Flowers of the Sky

by Richard A. Proctor

XI
THE RINGED PLANET SATURN.

Very different from the ruddy planet which approached so closely to him in November, 1877, is Saturn, the ringed world, the most wonderful of all the planets if the complexity of the system attending on him is considered, and in size inferior only to the giant Jupiter.

It will have been noticed, perhaps, by those who are familiar with the aspect of the planets, that the contrast between Mars and Saturn during their late approach to us was not only greater than usual, but greater than was to be expected even when account was taken of the unusual lustre of Mars. I have often wondered whether the ancient astronomers were ever perplexed by the varying lustre of Saturn. They recognised the fact that Mars has an orbit of great eccentricity (see the picture of the orbits of Mars, Venus, etc., at page [156]); and there was nothing in the varying lustre of Mars which could not be perfectly well explained by his known variations of distance, whether the Ptolemaic or the Copernican system were accepted. But with Saturn the case is different. His distance at successive returns to our midnight skies is subject to moderate changes only. Yet his brilliancy varies in a remarkable manner. We now know that those changes are due to the opening and closing of that marvellous system of rings which renders this planet the most beautiful of all the objects of telescopic observation which the heavens present to us. When the edge of the rings is turned towards the earth, we see only the most delicate thread of light on either side of the planet's disc. But when the rings are opened out to their full extent they reflect towards us as much light as we receive from the disc. At such times the planet presents a much more brilliant appearance than when the ring is turned nearly edgewise; in fact to the naked eye he seems very nearly twice as bright. Now at present the rings are turned nearly edgewise towards the earth. In July and August, 1869, the planet presented in the telescope the appearance presented in [fig. 30], where it will be seen that the shorter axis of the oval into which any one of the ring-outlines is thrown is nearly equal to half the larger axis. Since then the rings have been slowly closing up; and at present the rings are so little open that the corresponding shorter axis, if it could be directly seen, would appear to be about one-sixteenth only of the larger axis. The rings were turned exactly edgewise towards the sun at two in the afternoon, on St. Valentine's day, 1878, according to calculations which I made in 1864, and published in a table under the head "Passages of the Rings plane through the Sun between the years 1600 and 2000," in my treatise entitled "Saturn and its System." The Nautical Almanac for 1878, indeed makes the passage of the rings plane through the sun occur somewhat earlier, stating that at noon on February 14 the sun's centre would pass south of the ring's horizon by about one-fifth of its apparent diameter (as seen by us). But my own calculation took into account certain small details which, in matters of this sort, the Nautical Almanac computers neglect. After all, it mattered very little to terrestrial observers whether the sun's light passed from the northern to the southern side of the rings a few hours earlier or later: the moment when it passed could not possibly be observed, even if it had occurred during the night hours. In the present instance it occurred at midday, and unfortunately none of the interesting phenomena presented in powerful telescopes when the rings are turned edgewise to the sun or earth could be observed, for they occurred when Saturn and the sun were nearly in the same part of the heavens, and when the planet therefore was utterly lost in the splendour of the solar rays.

Fig. 30.—The planet Saturn in July and August, 1869.

But now let us briefly consider what is known or may be surmised respecting the noble planet which was so far outshone in November, 1877, by the comparatively minute orb of Mars.

Saturn travels at a distance from the sun exceeding rather more than nine and a half times that of our own earth. The second figure of orbits (see page 157) shows the wide span of his orbit compared with the earth's, and yet it will be seen that the orbits of Uranus and Neptune, planets unknown to the ancients, are so wide that the path of Saturn becomes in turn small by comparison.