Astronomers have often led the search for understanding and new knowledge. Planets move across the night sky quite differently from stars. Even the word "planet" comes from the Arabic for "wanderer". How do planets move?
last "naked-eye" astronomer
lifetime of careful data
1. Each planet moves in an elliptical orbit with the Sun at one focus.
2. The line from the Sun to any planet sweeps out equal areas in equal time intervals.
3. For each planet, the square of the period is proportional to the cube of the average radius.
(C) 2001, Doug Davis; all rights reserved