Curated Video
Why Are Planetary Orbits Elliptical?
Planetary orbits are elliptical because gravitational interaction over time changes the delicate balance of mass, velocity and distance from the star which otherwise keeps planetary orbit circular. For a very long time, from Ptolemy to...
University of Oregon
Electronic Universe: Kepler's Laws of Planetary Motion
From the The Electronic Universe Project web site. This page states and explains Kepler's three laws of planetary motion. Includes some downloadable movie clips and associated sound files. Discusses Ptolemy's attempt to explain...