Description

This applet illustrates the property of ellipses that the sum of distances between the two focii and the distances from each focus to a point on the ellipse remains a constant.

Purpose

The dynamic nature of this simulation shows how the sum of these distances remains constant for all points on the ellipse. As the animation runs, it shows the sum of distances as a point moves around the perimeter of the ellipse. As the point moves, the three distance components in the sum continually change as the sum of all three remains the same. This property would be difficult to visulize without animation. In addition, this simulation allows the user to change the eccentricity of the ellipse to show that this property is true for all ellipses, regardless of their shape or size.