fore the sun. Thus the earth's motion
seems to make the sun traverse a regular circle among the stars
once a year: but it is not the sun that moves.

[Illustration: Fig. 43.]

There are certain stars that have such irregular, uncertain, vagarious
ways that they were called vagabonds, or planets, by the early
astronomers. Here is the path of Jupiter in the year 1866 (Fig.
44). These bodies go forward for awhile, then stop, start aside,
then retrograde, [Page 112] and go on again. Some are never seen far
from the sun, and others in all parts of the ecliptic.

[Illustration: Fig. 44.]

First see them as they stand to-day, as in Fig. 45. The observer
stands on the earth at A. It has rolled over so far that he cannot
see the sun; it has set. But Venus is still in sight; Jupiter is
45 deg. behind Venus, and Saturn is seen 90 deg. farther east. When A has
rolled a little farther, if he is awake, he will see Mars before
he sees the sun; or, in common language, Venus will set after,
and Mars rise before the sun. All these bodies at near and far
distances seem set in the starry dome, as the different stars seem
in Fig. 42, p. 110.

[Illustration: Fig. 45. Showing Position of Planets.]

The mysterious movements of advance and retreat are rendered
intelligible by Fig. 46. The planet Mercury is at A, and, seen from
the earth, B is located at _a_, [Page 113] on the background of the
stars it seems to be among. It remains apparently stationary at _a_
for some time, because approaching the earth in nearly a straight
line. Passing D to C, it appears to retrograde among the stars to
_c_; remains apparently stationary for some time, then, in passing
from C to E and A, appears to pass back among the stars to _a_. The
progress of the earth, meanwhile, although it greatly retards the
apparent motion from A to C, greatly hastens it from C to A.

[Illustration: Fig. 46.--Apparent Movements of an Inferior Planet.]

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 deg. from the sun, and Mercury never more
than 30 deg.; 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 tra