Susanne Caldwell, age 12, of West Asheville, North Carolina, for her question:

How do they determine the start of the seasons?

This tricky job is based on accurate records of the motions of the sun across our skies. Astronomers chart the heavens with a network of lines like the lines of latitude and longitude on the face of the globe. The celestial equator is a heavenly circle directly above the earth's equator. The plane of the earth's orbit is called the ecliptic. It too is a curved line through the heavens. Since the earth's orbit is on the same plane as the sun, the yearly path of the sun marks the ecliptic across the starry heavens.

The ecliptic and the celestial equator are not parallel circles because the earth's axis is inclined to the plane of its orbit at an angle of 23 1/2 degrees. The ecliptic is tilted at the same angle to the celestial equator. Throughout the year, the sun movers all around the ecliptic. But the two celestial circles touch only at two points. The spring or fall season begins when the sun crosses one of these two points. It marks the moment of midsummer when it reaches the ecliptic point farthest north of the celestial equator. It marks the moment of midwinter at the point where the ecliptic is farthest south of the celestial equator.