Conic Sections: Ellipses

The most familiar type of conic section is the Ellipse, which can be easily drawn with some simple tools (Figure a.).

    Click on image to enlarge - From Seeds (1998)

Note some properties of the ellipse:

• From Figure a. it is apparent that the distance from one focus, to a point in the orbit, to the other focus is a constant.
• From Figure b. it appears that areas encompassed between two points in an orbit are equal, no matter where in the orbit.

And some mathematical properties of the ellipse:

• Eccentricity, e=c/a, where a is the semimajor axis, and c is the distance from the center of the ellipse to the foci.

Eccentricity describes the degree of non-circularity of an ellipse:  The higher the eccentricity, the more non-circular is the ellipse.