orbits are within that of the earth-viz., Mercury, Venus; (2) Superior, or those whose orbits are beyond that of the earth-Mars, Jupiter, Saturn, Uranus, Neptune.

MOTIONS OF A PLANET AS SEEN FROM THE SUN.Could we stand at the sun and watch the movements of the planets, they would all be seen to be revolving with different velocities in the order of the zodiacal signs. But to us, standing on one of the planets, itself in motion, the effect is changed. To an observer at the sun all the motions would be real, while to us many are only apparent. The position of a planet, as seen from the centre of the sun, is called its heliocentric place; as seen from the centre of the earth, its geocentric place. When Venus is at inferior conjunction, an observer at the sun would see it in the opposite part of the heavens from that in which it would appear to him if viewed from the earth.

MOTIONS OF AN INFERIOR PLANET.-An inferior planet is never seen by us in the part of the sky opposite to the sun at the time of observation. It cannot recede from him as much as 90°, or the circumference, since it moves in an orbit entirely enclosed by the orbit of the earth. Twice in every revolution it is in conjunction (6) with the sun,-an inferior conjunction (A) when it comes between the earth and the sun, and a superior conjunction (B) when the sun lies between it and the earth. (See Fig. 19.)

When the planet attains its greatest distance east or west (as we see it) from the sun, it is said to be at its greatest elongation, or in quadrature (□).

Fig. 19.
B

C

D

E

QUADRATURE AND CONJUNCTION.

When passing from B to A it is east of the sun, and from A to B it is west of the sun. When east of the sun, it sets later than the sun, and hence is "evening star" when west of the sun, it rises earlier than the sun, and hence is "morning star." An inferior planet is never visible when in superior conjunction, as its light is then lost in the greater brilliancy of the sun.

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.

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 a 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

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 him180°. 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 0the 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