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 free of perturbations, and if its inclination were zero. Both the mean longitude and the true longitude of the body in such an orbit would change at a constant rate over time. But if the orbit is eccentric and departs from circularity (and let it still be supposed free from any perturbations), then the orbit would become a Keplerian ellipse, and then the progress of the orbiting body in true longitude along this orbit would no longer change at a constant rate over time. The mean longitude then becomes an abstracted quantity, still proportional to the time, but now only indirectly related to the position of the orbiting body: the difference between the mean longitude and the true longitude is usually called the equation of the center. In such an elliptical orbit, the only times when the mean longitude is equal to the true longitude are the times when the orbiting body passes through periapsis (or pericenter) and apoapsis (or apocenter).

In an orbit that is undergoing perturbations, an osculating orbit together with its (elliptical) osculating elements can still be defined for any point in time along the actual orbit. For each successive set of osculating elements, a mean longitude can be defined, as in the unperturbed case. But here, the changes in mean longitude over time will not only be those due to some constant rate over time; there will also be superimposed perturbations (and the rate itself is also perturbed). A set of mean elements can still be defined for such an orbit, after abstracting the perturbational variations with time. The term 'mean longitude' was already used for the unperturbed and osculating cases, and the corresponding mean longitude member in a set of mean elements, after abstraction of the periodic variations, is sometimes therefore called the 'mean mean longitude'. To arrive at a true longitude from a mean mean longitude, the perturbational terms must be applied as well as the equation of the center.

Calculation

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

$L=M %2B \varpi = M %2B \Omega %2B \omega\,$

where:

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

Orbits

General	Box Capture Circular Elliptical / Highly elliptical Escape Graveyard Hyperbolic trajectory Inclined / Non-inclined Osculating Parabolic trajectory Parking Synchronous (semi sub)

Geocentric	Geosynchronous Geostationary Sun-synchronous Low Earth Medium Earth High Earth Molniya Near-equatorial Orbit of the Moon Polar Tundra Two-line elements

About other points	Areosynchronous Areostationary Halo Lissajous Lunar Heliocentric Heliosynchronous

Parameters

Classical	$i\,\!$ Inclination $\Omega\,\!$ Longitude of the ascending node $e\,\!$ Eccentricity $\omega\,\!$ Argument of periapsis $a\,\!$ Semi-major axis $M_o\,\!$ Mean anomaly at epoch

Other	$\nu\,\!$ True anomaly $b\,\!$ Semi-minor axis $\epsilon\,\!$ Linear eccentricity $E\,\!$ Eccentric anomaly $L\,\!$ Mean longitude $l\,\!$ True longitude $T\,\!$ Orbital period

Maneuvers

Bi-elliptic transfer Delta-v budget Geostationary transfer Gravity assist Gravity turn Hohmann transfer Low energy transfer Oberth effect Inclination change Phasing Rendezvous Transposition, docking, and extraction Collision avoidance (spacecraft)

Other orbital mechanics topics

Apsis Celestial coordinate system Characteristic energy Direct motion Epoch Ephemeris Equatorial coordinate system Ground track Interplanetary Transport Network Kepler's laws of planetary motion Lagrangian point n-body problem Orbit equation Orbital speed Orbital state vectors Perturbation Retrograde motion Specific orbital energy Specific relative angular momentum

List of orbits