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 is a hybrid angle.
Definition
Define a reference direction, ♈, along the ecliptic. Typically, this is the direction of the vernal 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,
.
Define the angular distance along the plane of the orbit from the ascending node to the pericenter as the argument of the pericenter,
.
Define the mean anomaly,
, 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,
, is the angular distance the body would have from the reference direction if it moved with uniform speed,