Popular Science Monthly/Volume 11/October 1877/Mars and its Satellites
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Mars and its Satellites
By Daniel Kirkwood
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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. 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 those of 1830, gave 24h 37m 23.7s. In regard to the exact period of rotation and the slight discrepancies between the results obtained by different observers, Prof. O. M. Mitchel remarks as follows:
8h 55m, making the corrections due to geocentric longitude, phase, and aberration, I find the period of rotation to be 24h 37m 20.6s, differing by only two seconds from Madler's period as last corrected."
Finally, Richard A. Proctor, Esq., by an exhaustive discussion of all the observations, has determined the period to be 24h 37m 22.735s.
The diameter of Mars is about 4,700 miles. Its surface is rather more than one-third that of the earth, while its volume is to that of our planet in the ratio of two to nine.
The persevering labors of Beer and Mädler proved beyond question that many of the lineaments observed in the aspect of this planet are permanent in their character, and not merely atmospheric. The same spots, with the same general outlines, and the same varieties of color, have been noticed at successive oppositions; not always, it is true, with precisely the same distinctness, but without any other changes than such as might be attributed to atmospheric variations. Two white circular spots are observed in the polar regions, which increase during the winter, and decrease in the summer, of each hemisphere respectively, and which may, therefore, be regarded as polar snows. These spots were noticed by Maraldi as early as 1716; their connection, however, with the change of seasons was first shown by Sir William Herschel. The same astronomer found the inclination of the axis of Mars to the plane of its orbit to be 61° 18'. The Martial tropics are therefore 28° 42' from the equator, making the torrid zone 10° wider than that of the earth. In so far, then, as climatic changes are dependent on the obliquity of the planets, the seasons of Mars may not differ, perhaps, except in their duration, very greatly from our own.
The Satellites of Mars.—We come now to the history of one of the most interesting discoveries of the nineteenth century. With the single exception of our own moon, Mars is the most favorably situated of all the heavenly bodies for telescopic observation. The most careful scrutiny, however, for more than two centuries, had failed to furnish any indication of the existence of a satellite. The opposition of Mars in August, 1877, occurred when the planet was very near its perihelion. The body was, therefore, in the best possible position for close examination. At the approach of this favorable epoch the new twenty-six inch refractor of the Naval Observatory at Washington, under the skillful direction of Prof. Asaph Hall, was turned upon the planet. On the night of August 11th a small star was observed near the disk of Mars, but its true character was not then suspected, or at least not determined. On Thursday night, the 16th, at 11h 42s, Prof. Hall again noticed a star of the thirteenth or fourteenth magnitude, very close to Mars, and measured its apparent distance from the planet. On the same night, about two o'clock, he again examined the planet, and to his great surprise found that the small star had moved in company with Mars. He had therefore discovered a Martial satellite. On Friday morning the observations were submitted to Prof. Simon Newcomb, who, from the data furnished by a watch of five hours, calculated the time of revolution, which he fixed as a first approximation at 31 or 32 hours. This showed that the satellite must pass behind Mars some time during the following night. It was accordingly invisible when first looked for in the evening, but, as predicted by Newcomb, it reappeared about one o'clock. On Saturday morning the discovery was made known to Admiral Rodgers, the superintendent of the observatory. It was determined, however, to wait for another observation before formally announcing so important a discovery. On Saturday evening the satellite was again found very nearly in its predicted place, and its exact position was measured by several astronomers.
About four o'clock on the morning of August 18th Prof. Hall discovered a second satellite, interior to the orbit of the first, and of about the same apparent magnitude. The astronomers of Europe were officially notified of the facts by the following dispatch:
The statement of fifty seconds as the distance of the inner satellite was subsequently found to be quite erroneous.
On Monday, August 21st, Rear-Admiral Rodgers, superintendent of the observatory, communicated the discovery, together with Prof. Newcomb's approximate circular elements of the orbits, to the Hon. R. W. Thompson, Secretary of the Navy.
Distances from the Centre of Mars.—The distance of the inner satellite from the centre of the primary is about 5,700 miles; that of the outer, 14,200. The distance of the former from the surface of Mars is but 3,300 miles—no greater, in fact, than that of London from New York. The apparent magnitude of Mars as seen from this satellite is 2,000 times greater than that of the sun or moon as seen from the earth.
Periods of the Satellites.—Prof. Newcomb gives 30h and 14m as the period of the outer satellite, and 7h and 38m as that of the inner. Both move, like our moon, from west to east. The period of the inner is less, while that of the outer is greater than a Martial day. It is obvious, therefore, that, as seen from the surface of the planet, the apparent motion of the satellites will be in opposite directions, the inner rising in the west and setting in the east; the outer rising in the east and setting in the west—so that the phenomenon of two moons meeting in mid-heaven will be to the Martialists no unusual occurrence.
The Mass of Mars.—Before the discovery of these satellites the determination of the mass of Mars was a problem of great difficulty, the body being too small to have much effect in disturbing the motions of other planets. The value assigned by Burckhardt was 2680337 and that of the sun being unity. The difficulty of the problem is now happily removed, and Newcomb has found from the elements of the exterior satellite a value 3090000; mass less than Burckhardt's in the ratio of six to seven.
The Bearing of the Discovery on the Nebular Hypothesis.—The inner satellite of Mars completes three orbital revolutions in less than a Martial day. This anomalous fact in the planetary system would seem, at first view, to be utterly inconsistent with the nebular hypothesis. The question is one of more than ordinary interest, but its discussion may well be deferred until we shall have obtained more exact information in regard to the Martial system.
- This value corresponds to a solar parallax of 8.88".
- Sidereal Messenger, p. 101.