fixed star carefully noted, it will be found that in the
course of a short time its real movement has been eastward, and that its
position with regard to the fixed star has changed, although the
revolution of the earth has appeared to carry both westward.

Phobos is 36 miles in diameter. Its actual period of revolution round
the planet is 7 hours and 39 minutes, but, owing to the movement of Mars
on its axis in the same direction, it appears to take a few minutes
over 11 hours to complete one revolution.

Near the equator, Phobos is seen above the horizon for about 4-1/4
hours, and is below it about 6-3/4 hours. According as the place from
which it is viewed is farther from the equator so will the time of
visibility of Phobos be decreased, until when latitude 69 deg. is reached
in either hemisphere, it will cease to become visible at all. This is
owing to its nearness to the planet; and, Mars being small, the curve
of its sphere is sharp, so that the horizon is more limited than on the
earth, and the satellite is shut out from view anywhere above latitude
69 deg. by the body of the planet.

Another peculiarity is that, when in the zenith, Phobos appears twice as
large in area as it does when near the horizon, and notwithstanding its
very small size, Phobos appears rather larger than our moon, because it
is so near to the planet.

The length of the Martian "night" is about 12 hours and 20 minutes, and
during this very short time Phobos may be seen to rise in the west, set
in the east, and rise again once more in the west. Consequently it will
be evident that it must travel very rapidly across the sky. It really
moves over a space of 32-1/2 deg. in a single hour--a great contrast to
the slow and stately movement of our moon, which only passes over half a
degree in an hour.

Moreover, Phobos may be seen to rise as a new moon, pass through its
phases to the full, wane, and again become new, all in the course of a
single Martian night; or it may be seen twice full and once new during
the same time.

Even this does not exhaust the list of phenomena, for, being so close
to Mars, Phobos is very frequently eclipsed by the shadow of the planet.
On the other hand, the sun may be eclipsed by Phobos something like
fourteen hundred times in the course of a Martian year; and, as already
mentioned, the other satellite is often occulted by Phobos--sometimes
when both may be only at the half full phase, and these occultations
look