Astrophysics (Index)About

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 - 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:


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

Referenced by pages:
circumbinary planet
Holmberg radius (RH)
mean anomaly
moon
near-Earth object (NEO)
OGLE-2005-BLG-3290Lb
Planet Nine
Laplace radius (rL)
Titius-Bode law
trans-Neptunian object (TNO)

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