equation of center
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equation of centerThe true anomaly minus the mean anomaly of a body moving in an elliptical orbit. It is the difference between the actual angular position of the body and the position it would have if its angular motion were uniform. It is a major inequality in the Moon's motion, being a direct result of the large eccentricity (0.0549) of the Moon's orbit and hence of its varying orbital velocity (see Kepler's laws).
Collins Dictionary of Astronomy © Market House Books Ltd, 2006