Next outside Jupiter, on the confines of the ancient planetary system,
revolves another grand planet, called Saturn. His distance from us is
sometimes nearly a thousand millions of miles, and he requires more
than a quarter of a century for the completion of each revolution.
Sometimes people do not pronounce the names of the planets quite
correctly. I have heard of a gardener who has a taste for astronomy,
and sometimes begins to talk about the planets Juniper and Citron.
Probably you will know what he meant to say. The ancients had
discovered Saturn to be a planet, for though he looked like a star, yet
his movement through the constellations could not escape their notice
when attention was paid to the heavens.
[Illustration: FIG. 68.--Saturn and the Earth compared.]
In the matter of size Saturn is only surpassed by Jupiter among the
planets. He is about 600 times as large as the earth; the small object,
E, shown in Fig. 68, represents our earth in its true comparative size
to the ringed planet; but Saturn is so far off, that even at his best
he is never so bright as Venus, or Mars, or Jupiter become when they
are favorably situated. On the globe of Saturn we can sometimes see a
few bands, but they are faint compared with those on Jupiter. There
is, however, no doubt that what we see upon Saturn is a dense mass of
cloud. Indeed, he can have comparatively little solid matter inside,
for this planet does not weigh so much as a ball of water the same size
would do. Saturn, like Jupiter, must be highly heated in his interior.
The ring, or rather series of rings, by which the planet is surrounded
are also shown in Fig. 68: these appendages are not fastened to the
globe of Saturn by any material bonds; they are poised in space,
without any support, while the globe or planet proper is placed
symmetrically in the interior.
I have made a model which shows Saturn with his rings, but it is
necessary for me to fasten the rings by little pieces of wire to the
globe, for there is no mechanical means by which the rings of the model
could be poised without support, as they are around the planet. If
we throw the beam of the electric lamp on the little planet, we see
the shadow which the planet casts on its ring. Similar shadows can be
observed in the actual Saturn of the sky, and this is a proof that
the planet does not shine by its own light, but by the light of the
sun which falls upon it. Here again we illustrate the wide difference
between a planet and a star, for were our sun to be put out, Saturn
and all the other planets in the sky would vanish from sight, while
the stars would, of course, twinkle on as before. There are three
rings round Saturn; they all lie in the same plane, and they are so
thin, that when turned edgewise towards us the whole system almost
disappears, except in very powerful telescopes. The outer and the
inner bright rings are divided by a dark line, which can be traced
entirely round. At the inner edge of the inner ring begins that strange
structure called the _crape_ ring, which extends halfway towards the
globe of the planet. The most remarkable point about the crape ring is
its semi-transparency, for we can sometimes see the globe of the planet
through this strange curtain. The crape ring can only be observed with
a powerful telescope. The other two rings are within the power of very
moderate instruments.