MAES AND HIS MOONS. 85 



moved from the sun — to get on with less than two of these satellites." 

 (" (Euvres 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 un- 

 proved 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 power- 

 ful 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 de- 

 tected 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 glar- 

 ing 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 dis- 

 covered 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.* 



* 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, IS??. 

 p. 129.) 



