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Variation in astronomy is used to refer to a perturbation of the orbit of a body, such as a planet or satellite.
Specifically, in the case of the motion of the Moon around the Earth, the Variation is a type of perturbation caused by the action of the Sun. The effect is to distort the Moon's orbit into an ellipse centred on the Earth with the major axis perpendicular to a line drawn between the Earth and Sun.
The Variation has a period of half a synodic month and causes the Moon's ecliptic longitude to vary by 0.66 degrees.
Note that the distortion of the Moon's orbit caused by the Variation is a different effect from normal undisturbed elliptical motion of a body. The Variation effect would still occur if the undisturbed motion of the Moon had an eccentricity of zero (i.e. circular).
The Variation is the second-largest solar perturbation of the Moon's orbit after the Evection.
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See also
References
- Brown, E.W. An Introductory Treatise on the Lunar Theory. Cambridge University Press, 1896 (republished by Dover, 1960).
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