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.
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 from ♈︎ to the ascending node, then up along the plane of the body's orbit to its mean position.
Sometimes the value defined in this way is called the "mean mean longitude", and the term "mean longitude" is used for a value that does have short-term variations (such as over a synodic month or a year in the case of the moon) but does not include the correction due to the difference between true anomaly and mean anomaly.
Also, sometimes the mean longitude (or mean mean longitude) is considered to be a slowly varying function, modeled with a Maclaurin series, rather than a simple linear function of time.
Discussion.
Mean longitude, like mean anomaly, does not measure an 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.
Formulae.
From the above definitions, define the "longitude of the pericenter"
"ϖ" = "Ω" + "ω".
Then mean longitude is also
"L" = "ϖ" + "M".
Another form often seen is the "mean longitude at epoch", "ε". This is simply the mean longitude at a reference time "t"0, known as the epoch. Mean longitude can then be expressed,
"L" = "ε" + "n"("t" − "t"0), or
"L" = "ε" + "nt", since "t" = 0 at the epoch "t"0.
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.
References.
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