Martha Stevens, age 12, of Richmond, Virginia, for her question:

Does the moon see a half and full earth?

Yes, a visitor to the moon may watch the changing phases of the earth, somewhat as we earthlings watch the changing lunar phases. However, there are a couple of variations in the program. In the first place, the earth is never visible from the far side of the moon. We cannot see it and it cannot see us. Hence the earth phases can be observed only on the side of the moon that faces us.

As we know, the lunar phases reveal the sunlit sections on this side of the moon. They change from quarter, to half to full as daylight creeps across the lunar landscape. The cycle of lunar phases takes about a month, because this is the length of the moon's day and night period. From the moon the earth phases also reveal the sunlit zones. But the complete series of changes takes only 24 hours. This is the time it takes the earth to spin through its day and night period.