When the planet is between the earth and the sun, as at P, it is said to be in inferior conjunction.
When it is in the same direction as the sun, but beyond it, as at P'', it is said to be in superior conjunction.
When the planet is at such a point in its orbit that a line drawn from the earth to it would be tangent to the orbit, as at P' and P''', it is said to be at its greatest elongation.
Fig. 145.
126. Apparent Motion of an Inferior Planet.—When the planet is at P, if it could be seen at all, it would appear in the heavens at A. As it moves from P to P', it will appear to move in the heavens from A to B. Then, as it moves from P' to P'', it will appear to move back again from B to A. While it moves from P'' to P''', it will appear to move from A to C; and, while moving from P''' to P, it will appear to move back again from C to A. Thus the planet will appear to oscillate to and fro across the sun from B to C, never getting farther from the sun than B on the west, or C on the east: hence, when at these points, it is said to be at its greatest western and eastern elongations. This oscillating motion of an inferior planet across the sun, combined with the sun's motion among the stars, causes the planet to describe a path among the stars similar to that shown in Fig. 145.
Fig. 146.
127. Phases of an Inferior Planet.—An inferior planet, when viewed with a telescope, is found to present a succession of phases similar to those of the moon. The reason of this is evident from Fig. 146. As an inferior planet passes around the sun, it presents sometimes more and sometimes less of its bright hemisphere to the earth. When the earth is at T, and Venus at superior conjunction, the planet turns the whole of its bright hemisphere towards the earth, and appears full; it then becomes gibbous, half, and crescent. When it comes into inferior conjunction, it turns its dark hemisphere towards the earth: it then becomes crescent, half, gibbous, and full again.