The eccentricity of an ellipse is a measure of how much it deviates from being a perfect circle. It is defined as the ratio of the distance between the center of the ellipse and one of its foci, to the length of the semi-major axis.

For Earth, the eccentricity is approximately 0.0167. This means that Earth's shape is very close to being a perfect sphere, but it is slightly flattened at the poles and bulges at the equator.