05/02/1997 07:00:00

Question of the Week: Does the earth keep a constant distance from the sun? If not, will the earth get closer to the sun and become more warm?

Question: Does the earth keep a constant distance from the sun? If not, will the earth get closer to the sun and become more warm?

Submitted by Steven S. Showers Newbury Park, California

Answered by Andrew Ingersoll, professor of planetary science, Caltech.

Earth has an eccentric orbit, which means that it moves in a path that is slightly oval in shape. Contrary to what you'd expect, Earth gets closest to the sun every December, and farthest from the sun every June. We in the northerhe maximum eccentricity is about 5 percent and the minimum is near zero, when the orbit is nearly circular. This cycle can be calculated for millions of years, and we know that the glaciers also have cycles of about 100,000 years. The question is whether the glaciers are tied to changes in Earth's eccentricity.

So the bottom line is that Earth does get closer and farther, and it does affect the climate. But the mechanism is not all that clear. Averaged over a year, the distance from the Earth the Sun changes very little, even over billions of years (the Earth is 4.5 billion years old).

What is more important to climatic changes over the eons is the fact that the sun is getting brighter. Earth now gets about 20 to 25 percent more sunlight than it did four billion years ago.

Written by Robert Tindol