ing any action.

CHAPTER XXII

CELESTIAL PHENOMENA SEEN FROM MARS--M'ALLISTER RECEIVES A PRACTICAL
LESSON IN GRAVITATION

Mars is really an ideal world for an astronomer to live in, its skies
being so clear, the air so thin and pure, and the stars shining so
brilliantly.

Besides these advantages, the rapid movements of the two satellites of
the planet result in a constant succession of celestial phenomena which
afford very frequent opportunities for most interesting observations.
Changes in the phases of the two moons, eclipses, occultations,
transits, &c., are constantly occurring, so there is nearly always
something to attract our attention to the Martian sky.

We have already seen several of these phenomena, and I will now describe
what we have observed.

Early one evening when we were out with Merna, we looked up at the sky
and saw the two moons a considerable distance apart, but approaching
each other from opposite directions, Phobos appearing to move very
rapidly. Both were near the full phase, Deimos being more nearly full
than Phobos; and we watched them drawing closer and closer together till
Phobos passed right in front of Deimos so as to hide it entirely. This
is termed an occultation; and both the satellites had become full when
the occultation occurred; but when they were again clear of each other
both were beginning to wane.

This sight may be seen anywhere near the Martian equator about every ten
hours.

The movements of Phobos seemed very peculiar to us who had lived upon
the earth and seen all the celestial bodies appearing to move in the
same direction.

I have already alluded to the fact that Phobos is only 3700 miles above
the surface of Mars, and moves so rapidly that it makes more than three
complete revolutions round the planet whilst the latter is turning only
once on its axis.

The effect of this very rapid revolution of the satellite, which has no
counterpart, so far as we know, in our Solar system is that, instead of
rising in the east and setting in the west as all the other heavenly
bodies appear to do, Phobos appears to rise in the west, cross the sky,
and set in the east.

The moon and planets all actually move from west to east; the apparent
reverse of this being caused by the more rapid movement of the earth on
its axis, giving the other bodies the appearance of moving from east to
west. If, however, our moon is closely watched, and its position with
regard to a