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PLANETS AND THEIR SATELLITES

the exact nature of its apparent motion let the inner circle of the diagram represent the orbit of Mercury and the outer one that of the earth. When the earth is at E, and Mercury at M, the latter is in inferior conjunction with the sun. At the end of three months it will have returned to the point M, but it will not yet be in conjunc-

Fig. 28.—Conjunctions of Mercury with the Sun.

tion, because, in the meantime, the earth has moved forward in its orbit. When the earth reaches a certain point F, Mercury will have reached the point N and will again be in inferior conjunction. This revolution from one inferior conjunction to another is called the synodic revolution of the planet. In the case of Mercury this is somewhat less than one third more than the time of actual

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