We know planets revolve around the Sun...but what ellipses do they travel? The answer is based on a simple mathematical equation. Earth science teacher David Gibson put the “eccentricity equation” into motion with this demonstrator, which you can use to graphically illustrate the orbits of the planets in our solar system and explain how the planets remain in orbit. This popular teaching aid fits on any standard overhead projector. Includes: teacher’s guide with background information, student activities and facts on the solar system. Grades 9-12.