The equation of the center, in astronomy and elliptical motion, is equal to the true anomaly minus the mean anomaly, i.e. the difference between the actual angular position in the elliptical orbit and the position the orbiting body would have if its angular motion was uniform. It arises from the ellipticity of the orbit, is zero at pericenter and apocenter, and reaches its greatest amount nearly midway between these points.
The "equation" in the present sense comes from astronomy. It was specified and used by Kepler, as that variable quantity determined by calculation which must be added or subtracted from the mean motion to obtain the true motion. It is based on aequatio, -onis, f. in Latin. In the expression "equation of time" used in astronomy, the term "equation" has a similar meaning.
Read more about Equation Of The Center: Analytical Expansions, Moon's Equation of The Center
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