MARS. AND HIS MOONS. 83 



employed. Accordingly, ever since the invention of the telescope, 

 Mars has been a favorite object of observation. The largest and most 

 powerful instruments have been employed to scrutinize this planet, 

 and the varied physical details of its surface have been most carefully 

 mapped by many astronomers. 



When, therefore, it was announced two years ago* that the Ameri- 

 can astronomer. Hall, had discovered two satellites belonging to Mars, 

 we ought not to be surprised at the astonishment with which the 

 news was received by the scientific world. Moreovei', there can be 

 no question that for more than two centui'ies past astronomers have 

 recognized the probability of the existence of satellites to this planet. 

 In fact, analogy would lead us to expect that Mars would be furnished 

 with one or more moons ; for, being situated at a greater distance 

 from the sun than the earth, it seems more especially to need such 

 luminaries to cheer its dark nights. Under the influence of these an- 

 ticipations, the astronomers, who have so carefully studied the physi- 

 cal features of Mars, have doubtless been looking for these satellites. 

 In fact, many of them have contended that the failure to discover 

 them is not by any means a conclusive proof of their non-existence ; 

 since, Mars being a very small planet, we might expect his moons to 

 be proportionally small, in which case they miglit escape detection 

 by the telescope. Thus, for example, the second satellite of Jupiter 

 is only about the forty-second part of the diameter of the planet ; and 

 a satellite which would only be the forty-second part of the diameter 

 of Mars would be about one hundred miles in diameter. At the least 

 distance of the earth from Mars a satellite of this dimension would 

 subtend an angle of less than one half of a second ; so that, even in 

 the most favorable position of Mars, powerful telescopes might fail to 

 reveal such an object, especially if it do not recede far from the disk 

 of the planet. 



Thus, Thomas Dick (" Celestial Scenery," American edition, p. 

 123, 1838) remarks in relation to this question : " If such a satellite 

 exist, it is highly piobable that it will revolve at the nearest possible 

 distance from the planet, in order to afford it the greatest quantity of 

 light ; in which case it would never be seen beyond two minutes of a 

 degree from the margin of the planet, and that only in certain favor- 

 able positions. If the plane of its orbit lay nearly in a line with our 

 axis of vision, it would frequently be hidden either by the interposition 

 of the body of Mars or by transiting its disk. It is therefore pos- 

 sible, and not at all improbable, that Mars may have a satellite, al- 

 though it has not yet been discovered. It is no argument for the non- 



* It was on the memorable night of the 11th of August, 1877, that Professor Asaph 

 Hall, of the Naval Observatory at Washington, caught the first glimpse of these diminu- 

 tive companions of Mars. The intervention of unfavorable weather kept him in a state 

 of anxious suspense, and postponed, for a period of five days, the complete verification 

 of his great discovery. 



