In geometry, the semi-major axis (also semimajor axis) a applies to ellipses and hyperbolas.
The semi-major axis of an ellipse is one half of the major axis running from the center, through a focus, and to the edge of the ellipse. The major axis is the longest line that runs through the center and both foci of an ellipse, its ends being at the widest points of the shape.
- al = b2.
A parabola can be obtained as the limit of a sequence of ellipses where one focus is kept fixed as the other is allowed to move arbitrarily far away in one direction, keeping l fixed. Thus a and b tend to infinity, a faster than b.
The semi-major axis is the mean value of the smallest and largest distance from one focus to the points on the ellipse. Now consider the equation in polar coordinates, with one focus at the origin and the other on the positive x-axis,
The mean value of and , is
The semi-major axis of a hyperbola is one half of the distance between the two branches; if this is in the x-direction the equation is:
In terms of the semi-latus rectum and the eccentricity we have
- is the length of the orbit's semi-major axis
- μ is the standard gravitational parameter
Note that for all ellipses with a given semi-major axis, the orbital period is the same, regardless of eccentricity.
In astronomy, the semi-major axis is one of the most important characteristics of an orbit, along with its orbital period. For solar system objects, the semi-major axis is related to the period of the orbit by Kepler's third law (originally empirically derived),
where G is the gravitational constant, and M is the mass of the central body, and m is the mass of the orbiting body. Typically, the central body's mass is so much greater than the orbiting body's, that m may be ignored. Making that assumption and using typical astronomy units results in the simpler form Kepler discovered.
It is often said that the semi-major axis is the "average" distance between the primary (the focus of the ellipse) and the orbiting body. This is not quite accurate, as it depends over what the average is taken.
- averaging the distance over the eccentric anomaly (q.v.) indeed results in the semi-major axis.
- averaging over the true anomaly (the same angle, measured at the focus) results, oddly enough, in the semi-minor axis .
- averaging over the mean anomaly (the fraction of the orbital period that has elapsed since pericentre, expressed as an angle), finally, gives the time-average (which is what "average" usually means to the layman): .
- the time-average of r-1 is a-1
Energy; calculation of semi-major axis from state vectors
- is orbital velocity from velocity vector of an orbiting object,
- is cartesian position vector of an orbiting object in coordinates of a reference frame with respect to which the elements of the orbit are to be calculated (e.g. geocentric equatorial for an orbit around Earth, or heliocentric ecliptic for an orbit around the Sun),
- is the gravitational constant,
- the mass of the central body.
Note that for a given central body and total specific energy, the semi-major axis is always the same, regardless of eccentricity. Conversely, for a given central body and semi-major axis, the total specific energy is always the same.
The International Space Station has an orbital period of 91.74 minutes, hence the semi-major axis is 6738 km  . Every minute more corresponds to ca. 50 km more: the extra 300 km of orbit length takes 40 seconds, the lower speed accounts for an additional 20 seconds.
- Jeremy B. Tatum, Celestial Mechanics, Chapter 9 - The Two Body Problem in Two Dimensions (2004)
- Darren M. Williams, Average distance between a star and planet in an eccentric orbit, American Journal of Physics, November 2003, Volume 71, Issue 11, pp. 1198-1200