| MARS AND ITS SATELLITES. |
By Professor DANIEL KIRKWOOD.
THE recent and wholly unexpected discovery of two Martial satellites has awakened a new and lively interest in all that relates to our neighboring planet. Its telescopic aspect and the probable nature of its physical constitution are especially worthy of renewed consideration.
The mean distance of Mars from the sun is 139,877,000 miles.[1] Its orbit deviates more from the circular form than that of any other principal planet with the exception of Mercury; its eccentricity being 0.09326. The difference, therefore, between its greatest and-least distance amounts to about 27,000,000 miles. But the eccentricity, though great, is nevertheless increasing; and, when it shall have attained its superior limit, the aphelion distance will be 196,000,000 miles. This is greater than the perihelion distance of many asteroids. Mars, therefore, occasionally invades the cluster of minor planets. Is it not possible that his superior force may attach some of its members to his retinue of satellites?
Mars was the first planet to exhibit indications of an axial revolution. As early as 1636 Fontana, a Neapolitan astronomer, had an imperfect view of a spot on the planet's disk. He reobserved the same figure in 1638, and from the changes noticed in its position and aspect he inferred the probability of the planet's rotation. He seems, however, to have made no effort to determine its period. Dr. Hooke, in 1666, noticed some well-defined spots, which he found to change their appearance on the surface, to disappear and return at regular intervals; whence he inferred that the planet completes a rotation either in twelve or twenty-four hours. During the same year Cassini observed spots on each hemisphere of the planet, from the motions of which he concluded the period of rotation to be 24h 40m. In 1704 Maraldi, the nephew and pupil of Cassini, made a series of observations on the spots, from which he deduced a period of 24h 39m. In 1719 he renewed his observations under favorable circumstances, and obtained a period precisely equal to that originally found by Cassini. In order to determine the exact period of rotation. Sir William Herschel undertook a series of observations in 1777, which he again resumed in 1779. From the changes which he observed in the appearances of the planet he fixed the time of revolution at 24h 39m 21.67s. The determination by Kunowsky in 1821 gave 24h 36m 40s The observations of Beer and Mädler in 1830 indicated a period of 24h 37m 10s. Their observations, however, in 1832, combined with
- ↑ This value corresponds to a solar parallax of 8.88".
