semi-major axis

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

Where:

• T is the orbital period.
• μ is the standard gravitational parameter of the body orbited.