cease, and
the object seems to rest; but is it going to remain at rest for ever?
Has its long journey been finished? For many nights this seems to be the
case, but at length the astronomer suspects that the planet must be
commencing to move backwards. A few nights more, and the fact is
confirmed beyond possibility of doubt, and the extraordinary discovery
of the direct and the retrograde movement of Mars has been accomplished.

[Illustration: Fig. 49.--The Apparent Movements of Mars In 1877.]

In the greater part of its journey around the heavens Mars seems to move
steadily from the west to the east. It moves backwards, in fact, as the
moon moves and as the sun moves. It is only during a comparatively small
part of its path that those elaborate movements are accomplished which
presented such an enigma to the primitive observer. We show in the
adjoining picture (Fig. 49) the track of the actual journey which Mars
accomplished in the opposition of 1877. The figure only shows that part
of its path which presents the anomalous features; the rest of the orbit
is pursued, not indeed with uniform velocity, but with unaltered
direction.

This complexity of the apparent movements of Mars seems at first sight
fatal to the acceptance of any simple and elementary explanation of the
planetary motion. If the motion of Mars were purely elliptic, how, it
may well be said, could it perform this extraordinary evolution? The
elucidation is to be found in the fact that the earth on which we stand
is itself in motion. Even if Mars were at rest, the fact that the earth
moves would make the planet appear to move. The apparent movements of
Mars are thus combined with the real movements. This circumstance will
not embarrass the geometer. He is able to disentangle the true movement
of the planet from its association with the apparent movement, and to
account completely for the complicated evolutions exhibited by Mars.
Could we transfer our point of view from the ever-shifting earth to an
immovable standpoint, we should then see that the shape of the orbit of
Mars was an ellipse, described around the sun in conformity with the
laws which Kepler discovered by observations of this planet.

Mars takes 687 days to travel round the sun, its average distance from
that body being 141,500,000 miles. Under the most favourable
circumstances the planet, at the time of opposition, may approach the
earth to a distance not greater than about 35,500,000 mil