Kepler's First Law states hat all planets move in ellipses with eccentricity e (where 0 < e < 1). The distance from the center of the ellipse to either focus is the foci length c. The semi-major axis a is half of the long dimension of the ellipse. Thus, the ellipse's e is the ratio of c/a. All planetary orbits in our solar system are "close to being circles" with low eccentricities.