Cathy Jolly, Age 10, Of Burlington, N.C., for her question:

How did the prime meridian get its name?

The word prime is coined from o1der words meaning the first hour of the day. The original meaning of meridian was midday. In astronomy the meridian is a great circle through the sky. It passes directly overhead and through both the north and south poles. In geography the meridians that girdle the g1obe are below the great meridians in the sky. They are the man made lines of longitude.

The hour of midday occurs when the sun crosses the meridian above our heads, and as the earth rotates the moment of midday sweeps all around the globe. But one meridian was needed as the noon hour of each calendar day. The prime meridian that runs through Greenwich, London, was chosen for the job, and the noon hour of universal time occurs when the sun crosses this meridian.