ESS Regents Practice Exam 2026 - Free Regents Practice Questions and Study Guide

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In the eccentricity formula e = distance between major foci divided by length of the major axis, what distance is in the numerator?

Distance between the two major foci

Eccentricity tells how stretched an ellipse is, and it comes from the ratio of a distance tied to the foci to the length of the major axis. The distance in the numerator is the distance between the two major foci. Along the major axis, the two foci sit, so that distance is 2c if c is the distance from the center to a single focus. The length of the major axis is 2a, where a is the semi-major axis. So e = (2c)/(2a) = c/a, which is the standard form. Therefore, the distance in the numerator must be the distance between the two major foci.

Distance between the center and a focus

Distance from the center to a vertex

Distance between perihelion and aphelion