Kepler (1571 - 1630) announced his three famous laws of planetary motion in 1609:

• Each planet or comet moves in an elliptical orbit, with the sun at one focus of the ellipse.
• A line from the sun to a given planet sweeps out equal areas in equal time.
• The periods of the planets are proportional to the 3/2 power of the major axis length of their orbits.

This animation illustrates the first two laws.

Click on [pause] to stop the animation.