Saturn, July 2, 1894. E. E. Barnard, 36-inch Equatorial.
Saturn is the least dense of all the planets; in fact, this enormous globe, nine times the diameter of the earth, would float in water. This fact of extremely low density at once suggests a state of matters similar to that already seen to exist, in all likelihood, in the case of Jupiter; and all the evidence goes to support the view that Saturn, along with the other three large exterior planets, is in the condition of a semi-sun.
The globe presents, on the whole, similar characteristics to those already noticed as prevailing on Jupiter, but, as was to be expected, in a condition enfeebled by the much greater distance across which they are viewed and the smaller scale on which they are exhibited. It is generally girdled by one or two tropical belts of a grey-green tone; the equatorial region is yellow, and sometimes, like the corresponding region of Jupiter, bears light spots upon it and a narrow equatorial band of a dusky tone; the polar regions are of a cold ashy or leaden colour. Professor Barnard's fine drawing (Plate [XXIV.]) gives an admirable representation of these features as seen with the 36-inch Lick telescope. Altogether, whether from greater distance or from intrinsic deficiency, the colouring of Saturn is by no means so vivid or so interesting as that of his larger neighbour.
The period of rotation was, till within the last few years, thought to be definitely and satisfactorily ascertained. Sir William Herschel fixed it, from his observations, at ten hours sixteen minutes. Professor Asaph Hall, from observations of a white spot near the equator, reduced this period to ten hours fourteen minutes twenty-four seconds. Stanley Williams and Denning, in 1891, reached results differing only by about two seconds from that of Hall; but the former, discussing observations of 1893, arrived at the conclusion that there were variations of rotation presented in different latitudes and longitudes of the planet's surface, the longest period being ten hours fifteen minutes, and the shortest ten hours twelve minutes forty-five seconds. Subsequently Keeler obtained, by spectroscopic methods, a result exactly agreeing with that of Hall. It appeared, therefore, that fairly satisfactory agreement had been reached on a mean period of ten hours fourteen minutes twenty-four seconds.
In 1903, however, a number of bright spots appeared in a middle north latitude which, when observed by Barnard, Comas Solà, Denning, and other observers, gave a period remarkably longer than that deduced from spots in lower latitudes—namely, about ten hours thirty-eight minutes. Accordingly, it follows that the surface of Saturn's equatorial regions rotates much more rapidly than that of the regions further north—a state of affairs which presents an obvious likeness to that prevailing on Jupiter. But in the case of Saturn the equatorial current must move relatively to the rest of the surface at the enormous rate of from 800 to 900 miles an hour, a speed between three and four times greater than that of the corresponding current on Jupiter!
The resemblance between the two great planets is thus very marked indeed. Great size, coupled with small density; very rapid rotation, with its accompaniment of large polar compression; and, even more markedly in the case of the more distant planet than in that of Jupiter, a variety of rotation periods for different markings, which indicates that these features have been thrown up from different strata of the planet's substance—such points of likeness are too significant to be ignored. It is not at all likely that Saturn has any solidity to speak of, any more than Jupiter; the probabilities all point in the direction of a comparatively small nucleus of somewhat greater solidity than the rest, surrounded by an immense condensation shell, where the products of various eruptions are represented.
Were this all that can be said about Saturn, the planet would scarcely be more than a reduced and somewhat less interesting edition of Jupiter. As it is, he possesses characteristics which make him Jupiter's rival in point of interest, and, as a mere telescopic picture, perhaps even his superior. When Galileo turned his telescope upon Saturn, he was presented with what seemed an insoluble enigma. It appeared to him that, instead of being a single globe, the planet consisted of three globes in contact with one another; and this supposed fact he intimated to Kepler in an anagram, which, when rearranged, read: 'Altissimum planetam tergeminum observavi'—'I have observed the most distant planet to be threefold.' Under better conditions of observation, he remarked subsequently, the planet appeared like an olive, as it still does with low powers. This was sufficiently puzzling, but worse was to follow. After an interval, on observing Saturn again, he found that the appearances which had so perplexed him had altogether disappeared; the globe was single, like those of the other planets. In his letter to Welser, dated December 4, 1612, the great astronomer describes his bewilderment, and his fear lest, after all, it should turn out that his adversaries had been right, and that his discoveries had been mere illusions.
Then followed a period when observers could only command optical power sufficient to show the puzzling nature of the planet's appendages, without revealing their true form. It appeared that Saturn had 'ansæ,' or handles, on either side of him, between which and his body the sky could be seen; and many uncouth figures are still preserved which eloquently testify to the bewilderment of those who drew them, though some of them are wonderfully accurate representations of the planet's appearance when seen with insufficient means. The bewilderment was sometimes veiled, in amusing cuttle-fish fashion, under an inky cloud of sesquipedalian words. Thus Hevelius describes the aspects of Saturn in the following blasting flight of projectiles: 'The mono-spherical, the tri-spherical, the spherico-ansated, the elliptico-ansated, and the spherico-cuspidated,' which is very beautiful no doubt, but scarcely so simple as one could wish a popular explanation to be.