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.