The outer satellite would, under like positions, present 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, respectively, 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 orbital-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 necessarily 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 composed of clouds of independently revolving minute satellites, 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-formation, but by the appropriation to himself of a couple of the numerous asteroids or planetoids, some of which, in their perihelion excursions, 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 periods than at present, but that the retarding influence of a resisting medium, on such small 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
