It is rather the instantaneous value which satisfies the above conditions as calculated from the current gravitational and geometric circumstances of the body's constantly-changing, perturbed orbit.
In systems with more mass, bodies will orbit faster, in accordance with Newton's law of universal gravitation.
Kepler's 2nd law of planetary motion states, a line joining a planet and the Sun sweeps out equal areas in equal times,[6] or for a two-body orbit, where dA/dt is the time rate of change of the area swept.
Substituting and rearranging, mean motion can also be expressed, where the −2 shows that ξ must be defined as a negative number, as is customary in celestial mechanics and astrodynamics.
The masses of the planets are all much smaller, m ≪ M. Therefore, for any particular planet, and also taking the semi-major axis as one astronomical unit, The Gaussian gravitational constant k = √G,[12][13][note 2] therefore, under the same conditions as above, for any particular planet and again taking the semi-major axis as one astronomical unit, Mean motion also represents the rate of change of mean anomaly, and hence can also be calculated,[14] where M1 and M0 are the mean anomalies at particular points in time, and Δt (≡ t1-t0) is the time elapsed between the two.
For Earth satellite orbital parameters, the mean motion is typically measured in revolutions per day.