Mean longitude

In astrodynamics or celestial dynamics mean longitude is the longitude at which an orbiting body could be found if its orbit were circular and its inclination were zero. The mean longitude changes at a constant rate over time. The only times when it is equal to the true longitude are at periapsis and apoapsis.

Calculation

The mean longitude $L\,$ can be calculated as follows:

$L=M+{\bar {\omega }}=M+\Omega +\omega \,$

where:

• $M\,$ is orbit's mean anomaly,
• ${\bar {\omega }}\,$ is longitude of the orbit's periapsis,
• $\Omega \,$ is the longitude of ascending node and
• $\omega \,$ is the argument of periapsis.