interest, is also noteworthy as the first of a series of discoveries of satellites of the outer planets. The satellites of Mars are difficult to observe, on account not merely of their faintness, but of their proximity to the planet, the light of which is so bright as to nearly blot out that of the satellite. Intrinsically the inner satellite is brighter than the outer one, but for the reason just mentioned it is more difficult to observe. The names given them by Hall were Deimos for the outer satellite and Phobos for the inner one, derived from the mythological horses that drew the chariot of the god Mars. A remarkable feature of the orbit of Phobos is that it is so near the planet as to perform a revolution in less than one-third that of the diurnal rotation of Mars. The result is that to an inhabitant of Mars this satellite would rise in the west and set in the east, making two apparent diurnal revolutions every day. The period of Deimos is only six days greater than that of a Martian day; consequently its apparent motion around the planet would be so slow that more than two days elapse between rising and setting, and again between setting and rising.
 |
| Fig. 3. |
Owing to the minuteness of these bodies it is impossible to make any measures of their diameters. These can be inferred only from their brightness. Assuming them to be of the same colour as Mars, Lowell estimates them to be about ten miles for Deimos and somewhat more for Phobos. But these estimates are uncertain, not only from the somewhat hypothetical character of the data on which they rest, but from the difficulty of accurately estimating the brightness of such an object in the glare of the planet.
A long and careful series of observations was made upon these bodies by other observers. Later, especially at the very favourable oppositions of 1892 and 1894, observations were made by Hermann Struve at Poulkova, who subjected all the observations up to 1898 to a very careful discussion. He showed that the inclination of the planes of the orbits to the equator of the planet is quite small, thus making it certain that these two planes can never wander far from each other. In the following statement of the numerical elements of the entire system, Struve’s results are given for the satellites, while those of Lowell are adopted for the position of the plane of the equator.
The relations of the several planes can be best conceived by considering the points at which lines perpendicular to them, or their poles, meet the celestial sphere. By theory, the pole of the orbital plane of each satellite revolves round the pole of a certain fixed plane, differing less from the plane of the equator of Mars the nearer the satellite is to Mars. Lowell from a combination of his own observations with those of Schiaparelli, Lohse and Cerulli, found for the pole of the axis of rotation of Mars[1]:—
Tilt[2] of Martian Equator to Martian ecliptic, 23°. 59′. Hermann Struve, from the observations of the satellites, found theoretically the following positions of this pole, and of those of the fixed planes of the satellite orbits for 1900:—
| Pole of Mars: R.A. | = 317.25° | Dec. = 52.63° |
| Pole of fixed plane for Phobos | = 317.24° | = 52.64° |
| Pole of fixed plane for Deimos | = 316.20° | = 53.37° |
Lowell’s position of the pole is that now adopted by the British Nautical Almanac.
The actual positions of the poles of the satellite—orbits revolve around these poles of the two fixed planes in circles. Putting N for the right-ascensions of their nodes on the plane of the terrestrial equator, and J for their angular distance from the north terrestrial pole, N, and J, for the corresponding poles of the fixed planes, and t for the time in years after 1900, Struve’s results are:—
(N − N1) sin J = 97.6′ sin (356.8° − 6.375° t)
(N − N1) sin J = 53.1′ sin (257°.1′ − 158.0° t)
The other elements are:—
| Deimos. | Phobos. | |
| Mean long. 1894, Oct. 0.0 G.M.T | 186.25° | 296.13° |
| Mean daily motion (tropical) | 285.16198° | 1128.84396° |
| Mean distance (Δ = 1) | 32.373″ | 12.938″ |
| Long. of pericentre, (π + N) | 264° + 6.375° t | 14° + 158.0° t |
| Eccentricity of orbit | 0.0031 | 0.0217 |
| Epoch for t | 1900.0 | 1900.0 |
Bibliography.—Flammarion, La Planète Mars et ses conditions d’habitilité (Paris, 1892), embodies so copious a résumé of all the publications and drawings relating to Mars up to 1891 that there is little occasion for reference in detail to early publications. Among the principal sources may be mentioned the Monthly Notices and Memoirs of the Royal Astronomical Society, the publications of the Astronomical Society of the Pacific, especially vols. vi., viii. and ix., containing observations and discussions by the Mt Hamilton astronomers, and the journals, Sidereal Messenger, Astronomy and Astrophysics and Astrophysical Journal. Schiaparelli’s extended memoirs appeared under the general title Osservazioni astronomiche e fisiche sull’ asse di rotazione e sulla topografia del pianeta Marte, and were published in different volumes of the Memoirs of the Reale Accademia dei Lincei of Rome. The observations and drawings of Lowell are found in extenso in Annals of the Lowell Observatory. Lowell’s conclusions are summarized in Mars and its Canals, by Percival Lowell (1906), and Mars as the Abode of Life (1909). In connexion with his work may be mentioned Mars and its Mystery, by Edward S. Morse (Boston, 1906), the work of a naturalist who made studies of the planet at the Lowell Observatory in 1905. Brief discussions and notices will also be found in the Lowell Observatory Bulletins. The optical principles involved in the interpretations of the canals are discussed in recent volumes of the Monthly Notices, R.A.S., and in the Astrophysical Journal. In 1907 the veteran A. R. Wallace disputed Lowell’s views vigorously in his Is Mars Habitable? and was briefly answered by Lowell in Nature, who contended that Wallace’s theory was not in accord with celestial mechanics. (S. N.)
MARSALA, a seaport of Sicily, in the province of Trapani, 19 m. by rail S. of Trapani. Pop. (1881), 19,732; (1901), 57,567. The low coast on which it is situated is the westernmost point of the island. The town is the seat of a bishop, and the cathedral contains 16 grey marble columns, which are said to have been intended for Canterbury Cathedral in England, the vessel conveying them having been wrecked here. The town owes its importance mainly to the trade in Marsala wine.
Marsala occupies the site of Lilybaeum, the principal stronghold of the Carthaginians in Sicily, founded by Himilco after the abandonment of Motya. Neither Pyrrhus nor the Romans were able to reduce it by siege, but it was surrendered to the latter in 241 B.C. at the end of the First Punic War. In the later wars it was a starting point for the Roman expeditions against Carthage;
