Yet this seeming immobility from moment to moment takes place in what is really a seething furnace, the fiery glow of which we catch below the vast ebullition of cloud in the cherry hue of its darker portions. Distance has merged the turmoil into the semblance of quiescence and left only its larger secular changes to show. Even so the Colorado River from the brink of the Grand Cañon is seen apparently at rest, the billows of its rapids so stereotyped to stability one takes the rippled sand bank for the river and the billows of the river for the ripple marks of its banks.
At twice the distance of Jupiter we cross the orbit of Saturn. Here the ringed planet, with an annual sweep of twenty-nine and a half of our years, pursues his majestic circuit of the Sun. Diademed with three or more circlets of light and diamonded by ten satellites, he rivals in his cortège that of his own lord. In some ways his surpasses the Sun’s. For certainly his retinue is the more spectacular of the two; the more so that it is much of it fairly comprised within a single glance. Very impressive Saturn is as, attended thus, he sails into the field of view.
Saturn—A drawing by Dr. Lowell,
showing agglomerations.
In our survey we may best begin with his globe. If Jupiter’s compression is striking, Saturn’s is positively startling when well displayed. This happens but at rare intervals. As the plane of his equator is almost exactly that of the rings, the flattening is conspicuous only on those occasions when the rings disappear because their plane passes through the line of sight. Seen at such times the effect of the discrowned orb is so strange as to suggest delusion. This occurred two years ago in 1907, and when the planet was picked up by its position and entered the field unheralded by its distinctive appendage, it was almost impossible to believe there had not been some mistake and a caricatured Jupiter had taken its place. For the flattening outdoes that of Jupiter as 3 to 2, being ⅒ of the equatorial diameter. Such a bulging almost suggests disruption and is due to the extreme lightness of the planet’s substance, which is actually only 0.72 of that of water. Like Jupiter, the disk exhibits belts, though very much fainter, and, like his, these are of a cherry red. As the planet’s albedo is even greater, 0.78 of absolute whiteness, as deduced from H. Struve’s measures of the diameter, the same suspicion of shining, at least in part, from inherent light, applies equally to him. But it is practically certain that in neither case does this light equal that of the planet’s clouds, or add anything to them. Both planets are red-hot, not white-hot. The determination of the albedo depends upon that of the diameter, and an increase in the latter would lower the albedo to that of cloud.
His most unique possession are his rings. Broad, yet tenuous, they weigh next to nothing, being, as Struve has dubbed them, “Immaterial light.” Nevertheless, it is not their lightness but their make-up that prevents from lying uneasy the head that wears this crown.
The mechanical marvel was not appreciated by early astronomers, who took it for granted that they were what they seemed, solid, flat rings, all of a piece. Even Laplace considered it sufficient to divide them up concentrically to insure stability. To Edouard Roche of Montpellier, as retiringly modest as he was penetratingly profound, is due the mathematical detection that to subsist they must be composed of discrete particles,—brickbats, Clerk Maxwell called them, when, later, unaware of Roche’s work, he proved independently the same thing in his essay on Saturn’s rings. Peirce, too, in ignorance of Roche, had half taken the same step a little before, showing that they must at least be fluid. Then in 1895 Keeler ingeniously photographed the spectrum of both ball and rings to the revealing of velocities in the line of sight of the different portions of the spectrum exactly agreeing with the values mechanics demanded.
The rings have usually been considered to be flat. At the time of their disappearance, however, knots have been seen upon them. It is as if their filament had suddenly been strung with beads. At the last occurrence of the sort in 1907, these beads were particularly well seen at several observatories, and were critically studied at Flagstaff. In connection with a new phenomenon detected there, that of a dark core in the shadow the rings threw across the planet’s face, an explanation suggested itself to account for both them and it: to wit, that the rings were not really flat, but tores; rings, that is, like an anchor ring, any cross-section of which would be of the nature of an oval flattened on its inner side. The cogency of the explanation consisted in its solution not only of the appearances but of the cause competent to bring those appearances about.
For measurement showed that the knots were permanent in position, which, since the ring revolved, indicated that they extended all round it in spite of their not seeming to do so, and that their distances from Saturn were just what this cause should produce.