Mean longitude
Mean longitude is the ecliptic longitude at which an orbiting body could be found if its orbit were circular and free of perturbations. While nominally a simple longitude, in practice the mean longitude does not correspond to any one physical angle. Definition Define a reference direction, ♈︎, along the ecliptic. Typically, this is the direction of the March equinox. At this point, ecliptic longitude is 0°. The body's orbit is generally inclined to the ecliptic, therefore define the angular distance from ♈︎ to the place where the orbit crosses the ecliptic from south to north as the longitude of the ascending node, Ω. Define the angular distance along the plane of the orbit from the ascending node to the pericenter as the argument of the pericenter, ω. Define the mean anomaly, M, as the angular distance from the pericenter which the body would have if it moved in a circular orbit, in the same orbital period as the actual body in its elliptical orbit. From these definitions, the mean longitude, L, is the angular distance the body would have from the reference direction if it moved with uniform speed, L = Ω + ω + M, measured along the ecliptic