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disadvantage is counterbalanced by the fact that the planet continually comes nearer during the interval, so that we get a better view of whatever portion of the illuminated hemisphere may be visible to us. Its apparent form and size at different times during its synodic revolution go through a series of changes similar to those shown in the next chapter in the case of Venus.

The question whether Mercury has an atmosphere is also one on which opinions differ, the prevailing opinion being in the negative. It seems quite certain that, if it has one, it is too rare to reflect the light of the sun.

It will be readily seen that, if an inferior planet revolved around the sun in the same plane as the earth, we should see it pass over the sun's disk at every inferior conjunction. But no two planets revolve in the same plane. Of all the major planets the orbit of Mercury has the largest inclination to that of the earth. In consequence, when in inferior conjunction, it commonly passes a greater or less distance to the north or to the south of the sun. If, however, it chances to be near one of its nodes at the time in question, we shall see it as a black spot passing across the sun's disk. This phenomenon is called a transit of Mercury. Such transits occur at intervals ranging between three and thirteen years. They are observed with much interest by astronomers because it is possible to determine with great precision the time at which the planet enters upon the solar disk, and leaves it again. Knowing these times,