### semi-major axis

**(***a*)
(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 - e^{2})^{.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(a^{3}/μ)^{.5}

Where:

(*orbits,celestial mechanics,measure*)
http://en.wikipedia.org/wiki/Semi-major_axis

**Referenced by:**

circumbinary planet

Holmberg radius

mean anomaly

moon

Laplace radius (r_{L})

Titius-Bode law

trans-Neptunian object (TNO)

index