Mary Beth Spencer, age 12, of Tucson, Ariz., for her question:

Exactly how long is the solar year?

We take it for granted that a year is the time it takes our earth to orbit the sun. But the motions of heavenly bodies are never precise, besides which they tend to move into different positions in relation to each other. For these reasons there are several different periods of time called the year. The sidereal year marks the return of the earth to the same spot on its orbit. There is also the lunar year, the eclipse year and the solar year.

The solar year marks the time from one spring equinox to the next. It is known also as the tropical year, the natural year or the astronomical year. It keeps pace with the seasons, and our calendar is based upon it. The length of the solar year is 365 days, 5 hours, 48 minutes and 45.575 seconds. It is slightly more than 20 minutes longer than the true or single year in which the Earth completes one orbit of the sun.