For the explanation of the nature of Saturn’s rings we are indebted
to the calculations of mathematicians. You might have thought,
perhaps, that nothing would be simpler than to suppose the rings
were stiff plates made from solid material. But the question cannot
be thus settled. We know that the ring could not bear the strain
of the planet’s attraction upon it if it were a solid body. I may
illustrate the argument by familiar facts about bridges. Where the
span is but a small one, as, for instance, when a road has to cross
a railway, a canal, or a river, the arch is, of course, the proper
kind of structure. There is, for example, a specially beautiful arch
over the river Dee at Chester. But if the bridge be longer than this,
masonry arches are not suitable. Where a considerable span has to be
crossed, as at the Menai Straits, or a gigantic one, as at the Firth
of Forth, then arches have to be abandoned, and iron bridges of a
totally different construction have to be employed. Arches cannot be
used beyond a limited span, because the strain upon the materials
becomes too great for their powers of resistance to withstand. Each of
the stones in an arch is squeezed by intense pressure, and there is a
limit beyond which even the stoutest stones cannot be relied upon. As
soon, therefore, as the span of the arch is so great that the stones
it contains are squeezed as far as is compatible with safety, then the
limit of size for that form of arch has been reached.
Suppose that you stood on Saturn at his equator, and looked up at the
mighty ring which would stretch edgewise across your sky. It would rise
up from the horizon on one side, and, passing over your head, would
slope down to the horizon on the other. You would, in fact, be under an
arch of which the span was about 100,000 miles. Owing to the attraction
of Saturn, every part of that structure would be pulled forcibly
towards his surface, and thus the materials of the arch, if it were a
solid body, would be compressed with terrific force.
It does not really signify that the arch I am now speaking of is half
of a ring the other half of which is below the globe of the planet.
That is only a difference with respect to the support of two ends of
the arch, and does not affect the question as to the pressure upon
its materials; nor does the fact that the ring is revolving remove
the difficulty, though it undoubtedly lessens it. We know no solid
substance which could endure the pressure. Even the toughest steel
that ever was made would bend up like dough under such conditions. We
cannot, therefore, account for Saturn’s ring by supposing it to be a
solid, for no solid would be strong enough.
Do you not remember the old fable of the oak tree and the pliant
reed--how when the storm was about to arise the oak laughed at the
poor reed, and said it would never be able to withstand the blasts?
But matters did not so turn out. The mighty oak, which would not
yield to the storm, was blown down, while the slender reed bent to
the wind and suffered no injury. This gives us a hint as to the true
constitution of Saturn’s ring; it is not a solid body, trying to resist
by mere strength; it is rather to be explained as an excessively pliant
structure. Indeed, I ought not to call it a structure at all; it is
rather a multitude of small bodies not in the least attached together.
I do not know what the size of these bodies may be. For anything we can
tell, they may be no larger than the pebbles you find on a gravel walk.
Let us see how we could encircle our earth with rings like those which
surround Saturn. I shall ask you to be provided with a sufficiently
large number of pebbles, and you must also imagine that I have the
means of ascending high up into space, halfway from here to the moon.
Suppose I went up there and simply dropped the pebble, of course, it
would tumble straight down to the earth again. If, however, I threw it
out with proper speed and in the proper direction, I could start it
off like a little moon, and it would go on round and round our earth
in a circle. I mention a pebble, but really it is little matter what
the size of the object may be--it may be as small as a grain of shot
or as big as a cannon-ball. Now take another pebble. Cast it also in a
somewhat similar path, taking care, however, that the planes of the two
orbits shall be the same. Each of these little bodies shall pursue its
journey without interference from the other. Then proceed in the same
way with a third, a fourth, with thousands and millions and billions of
pebbles, until at last the small bodies will become so numerous that
they almost fill a large part of the plane with a continuous shoal.
Each little object, guided entirely by the earth’s attraction, will
pursue its path with undeviating regularity. Its neighbors will not
interfere with it, nor will it interfere with them. Let us circumscribe
the limits of our flat shoal of moonlets. We first take away all those
that lie outside a certain large circle; then we shall clear away
sufficient to make a vacant space between the outer ring and the inner
ring, and thus the two conspicuous rings have been made; at the inside
of the inner ring we shall take out numbers of pebbles here and there,
so as to make this part much less dense than the outer portions, and
thus produce a semi-transparent crape ring; then we shall clear away
those that come too close to the planet, and form a neat inner boundary.
Could we then view our handiwork from the standpoint of another planet,
what appearance would our earth present? The several pebbles, though
individually so small, would yet, by their countless numbers, reflect
the sun’s light so as to produce the appearance of a continuous sheet.
Thus we should find a large bright outer ring surrounding the earth,
separated by a dark interval from the inner ring, and at the margin of
the inner ring the pebbles would be so much more sparsely distributed
that we should be able to see through them to some extent. That
beautiful system of rings which Saturn displays is undoubtedly of a
similar character to the hypothetical system which I have endeavored to
describe. No other explanation will account for the facts, especially
for the semi-transparency of the crape ring. The separate bodies from
which Saturn’s rings are constituted seem, however, so small that we
are not able to see them individually. There are some other fine lines
running round the rings beside the great division, and these can also
be explained by the theory I have stated.
Saturn has other claims on our attention besides those of its rings.
It has an elaborate retinue of satellites--no fewer, indeed, than
nine; but some of them are very faint objects, and not by any means
so interesting as the system by which Jupiter is attended. The ninth
of these was discovered quite recently by Professor W. H. Pickering,
of Harvard College Observatory. This little moon, for which the name
“Phoebe” has been suggested, is further from the planet than any of the
others. It is a minute object shining as a star of the 15th or 16th
magnitude, and moves around the planet in a period of about sixteen
months.
Saturn was the last and outermost of the planets with which the
ancients were acquainted. Its path lay on the frontiers of the then
known solar system, and the magnificence of the planet itself, with
its attendant luminaries and its marvellous rings, rendered it worthy
indeed of a position so dignified. These five planets--namely, Mercury,
Venus, Mars, Jupiter, and Saturn--made up with the sun and the moon
the seven “planets” of the ancients. They were supposed to complete
the solar system, and, furthermore, the existence of other members was
thought to be impossible. In modern times it has been discovered that
there are yet two more planets. I do not now refer to those little
bodies which run about in scores between Mars and Jupiter. I mean two
grand first-class planets, far bigger than our earth. One of them is
Uranus, which revolves far outside Saturn, and the other is Neptune,
which is much further still, and whose mighty orbit includes the whole
planetary system in its circuit. To complete its journey round the sun
not less than 165 years is required.