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An ellipse's semi-major axis (a) is half the distance between the ellipse's furthest two points, or equivalently, from the ellipse's center to one of the points furthest from the center. The semi-minor axis (b) is an analogous distance from the ellipse's center to one of the ellipse's points nearest the center. They are mathematically related with e, the ellipse's eccentricity:
b = a (1 - e2)0.5
The semi-major axis is used in various orbital calculations. For example, for a non-circular (elliptical) orbit:
T = 2π(a3/μ)0.5
Where: