Jon Henderson, age 14, of Peoria, Ill., for his question:

What is a perihelion?

Perihelion and aphelion refer to points in a planetary orbit. The helion part of both words refers to the sun. Since no planetary orbit is a perfect circle, there must be one point where it comes nearest to and another where it is farthest from the sun. These two points are the perihelion and the aphelion of the orbit.

The earth's average distance from the sun is about 100,000 miles short of 93 million miles. It traverse the aphelion part of its orbit early in July. It is then at its greatest distance from the sun, and some 1.5 million miles farther away than the average distance. Early in January the earth travels the perihelion part of its orbit and comes its closest to the sun. The earth and the sun are then separated by the average distance, plus 1.5 million miles.