Edna Nelson, age 14, of Billings, Mont., for her question:

WHAT DID JOHANNES KEPLER DISCOVER?

Johannes Kepler was a German astronomer and mathematician who discovered three laws of planetary motion. Later on, his three laws formed an indispensable part of Sir Isaac Newton's discovery of universal gravitation.

Here are Kepler's three famous laws:

1.) Every planet follows an oval shaped path, or orbit, around the sun, called an ellipse. The sun is located at one point of this elliptical orbit.

2.) An imaginary line from the center of the sun to the center of a planet sweeps out the same area in a given time. This means that the planets move faster when they are closer to the sun.

3.) The time taken by a planet to make one complete trip around the sun is called its period. The squares of the periods of two planets are proportional to the cubes of their mean distance from the sun.

In Kepler's third law, he discovered that a planet that is four times as far away from the sun as another planet takes eight times as long to go around the sun.

Kepler lived from 1571 to 1630. After graduating from the University of Tubingen, and teaching for a time in Graz, he became the assistant to Tycho Brahe, the greatest astronomical observer before the introduction of the telescope.

After Brahe died, Rudolph II, the Holy Roman Emperor, appointed Kepler to be Brahe's successor as imperial mathematician.

Kepler made his most significant discoveries when he tried to find an orbit to fit all of Brahe's observations of planet Mars. Earlier astronomers thought a planet's orbit was a circle or a combination of circles.

But Kepler was skeptical of the circular orbit theory and he tried an ellipse in his calculations. The ellipse worked and Kepler successfully challenged a 2,000 year old assumption.

Another of Kepler's achievements was his suggestion of a better combination of lenses for telescopes.

A Polish astronomer named Nicolaus Copernicus is today considered to be the founder of modern astronomy. He was the scientist who developed the theory that Earth is a moving planet.

Copernicus didn't accept the 1,400 year old theory that Earth was the center of the universe.

The first astronomer to uphold openly Copernicus' theories was none other than Johannes Kepler.