Opaque and broad, is seen its arch to spread
Round the big globe, at stated periods led.”
This planet shines brighter than an ordinary first-magnitude star, and is a pretty conspicuous object, though less luminous than either Venus, Jupiter, or Mars. He emits a dull yellowish light, steadier than the sparkling lustre of Mercury or Venus.
The globe of Saturn is surrounded by a system of highly reflective rings, giving to the planet a character of form which finds no parallel among the other orbs of our system. His peculiar construction is well calculated to be attractive in the highest degree to all those who take delight in viewing the wonders of the heavens. Saturn is justly considered one of the most charming pictures which the telescope unfolds. A person who for the first time beholds the planet, encircled in his rings and surrounded by his moons, can hardly subdue an exclamation of surprise and wonder at a spectacle as unique as it is magnificent. Even old observers, who again and again return to the contemplation of this remarkable orb, confess they do so unwearyingly, because they find no parallel elsewhere; the beautifully curving outline of the symmetrical image always retains its interest, and refreshes them with thoughts of the Divine Architect who framed it!
The luminous system of rings attending this planet not only gratifies the eye but gives rise to entertaining speculations as to its origin, character, and purposes with regard to the globe of Saturn. Why, it has been asked, was this planet alone endowed with so novel an appendage? and what particular design does it fulfil in the economy of Saturn? It cannot be regarded as simply an ornament in the firmament, but must subserve important ends, though these may not yet have been revealed to the eye of our understanding.
Period &c.—Saturn revolves round the Sun in 10,759 days 5 hrs. 16 min., which is equal to nearly 29½ years. His mean distance from the Sun is 886,000,000 miles, but this interval varies from 841 to 931 millions, owing to the eccentricity of his orbit. When in opposition his apparent diameter reaches 20″·7, and declines to 15″ at the time of conjunction. The planet’s actual diameter is 75,000 miles, and his polar compression very considerable, viz. about 1/10, which exceeds that of any other planet. His synodic period is equal to 378 days; so that he comes into opposition with the Sun thirteen days later every year. The oblate figure of his disk is very noticeable when the rings are turned edgeways to the Earth and practically invisible; but when they are inclined the complete contour of the globe is lost, and the polar flattening becomes scarcely obvious.
“Square-shouldered” Aspect.—Sir W. Herschel, from observations in April 1805, said:—“There is a singularity which distinguishes the figure of Saturn from that of all the other planets.” On April 19 of the year named he described the planet as “like a parallelogram with the four corners rounded off deeply, but not so much as to bring it to a spheroid.” This gave the globe a “square-shouldered” aspect. But this curious figure appears to have been very rarely observed in subsequent years; and accurate measures with the micrometer were adduced in 1833-48 in proof that no such anomaly had a real existence. Dr. Kitchiner, commenting on Herschel’s remarks, said:—“I have occasionally observed this planet during thirty years, and I do not remember to have seen the body of it of this singular form except for a few months about September 1818.” But there is no doubt that occasionally the planet does assume an apparent form similar to that attributed to it by Herschel. In the autumn of 1880 I studied the visible appearance of Saturn by means of a 10-inch reflector, and recorded as follows:—“The S. pole, over which the dark belts lay, seemed compressed in the most remarkable manner; but where a bright belt intervened, in about lat. 45°, the contrary effect was produced. Here the limbs were apparently raised (by irradiation) above the spherical contour; so that the distorted image gave the planet that distinctly ‘square-shouldered’ aspect sometimes mentioned in text-books.” The explanation appears to me very simple. The singular figure is due to the contrasting effects of the belts. While the bright belt in lat. 45° causes a very evident shouldering-out of the limbs at its extremities, the dark belts nearer the pole and the equator act with opposite effect, for they apparently compress the disk where they meet the limbs, and thus the eye discerns a figure to all appearance distorted into the “square-shouldered” form. Mr. J. L. McCance confirmed these remarks by independent observations at the same period with a 10-inch reflector by Calver (‘Monthly Notices,’ vol. xli. pp. 84, 282).
Early Observations.—The appearance of Saturn offered a considerable difficulty to observers soon after the invention of the telescope. Galilei became greatly perplexed. He saw the planet, not as a circular globe like Jupiter, but distinctly elongated in shape, and conceived the appearance to be due to a central globe with smaller spheres hanging on the sides! He continued his observations, without, however, arriving at the solution of the mystery, until the malformation began to disappear; and in 1612 he was astonished to find the disk spherical. In his surprise, he asked—“Were the appearances indeed illusion and fraud, with which the glasses have so long deceived me, as well as many others to whom I have shown them?... The shortness of the time, the unexpected nature of the event, the weakness of my understanding, and the fear of being mistaken, have greatly confounded me.” Gassendi, in 1633, also announced that Saturn appeared to him to be closely attended by two globes of the same colour as the planet. Riccioli alleged that the planet was surrounded by a thin, plain, elliptic ring, connected with the sphere by two arms. None of Galilei’s contemporaries possessed the instrumental means to extricate him from his doubts; and it remained for Huygens, in 1654 (twelve years after the death of Galilei), to discover that Saturn “is surrounded by a slender flat ring, which in no part coheres with the body of the planet, and is inclined to the ecliptic”[42]. The same observer showed that the disappearance which had so puzzled Galilei arose from the varying inclination in the ring: at times it would become invisible, when presenting its narrow edge to the Earth, and this actually occurred again in 1671, as Huygens had predicted. In 1676 Cassini detected a belt upon the planet, and also a dark division in the ring. Dr. Smith’s ‘Optics’ (1738) thus alludes to these discoveries:—
