George ICuney, age 11, of Milwaukee, Wisconsin, for his question:

Where is a zenith?

The zenith is a point in the sky, directly above the top of your head. Wherever you go on land or sea, you take your own personal zenith with you. It is not quite the same zenith that belongs to your neighbor and even a little different from the friend who walks by your side. All this is true but your zenith does not remain unchanged.

The starry heavens surround our globe on every side, somewhat like the inside of an enormous sphere. We see only half of this celestial sphere at a time. But as the earth rotates, we get to see both halves during a calendar day. Overhead, the celestial view passes across the zenith, moment by moment. At the opposite point of the celestial sphere is the nadir, now out of sight. In twelve hours, this nadir point will be your zenith and your present zenith will be the nadir.