| has gloss | eng: In astrodynamics or celestial dynamics, mean longitude is the longitude at which an orbiting body could be found if its orbit were circular, and free of perturbations, and if its inclination were zero. Both the mean longitude and the true longitude of the body in such an orbit would change at a constant rate over time. But if the orbit is eccentric and departs from circularity (and let it still be supposed free from any perturbations), then the orbit would become a Keplerian ellipse, and then the progress of the orbiting body in true longitude along this orbit would no longer change at a constant rate over time. The mean longitude then becomes an abstracted quantity, still proportional to the time, but now only indirectly related to the position of the orbiting body: the difference between the mean longitude and the true longitude is usually called the equation of the center. In such an elliptical orbit, the only times when the mean longitude is equal to the true longitude are the times when the orbiting body passes through periapsis (or pericenter) and apoapsis (or apocenter). |
