Mark Kemper, age 12, of Dotham, Ala., for his question:

WHAT IS A HALO?

A halo is a circle of light. The word comes from the Greek name for a threshing floor on which oxen walked a circular path by going around and around the floor.

In art and religion, a halo is a ring of light appearing above the head of a saintly person. Few pictures of Christ or a saint have been painted since the 400s without a shining circle nearly surrounding the head.

In astronomy, halos are the luminous rings around the sun and moon. These halos are probably formed by light rays which are refracted or reflected and bent together, by particles of ice. Such halos are generally red on the inside, then orange, yellow and white on the outside.

Halos are often seen in the Arctic regions. A cross is sometimes seen inside the circle, stretching outside the halo. Large spots of lights called parhelia also appear on the bands of the cross. Parhelia are called mock suns or sundogs.

Coronas are often called halos, but they have different causes.