies agree
in so far as having clouds are concerned. What would you think when I
tell you that we were able to weigh Jupiter by the aid of his little
moons, of which I shall afterwards speak? These little bodies inform us
that Jupiter is about 300 times as heavy as our earth, and we have no
doubt about this, for it has been confirmed in other ways. But we have
found by actual measurement that Jupiter is 1200 times as big as the
earth, and therefore, if he were constituted like the earth, he ought
to be 1200 times as heavy. This is, I think, quite plain; for if two
cakes were made of the same material, and one contained twice the bulk
of the other, then it would certainly be twice as heavy. If there be
two balls of iron, one twice the bulk of the other, then, of course,
one has twice the weight of the other. But if a ball of lead have
twice the bulk of a ball of iron, then the leaden ball would be more
than twice as heavy as the iron, because lead is the heavier material.
In the same way, the weights of the earth and Jupiter are not what we
might expect from their relative sizes. If the two bodies were made
of the same materials and in the same state, then Jupiter would be
certainly four times as heavy as we find him to be. We are, therefore,
led to the belief that Jupiter is not a solid body, at least in its
outer portions. The masses of cloud which surround the planet seem
to be immensely thick, and as clouds are, of course, light bodies in
comparison with their bulk, they have the effect of largely increasing
the apparent size of Jupiter, while adding very little to his weight.
There is thus a great deal of mere inflation about this planet, by
which he looks much bigger than his actual materials would warrant if
he were constituted like the earth.
These facts suggest an interesting question. Why has Jupiter such an
immense atmosphere, if we may so call it? The clouds we are so familiar
with down here on the earth are produced by the heat of the sun, which
beats down upon the wide surface of the ocean, evaporates the water,
and raises the vapor up to where it forms the clouds. Heat, therefore,
is necessary for the formation of cloud; and with clouds so dense and
so massive as those on Jupiter, more heat would apparently be necessary
than is required for the moderate clouds on this earth. Whence is
Jupiter to get this heat? Have we not seen that the great planet is
far more distant from the sun than we are? In fact, the intensity of
the sun’s heat on Jupiter is not more than the twenty-fifth part of
what we derive from the same source. We can hardly believe that the
sun supplied the heat to make those big clouds on the great planet; so
we must cast about for an additional source, which can only be inside
the planet itself. So far as his internal heat is concerned, Jupiter
seems to be in much the same condition now as our earth was once, ages
ago, before its surface had cooled down to the present temperature.
As Jupiter is so much larger than the earth, he has been slower in
parting with his heat. The planet seems not yet to have had time to
cool sufficiently to enable water to remain on his surface. Thus the
internal heat of the planet supplies an explanation of his clouds. We
may also remark that as the present condition of Jupiter illustrates
the early condition of our earth, so the present condition of the earth
foreshadows the future reserved for Jupiter when he shall have had time
to cool down, and when the waters that now exist in the form of vapor
shall be condensed into oceans on his surface.