[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 transits sometimes take place.

[Page 114] 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.