Editing Mean motion (section)

{{Short description|Angular speed required for a body to complete one orbit}}
{{confusing|date=December 2018}}
In [[orbital mechanics]], '''mean motion''' (represented by ''n'') is the [[Angular frequency|angular speed]] required for a body to complete one orbit, assuming constant speed in a [[circular orbit]] which completes in the same time as the variable speed, [[elliptic orbit|elliptical orbit]] of the actual body.<ref>
{{cite book 
 | editor-last = Seidelmann
 | editor-first = P. Kenneth
 | editor2-last = Urban
 | editor2-first = Sean E.
 | title = Explanatory Supplement to the Astronomical Almanac
 | publisher = University Science Books, Mill Valley, CA
 | year = 2013
 | isbn = 978-1-891389-85-6
 | edition=3rd
|page=648}}
</ref>  The concept applies equally well to a small body revolving about a large, massive primary body or to two relatively same-sized bodies revolving about a common [[center of mass]]. While nominally a [[mean]], and theoretically so in the case of [[Two-body problem|two-body motion]], in practice the mean motion is not typically an [[average]] over time for the orbits of real bodies, which only approximate the two-body assumption. It is rather the instantaneous value which satisfies the above conditions as calculated from the current [[Gravitational constant|gravitational]] and [[ellipse|geometric]] circumstances of the body's constantly-changing, [[Perturbation (astronomy)|perturbed]] [[orbit]].

Mean motion is used as an approximation of the actual orbital speed in making an initial calculation of the body's position in its orbit, for instance, from a set of [[orbital elements]]. This mean position is refined by [[Kepler's equation]] to produce the true position.