the orbit of the Earth accurately drawn to scale. It will be seen
that while the orbit of the earth is very nearly circular, the orbit of
Mars has a very decided degree of eccentricity; indeed, with the
exception of the orbit of Mercury, that of Mars has the greatest
eccentricity of any orbit of the larger planets in our system.

The value of an opposition of Mars for telescopic purposes will vary
greatly according to circumstances. The favourable oppositions will be
those which occur as near as possible to the 26th of August. The other
extreme will be found in an opposition which occurs near the 22nd of
February. In the latter case the distance between the planet and the
earth is nearly twice as great as the former. The last opposition which
was suitable for the highest class of work took place in the year 1877.
Mars was then a magnificent object, and received much, and deserved,
attention. The favourable oppositions follow each other at somewhat
irregular intervals; the last occurred in the year 1892, and another
will take place in the year 1909.

The apparent movements of Mars are by no means simple. We can imagine
the embarrassment of the early astronomer who first undertook the task
of attempting to decipher these movements. The planet is seen to be a
brilliant and conspicuous object. It attracts the astronomer's
attention; he looks carefully, and he sees how it lies among the
constellations with which he is familiar. A few nights later he observes
the same body again; but is it exactly in the same place? He thinks not.
He notes more carefully than before the place of the planet. He sees how
it is situated with regard to the stars. Again, in a few days, his
observations are repeated. There is no longer a trace of doubt about the
matter--Mars has decidedly changed his position. It is veritably a
wanderer.

Night after night the primitive astronomer is at his post. He notes the
changes of Mars. He sees that it is now moving even more rapidly than it
was at first. Is it going to complete the circuit of the heavens? The
astronomer determines to watch the orb and see whether this surmise is
justified. He pursues his task night after night, and at length he
begins to think that the body is not moving quite so rapidly as at
first. A few nights more, and he is sure of the fact: the planet is
moving more slowly. Again a few nights more, and he begins to surmise
that the motion may cease; after a short time the motion does