Mean longitude is simply the mean longitude at a reference time "t", known as the epoch. Mean longitude can then be expressed, where "n" is the "mean angular motion" and "t" is any arbitrary time. In some sets of orbital elements, "ε" is one of the six elements. 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. From these definitions, the "mean longitude", "l", is the angular distance
Mean longitude angle between any physical objects. It is simply a convenient uniform measure of how far around its orbit a body has progressed since passing the reference direction. While mean longitude measures a mean position and assumes constant speed, true longitude measures the actual longitude and assumes the body has moved with its actual speed, which varies around its elliptical orbit. The difference between the two is known as the equation of the center. From the above definitions, define the "longitude of the pericenter" Then mean longitude is also Another form often seen is the "mean longitude at epoch", "ε". This