88 THE POPULAR SCIENCE MONTHLY. 



The outer satellite would, under like positions, jDresent apparent 

 diameters, respectively, of 24' and 28', or considerably less than the 

 apparent diameter of our moon. The nearest satellite of Jupiter 

 (having a diameter of 2,310 miles) would, in like positions, present to 

 an observer on the surface of that planet apparent diameters, respec- 

 tively, of 31' and 37'. 



As we have seen, 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." According to this hypothesis, the or- 

 JiYa^periods of the satellites should be approximately equal to the 

 rotation-periods of the primary at the epochs when the satellites were 

 thrown off from it. The acceleration of the rotation-period of the 

 primary, in consequence of its subsequent contraction, would neces- 

 sarily render its time of rotation less than the orbital-period of any 

 satellite. As far as yet known, the inner satellite of Mars affords the 

 only instance in which the rotation-period of the primary is greater 

 than the orbital-period of the secondary. 



It must be remembered, however, that if we regard the rings of 

 Saturn as comjjosed of clouds of independently revolving minute satel- 

 lites, those constituting the innermost portions of the inner ring must 

 revolve in less time than the rotation-period of that planet. Under 

 this view, therefore, the case of the inner satellite of Mars is not 

 unique. 



There are, however, several methods by which the apparently 

 anomalous fact may be accounted for consistently with the nebular 

 hypothesis : 



1. In the first place, it has been suggested that Mars may not have 

 obtained his satellites by means of the usual process of moon-forma- 

 tion, but by the appropriation to himself of a couple of the numer- 

 ous asteroids or planetoids, some of which, in their perihelion excur- 

 sions, approach comparatively near to Mars in his aphelion positions. 

 Thus, the planetoid called Phocea, when it is at its least distance and 

 Mars at his greatest distance from the sun, would only be about 

 11,000,000 miles from each other. It is, therefore, possible that some 

 of the planetoids, moving in orbits of greater eccentricity than any yet 

 discovered, may, at some former period, have approached so near Mars 

 as to have become permanently attached to it as satellites. 



2. In the second place, it is possible that these Martial moons may 

 have originally revolved in larger orbits, and therefore in longer peri- 

 ods than at present, but that the retarding influence of a resisting 

 medium on such synall masses might, in the course of myriads of ages, 

 have contracted their orbits and consequently shortened their orbital- 

 periods. In this connection it must be borne in mind that, according 

 to the nebular hypothesis. Mars must be a vastly older planet than 

 the earth ; so that this retardation may have been in progress for an 



