### 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

Moon

Laplace Radius (r_{L})

Titius-Bode Law

Trans-Neptunian Object (TNO)

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