Page:Astronomy for Everybody.djvu/213

Professor Hall assigned the name Deimos to the outer and Phobos to the inner, these being the attendants of Mars in ancient mythology. Phobos has the remarkable peculiarity that it revolves around the planet in less than nine hours, making its period the shortest of any yet known in the solar system. This is little more than one third the time of the planet's rotation on its axis. The consequence of this is that, to the inhabitants of the planet, its nearest moon rises in the west and sets in the east.

Deimos performs its revolution in 30 hours 18 minutes. The result of this rapid motion is that some two days must elapse between its rising and setting.

Phobos is only 3,700 miles from the surface of the planet. It must therefore be an interesting object to the inhabitants of Mars, if they have telescopes.

In size these bodies are the smallest visible to us in the solar system, with the possible exception of Eros and possibly some others of the fainter asteroids. From Professor Pickering's photometric estimates their diameter was estimated to be not very different from seven miles. Their apparent size as we view them is therefore not very different from that of a small apple hanging over the city of Boston, and seen with a telescope from the city of New York. In this respect they form a singular contrast to nearly or quite all of the other satellites, which are generally a thousand miles or more in diameter. The one exception to this is the fifth satellite of Jupiter, to be described in the chapter on Jupiter and its satellites. Although this is much less than a thousand miles in diam-