Micki Armstrong, age 13, of Mulvane, Kansas, for her question:

Is the earth's orbit a perfect circle?

The motions and schedules of the heavenly bodies are very precise. But rarely do their orderly patterns fit neatly into our man made systems of geometry. The earth's orbit is more circular than the orbits of most of the other planets. But it is not a perfect circle. Anon circular orbit is said to be eccentric and we measure this quality in numbers and decimals. The eccentricity of the earth's orbit is 0.017. This is very slight compared with Pluto's eccentricity of 0.248. Our somewhat oval, or elliptical, orbit makes our distance from the sun vary somewhat through the year.

Our average distance through the year is close to 93 million miles. But at perihelion, when we come closest to the sun, the distance is about 91.4 million miles. At aphelion, when we are farthest from the sun, the distance is 94.6 million miles. You would expect our summer season to occur during the closer perihelion phase. But it does not. On January 1, the sun is some three million miles closer than on July 1. The explanation of that situation, of course, belongs in the answer to a different question.