Longitude of the periapsis
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In astrodynamics, the longitude of the periapsis (symbolized 
) of an orbiting body is the longitude (measured from the point of the vernal equinox) at which the periapsis (closest approach to the central body) would occur if the body's inclination were zero. The longitude of periapsis is a compound angle, with part of it being measured in the plane of reference and the rest being measured in the plane of the orbit. Likewise, any angle derived from the longitude of periapsis (e.g. mean longitude and true longitude) will also be compound.
[edit] Calculation from state vectors
can be calculated from longitude of ascending node Ω and argument of periapsis ω:
which are derived from orbital state vectors.
