A Text-Book of Astronomy

by George C. Comstock

From theoretical reasons based upon the law of gravitation astronomers have held that the rings of Saturn could not possibly be solid or liquid bodies. The strains impressed upon them by the planet's attraction would tear into fragments steel rings made after their size and shape. Quite recently Professor Keeler has shown, by applying the spectroscope (Doppler's principle) to determine the velocity of the ring's rotation about Saturn, that the inner parts of the ring move, as Kepler's Third Law requires, more rapidly than do the outer parts, thus furnishing a direct proof that they are not solid, and leaving no doubt that they are made up of separate fragments, each moving about the planet in its own orbit, like an independent satellite, but standing so close to its neighbors that the whole space reflects the sunlight as completely as if it were solid. With this understanding of the rings it is easy to see why they are so thin. Like Jupiter, Saturn is greatly flattened at the poles, and this flattening, or rather the protuberant mass about the equator, lays hold of every satellite near the planet and exerts upon it a direct force tending to thrust it down into the plane of the planet's equator and hold it there. The ring lies in the plane of Saturn's equator because each particle is constrained to move there.

The division of the ring into two parts, an outer and an inner ring, is usually explained as follows: Saturn is surrounded by a numerous brood of satellites, which by their attractions produce perturbations in the material composing the rings, and the dividing line between the outer and inner rings falls at the place where by the law of gravitation the perturbations would have their greatest effect. The dividing line between the rings is therefore a narrow lane, 2,400 miles wide, from which the fragments have been swept clean away by the perturbing action of the satellites. Less conspicuous divisions are seen from time to time in other parts of the ring, where the perturbations, though less, are still appreciable. But it is open to some question whether this explanation is sufficient.

The curious darkness of the inner or crape ring is easily explained. The particles composing it are not packed together so closely as in the outer ring, and therefore reflect less sunlight. Indeed, so sparsely strewn are the particles in this ring that it is in great measure transparent to the sunlight, as is shown by a recorded observation of one of the satellites which was distinctly although faintly seen while moving through the shadow of the dark ring, but disappeared in total eclipse when it entered the shadow cast by the bright ring.

144. The ball of Saturn.—The ball of the planet is in most respects a smaller copy of Jupiter. With an equatorial diameter of 76,000 miles, a polar diameter of 69,000 miles, and a mass 95 times that of the earth, its density is found to be the least of any planet in the solar system, only 0.70 of the density of water, and about one half as great as is the density of Jupiter. The force of gravity at its surface is only a little greater (1.18) than on the earth; and this, in connection with the low density, leads, as in the case of Jupiter, to the conclusion that the planet must be mainly composed of gases and vapors, very hot within, but inclosed by a shell of clouds which cuts off their glow from our eyes.

Like Jupiter in another respect, the planet turns very swiftly upon its axis, making a revolution in 10 hours 14 minutes, but up to the present it remains unknown whether different parts of the surface have different rotation times.

145. The satellites.—Saturn is attended by a family of nine satellites, a larger number than belongs to any other planet, but with one exception they are exceedingly small and difficult to observe save with a very large telescope. Indeed, the latest one is said to have been discovered in 1898 by means of the image which it impressed upon a photographic plate, and it has never been seen.

Titan, the largest of them, is distant 771,000 miles from the planet and bears much the same relation to Saturn that Satellite III bears to Jupiter, the similarity in distance, size and mass being rather striking, although, of course, the smaller mass of Saturn as compared with Jupiter makes the periodic time of Titan—15 days 23 hours—much greater than that of III. Can you apply Kepler's Third Law to the motion of Titan so as to determine from the data given above, the time required for a particle at the outer or inner edge of the ring to revolve once around Saturn?

Japetus, the second satellite in point of size, whose distance from Saturn is about ten times as great as the moon's distance from the earth, presents the remarkable peculiarity of being always brighter in one part of its orbit than in another, three or four times as bright when west of Saturn as when east of it. This probably indicates that, like our own moon, the satellite turns always the same face toward its planet, and further, that one side of the satellite reflects the sunlight much better than the other side—i. e., has a higher albedo. With these two assumptions it is easily seen that the satellite will always turn toward the earth one face when west, and the other face when east of Saturn, and thus give the observed difference of brightness.