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Semi-Major Axis

(maximum radius of an ellipse)

An Ellipse's Semi-Major Axis (a) is half the distance between its furthest two points. The Semi-Minor Axis (b) is an analogous distance from the center to one of the nearest points on the ellipse. They are related:

b = a (1 - e2).5

Where e is the Eccentricity. The Semi-Major Axis is used in various orbital calculations. For example, for a non-circular (elliptical) orbit:

T = 2π(a3/μ).5


(orbits,celestial mechanics,measure)

Referenced by:
Circumbinary Planet
Holmberg Radius
Laplace Radius (rL)
Titius-Bode Law
Trans-Neptunian Object (TNO)