THE RINGED PLANET
Saturn is the most remote of all the planets that the ancient peoples
knew anything about. These anciently known planets are sometimes called
the lucid or naked-eye planets--five in number: Mercury, Venus, Mars,
Jupiter, and Saturn. Saturn shines as a first-magnitude star, with a
steady straw-colored light, and is at a distance of about 800 million
miles from the earth when best seen. Saturn travels completely round the
sun in a little short of thirty years, and the telescope, when turned to
Saturn, reveals a unique and astonishing object; a vast globe somewhat
similar to Jupiter, but surrounded by a system of rings wholly unlike
anything else in the universe, as far as at present known; the whole
encircled by a family of ten moons or satellites. The Saturnian system,
therefore, is regarded by many as the most wonderful and most
interesting of all the objects that the telescope reveals.
At first the flattening of the disk of Saturn is not easily made out,
but every fifteen years (as 1921 and 1936) the earth comes into a
position where we look directly at the thin edge of the rings, causing
them to completely disappear. Then the remarkable flattening of the
poles of Saturn is strikingly visible, amounting to as much as one-tenth
of the entire diameter. The atmospheric belt system is also best seen at
these times.
But the rings of Saturn are easily the most fascinating features of the
system. They can never be seen as if we were directly above or beneath
the planet so they never appear circular, as they really are in space,
but always oval or elliptical in shape. The minor axis or greatest
breadth is about one-half the major axis or length. The latter is the
outer ring's actual diameter, and it amounts to 170,000 miles, or two
and one-half times the diameter of Saturn's globe.
There are in fact no less than four rings; an outer ring, sometimes seen
to be divided near its middle; an inner, broader and brighter ring; and
an innermost dusky, or crape ring, as it is often called. This comes
within about 10,000 miles of the planet itself. After the form and size
of the rings were well made out, their thickness, or rather lack of
thickness, was a great puzzle.
If a model about a foot in diameter were cut out of tissue paper, the
relative proportion of size and thickness would be about right. In space
the thickness is very nearly 100 miles, so that, when we look at the
ring system edge-on, it becomes all but invisible except in very large
telescopes. Clearly a ring so thin cannot be a continuous solid object
and recent observations have proved beyond a doubt that Saturn's rings
are made up of millions of separate particles moving round the planet,
each as if it were an individual satellite.
Ever since 1857 the true theory of the constitution of the Saturnian
ring has been recognized on theoretic grounds, because Clerke-Maxwell
founded the dynamical demonstration that the rings could be neither
fluid nor solid, so that they must be made up of a vast multitude of
particles traveling round the planet independently. But the physical
demonstration that absolutely verified this conclusion did not come
until 1895, when, as we have said in a preceding chapter, Keeler, by
radial velocity measures on different regions of the ring by means of
the spectroscope, proved that the inner parts of the ring travel more
swiftly round the planet than the outer regions do. And he further
showed that the rates of revolution in different parts of the ring
exactly correspond to the periods of revolution which satellites of
Saturn would have, if at the same distance from the center of the
planet. The innermost particles of the dusky ring, for example, travel
round Saturn in about five hours, while the outermost particles of the
outer bright ring take 137 hours to make their revolution. For many
years it was thought that the Saturnian ring system was a new satellite
in process of formation, but this view is no longer entertained; and the
system is regarded as a permanent feature of the planet, although
astronomers are not in entire agreement as to the evolutionary process
by which it came into existence--whether by some cosmic cataclysm, or by
gradual development throughout indefinite aeons, as the rest of the
solar system is thought to have come to its present state of existence.
Possibly the planetesimal hypothesis of Chamberlin and Moulton affords
the true explanation, as the result of a rupture due to excessive tidal
strain.