semi-major axis

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:

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