When in inferior conjunction, it sometimes passes in front of the sun, and appears to us as a round black spot swiftly moving across his disk. This is called a transit.

B

+8

Fig. 20.
G

A

RETROGRADE MOTION.

Retrograde motion of an inferior planet.-Suppose the earth to be at A, and the planet at B. Now, while the earth is passing to F, the planet will pass to D-the arc AF being shorter than BD, because the nearer a planet is to the sun the greater its velocity. While the planet is at B, we locate it at C on the ecliptic, in Gemini; but at D, it appears to us to be at G, in Taurus. So that the planet has retrograded through an entire sign on the ecliptic, while its course all the while has been directly for

ward in the order of the signs; and to an observer at the sun, such would have been its motion.

Phases of an inferior planet.—An inferior planet presents all the phases of the moon. At superior conjunction, the whole illumined disk is turned toward us; but the planet is lost in the sun's rays: therefore neither Mercury nor Venus ever presents a full circular appearance, like the full moon. A little before or after superior conjunction, an inferior

Fig. 21.

E

PHASES OF AN INFERIOR PLANET.

W

planet may be seen with a telescope; but the whole of the light side is not turned toward us, and so the planet appears gibbous, like the moon between first quarter and full. In quadrature, the planet shows us only one-half its illumined disk; this decreases, becoming more and more crescent toward inferior conjunction, at which time the unillumined side is toward us.

MOTIONS OF A SUPERIOR PLANET.-The superior planet moves in an orbit which entirely surrounds

that of the earth. When the earth is at E (Fig. 22), the planet at L is said to be in opposition to the sun. It is then at its greatest distance from him— 180°. The planet is on the meridian at midnight while the sun is on the corresponding meridian on the opposite side of the earth; or the planet may be rising when the sun is just setting. When the planet is at N, it is in conjunction, and being lost in the sun's rays is invisible to us.

Retrograde motion of a superior planet.-Suppose the earth to be at E and the planet at L, and that we move on to G while the planet passes on to 0— the distance EG being longer than LO (just the reverse of what takes place in the movements of the inferior planets); at E, we should locate the planet at P on the ecliptic, in the sign Cancer; but at G, it would appear to us at Q, in the sign Gemini, having apparently retrograded on the ecliptic the distance PQ, while it was all the while moving on in the direct order of the signs. Now, suppose the earth moves on to I and the planet to U, we should then see it at the point W, further on in the ecliptic than Q, which indicates direct motion again, and at some point near Q the planet must have appeared without motion. After this, it will continue direct until the earth has completed a large portion of her orbit, as we shall easily see by imagining various positions of the earth and planet, and then drawing lines as we have just done, noticing whether they indicate direct or retrograde motion. The greater

the distance of a planet the less it will retrograde, as we shall perceive by drawing another orbit outside the one represented in the cut, and making the same suppositions concerning it as those we have already explained.

Fig. 22.

RETROGRADE MOTION OF A SUPERIOR PLANET.

SIDEREAL AND SYNODIC REVOLUTION.-The interval of time required by a planet to perform a revolution from one fixed star back to it again, is termed a sidereal revolution (sidus, a star).

1. The interval of time between two similar con

junctions of an inferior planet with the earth and sun is termed a synodic revolution. Were the earth at rest, there would be no difference between a sidereal and a synodic revolution, and the planet would come into conjunction twice in each revolution. Since, however, the earth is in motion, it follows that after the planet has completed its sidereal revolution, it must then overtake the earth before they can both come again into the same position with regard to the sun. The faster a planet moves, the sooner it can do this. Mercury, travelling at the greater speed and on an inner orbit, accomplishes it much quicker than Venus. The synodic period always exceeds the sidereal.

2. The interval between two successive conjunctions or oppositions of a superior planet is termed a synodic revolution. Since the earth moves so much faster than any superior planet, it follows that after it has completed a sidereal revolution it must then overtake the planet before they can come again into the same position with regard to the sun. The slower the planet moves, the sooner it can do this. Uranus, making a sidereal revolution in eighty-four years, can be overtaken more quickly than Mars, which makes one in less than two years. It consequently requires over a second revolution to catch up with Mars, of one to overtake Jupiter, and but little over of one to come up with Uranus. Indeed, the earth repasses Neptune in two days after it has finished a sidereal revolution.