The planet generally shines with the brilliancy of a moderate first-magnitude star, and with a dingy, reddish light, as if seen through a smoky atmosphere.
265. Volume and Density of Saturn.—The real diameter of Saturn is about seventy thousand miles, and its volume over seven hundred times that of the earth. The comparative size of the earth and Saturn is shown in Fig. 286. This planet is a little more than half as dense as Jupiter.
Fig. 286.
266. The Sidereal and Synodical Periods of Saturn.—Saturn makes a complete revolution round the sun in a period of about twenty-nine years and a half, moving in his orbit at the rate of about six miles a second. The planet passes from opposition to opposition again in a period of three hundred and seventy-eight days, or thirteen days over a year.
267. Physical Constitution of Saturn.—The physical constitution of Saturn seems to resemble that of Jupiter; but, being twice as far away, the planet cannot be so well studied. The farther an object is from the sun, the less it is illuminated; and, the farther it is from the earth, the smaller it appears: hence there is a double difficulty in examining the more distant planets. Under favorable circumstances, the surface of Saturn is seen to be diversified with very faint markings; and, with high telescopic powers, two or more very faint streaks, or belts, may be discerned parallel to its equator. These belts, like those of Jupiter, change their aspect from time to time; but they are so faint that the changes cannot be easily followed. It is only on rare occasions that the time of rotation can be determined from a study of the markings.
268. Rotation of Saturn.—On the evening of Dec. 7, 1876, Professor Hall, who had been observing the satellites of Saturn with the great Washington telescope (18), saw a brilliant white spot near the equator of the planet. It seemed as if an immense eruption of incandescent matter had suddenly burst up from the interior. The spot gradually spread itself out into a long light streak, of which the brightest point was near the western end. It remained visible until January, when it became faint and ill-defined, and the planet was lost in the rays of the sun.
From all the observations on this spot, Professor Hall found the period of Saturn to be ten hours fourteen minutes, reckoning by the brightest part of the streak. Had the middle of the streak been taken, the time would have been less, because the bright matter seemed to be carried along in the direction of the planet's rotation. If this motion was due to a wind, the velocity of the current must have been between fifty and a hundred miles an hour. The axis of Saturn is inclined twenty-seven degrees from the perpendicular to its orbit.
