moved from the sun—to get on with less than two of these satellites." ("Œuvres de Voltaire"—Micromegas, chapter iii.) How completely the recent discovery of the American astronomer has "turned the tables" on the renowned satirist of the last century! The previsions of those "excellent philosophers" who founded their conclusions upon analogical reasoning, although slumbering in the domains of the unproved for more than two centuries, have at last been verified by direct observation.
As the moons of Mars are very small objects, it is only under the most favorable circumstances that they can be seen by the most powerful telescopes. Mars is nearest to us when his opposition occurs, when he is near his perihelion; and the greatest possible proximity to us occurs when Mars is in opposition in perihelion and the earth is in aphelion at the same time. The oppositions of Mars near perihelion occur at intervals of fifteen and seventeen years successively. A very good opposition occurred in 1862, and a great many distinguished astronomers embraced the opportunity of scrutinizing Mars with the aid of excellent instruments. A still more favorable opportunity was presented in the summer of 1877, when Mars was nearer to us than it has been since 1845. It was at this time that Professor Asaph Hall was fortunate enough, by means of the new 26-inch refractor of the Naval Observatory at Washington, to discover two moons belonging to this planet. It is true that this was probably the first time that so powerful a telescope had ever been directed to the examination of Mars under similar favorable conditions; yet it is a significant fact that, since the announcement of the discovery, the satellites have been detected by means of telescopes of more moderate power. The secret of Professor Hall's discovery seems to have consisted in devising the means of cutting off, from the field of view of the telescope, the glaring light of Mars. In like manner, M. Henry, of the Observatory of Paris, on August 27, 1877, was able to see the satellites when Mars was screened from view. These diminutive moons nestle so closely to the planet that it is difficult to see them in the blaze of light reflected from Mars. Had similar means of screening the planet been employed, it is probable that one or both of these satellites might have been discovered in 1862.
The distance of the inner satellite from the center of the primary is about 2·73 times the radius of Mars; that of the outer one about 6·846 times the same radius. Assuming the diameter of Mars to be about 4,200 miles, these distances become, respectively, 5,733 and 14,376 miles from the center of Mars. The nearest satellite of Jupiter is distant about six times the radius of the primary, and the innermost satellite of Saturn is distant a little more than three times the radius of that planet.[1]
- ↑ The following table exhibits the mean distances of the satellites from the centers of the primaries, expressed in equatorial radii of the latter. ("Nature," December 13, 1877, p. 129.)
