ucture. The orbit of Mars is one of
remarkable proportions, and it was by the observations of this orbit
that the celebrated laws of Kepler were discovered. During the
occasional approaches of Mars to the earth it has been possible to
measure its distance with accuracy, and thus another method of finding
the sun's distance has arisen which, to say the least, may compete in
precision with that afforded by the transit of Venus. It must also be
observed that the greatest achievement in pure telescopic research which
this century has witnessed was that of the discovery of the satellites
of Mars.

To the unaided eye this planet generally appears like a star of the
first magnitude. It is usually to be distinguished by its ruddy colour,
but the beginner in astronomy cannot rely on its colour only for the
identification of Mars. There are several stars nearly, if not quite, as
ruddy as this globe. The bright star Aldebaran, the brightest star in
the constellation of the Bull, has often been mistaken for the planet.
It often resembles Betelgeuze, a brilliant point in the constellation of
Orion. Mistakes of this kind will be impossible if the learner has first
studied the principal constellations and the more brilliant stars. He
will then find great interest in tracing out the positions of the
planets, and in watching their ceaseless movements.

[Illustration: Fig. 48.--The Orbits of the Earth and of Mars, showing
the Favourable Opposition of 1877.]

The position of each orb can always be ascertained from the almanac.
Sometimes the planet will be too near the sun to be visible. It will
rise with the sun and set with the sun, and consequently will not be
above the horizon during the night. The best time for seeing one of the
planets situated like Mars will be during what is called its opposition.
This state of things occurs when the earth intervenes directly between
the planet and the sun. In this case, the distance from Mars to the
earth is less than at any other time. There is also another advantage in
viewing Mars during opposition. The planet is then at one side of the
earth and the sun at the opposite side, so that when Mars is high in the
heavens the sun is directly beneath the earth; in other words, the
planet is then at its greatest elevation above the horizon at midnight.
Some oppositions of Mars are, however, much more favourable than others.
This is distinctly shown in Fig. 48, which represents the orbit of Mars
and