A planet's elliptical orbit around a star

Uploaded for a blog post on the Antikythera Mechanism at http://alunsalt.com/2010/05/14/planet...

Cogs move in circles, but planets don't. Kepler found that small errors in Copernicus' model of the solar system could be solved if the planets moved in ellipses. For Earth the ellipse is so slight that it's only fractionally off a circle. Yet even this fraction could be observed by the ancient Greeks because a planet moves faster when it's closer to a star. But how do you simulate elliptical motion if you only have circular cogs?

The animation was made with the excellent My Solar System 2.02 at http://phet.colorado.edu/sims/my-sola...

For more info on Planetary Motion search for Kepler's Laws. http://www.youtube.com/results?search...