7o6 THE POPULAR SCIENCE MONTHLY. 



MAES AND ITS SATELLITES. 



By Peofessos DAKIEL KIEKWOOD. 



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 natui-e of its physical constitution are especially worthy of 

 renewed consideration. 



The mean distance of Mars from the snn is 139,877,000 miles.* 

 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 as- 

 teroids. 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 revolu- 

 tion. 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 jDrobability of the planet's rotation. He seems, 

 however, to have made no eflbrt 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 24'' 40". In 1704 

 Mai'aldi, the nephew and pupil of Cassini, made a series of observa- 

 tions on the spots, from which he deduced a period of 24'' sg*". 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 Her- 

 schel undertook a series of observations in 1777, which he again 

 resumed in 1779. From the changes which he observed in the ap- 

 pearances of the planet he fixed the time of revolution at 24^" 39"" 

 21.67'. The determination by Kunowsky in 1821 gave 24"' 36" 40\ 

 The observations of Beer and Mildler in 1830 indicated a period of 

 24h 3i^m -|^Qs_ Their observations, however, in 1832, combined with 



- This value corresponds to a solar parallax of 8.88". 



