Casio fx-CG10 Advanced Operation Manual page 106

Sample Solution—Earth's Revolution (continued)
Though we note slight rounding errors, relative to the distances we are
discussing, these are minor. What is noteworthy is how close to the center the
two foci appear to be.
f. What is the eccentricity of the ellipse? Discuss what that means in general
terms.
Press
The eccentricity of an ellipse is a number between 0 and 1. The closer the
eccentricity is to 0, the closer the ellipse is to a circle. The closer it is to 1, the
more the ellipse collapses on itself, making a very narrow ellipse. Here we see
the eccentricity is approximately 0.0167; the Earth's trip around the sun is very
close to circular.
Fostering Advanced Algebraic Thinking with Casio Technology
(G-Solv), and then (e) for eccentricity.