It is also apparent that Mercury and Venus, seen from the earth,
can never appear far from the sun. They must be just behind the
sun as evening stars, or just before it as heralds of the morning.
Venus is never more than 47° from the sun, and Mercury never more
than 30°; indeed, it keeps so near the sun that very few people
have ever seen the brilliant sparkler. Observe how much larger the
planet appears near the earth in conjunction at D than in opposition
at E. Observe also what phases it must present, and how transits
sometimes take place.

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The movement of a superior planet, one whose orbit is exterior
to the earth, is clear from Fig. 47. When the earth is at A and
Mars at B, it will appear among the stars at C. When the earth is
at D, Mars having moved more slowly to E, will have retrograded
to F. It remains there while the earth passes on, in a line nearly
straight, from Mars to G; then, as the earth begins to curve around
the sun, Mars will appear to retraverse the distance from F to
C, and beyond. The farther the superior planet is from the earth
the less will be the retrograde movement.

[Illustration: Fig. 47.--Illustrating Movements of a Superior Planet.]

The reader should draw the orbits in proportion, and, remembering
the relative speed of each planet, note the movement of each in
different parts of their orbits.

To account for these most simple movements, the earlier astronomers
invented the most complex and impossible machinery. They thought the