### semi-major axis

**(***a*)
(maximum radius of an ellipse)

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 - e^{2})^{0.5}

The semi-major axis is used in various
orbital calculations. For example, for
a non-circular (elliptical) orbit:

T = 2π(a^{3}/μ)^{0.5}

Where:

(*orbits,celestial mechanics,measure*)
**Further reading:**

http://en.wikipedia.org/wiki/Semi-major_axis

**Referenced by pages:**

circumbinary planet

Holmberg radius (R_{H})

mean anomaly

moon

near-Earth object (NEO)

OGLE-2005-BLG-3290Lb

Planet Nine

projected semi-major axis

Laplace radius (r_{L})

Titius-Bode law

trans-Neptunian object (TNO)

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