Although the planet Jupiter does not present such striking
features as Saturn, it is of even more interest to the amateur
astronomer, because he can study it with less optical power, and
see more of the changes upon its surface. Every work on astronomy
tells in a general way of the belts of Jupiter, and many speculate
upon their causes. The reader of recent works knows that Jupiter
is supposed to be not a solid mass like the earth, but a great
globe of molten and vaporous matter, intermediate in constitution
between the earth and the sun. The outer surface which we see is
probably a hot mass of vapor hundreds of miles deep, thrown up
from the heated interior. The belts are probably cloudlike forms
in this vaporous mass. Certain it is that they are continually
changing, so that the planet seldom looks exactly the same on two
successive evenings. The rotation of the planet can be very well
seen by an hour's watching. In two hours an object at the centre
of the disk will move off to near the margin.
The satellites of this planet, in their ever-varying phases, are
objects of perennial interest. Their eclipses may be observed with
a very small telescope, if one knows when to look for them.
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