The distance of the outer satellite from Mars's centre is about 14,300 miles, from Mars's surface about 12,000 miles. The inner travels at a distance of about 5,750 miles from the centre, and about 3,450 miles from the surface of Mars.
The motions of the satellites as seen from Mars must be very different from those of our own moon. Thus, our moon moves so slowly among the stars that she requires nearly an hour to traverse a distance equal to her own apparent diameter. The outer moon of Mars traverses a similar distance--that is, not her own apparent diameter, but an arc on the stellar heavens equal to our moon's apparent diameter--in about two and a half minutes, while the inner moon moves so rapidly as to traverse the same distance in about forty seconds. To both moons, therefore, but to the inner in particular, Job's description of our moon as "walking in brightness" would seem singularly applicable, so far at least as the rapidity of their motions is concerned. Their brightness, however, cannot be comparable to our moon's. For notwithstanding their much greater proximity, it is easily shown that they must present much smaller discs, and being illuminated by a more distant sun, their discs cannot shine so brightly as our moon's. That is, not only are the discs smaller, but their intrinsic brightness is less. Assuming the outer moon to be ten miles, the inner fifteen miles in diameter, it is easily shown that the two together, if full at the same time, can hardly give one-twelfth as much light to Martians as our moon gives to us.
Yet there can be no doubt that the Martian moons must be (or _have been_) most useful additions to the Martian skies. They do not give a useful measure of time intermediate in length between the day and the year, as our moon does; for the outer travels round the planet in about thirty and a quarter hours, the inner in about seven and a half hours. Nor can they exert an influence upon the Martian seas corresponding to that exerted by our own moon in generating the lunar tidal wave. But their motions must serve usefully to indicate the progress of time, both by night and by day, for they must be visible by day unless very close to the sun. They must be even more useful than our moon in indicating the longitude of ships at sea, seeing that the accuracy with which a moon indicates longitude is directly proportional to her velocity of motion among the stars.