Longitude of the periapsis
In
celestial mechanics, the
longitude of the periapsis (symbolized
ϖ) 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. For motion of a planet around the Sun, this position could be called
longitude of perihelion. 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.
Sometimes, the term
longitude of periapsis is used to refer to
ω, the angle between the ascending node and the periapsis. That usage
of the term is especially common in discussions of binary stars and
exoplanets.
[1] However, the angle ω is less ambiguously known as the
argument of periapsis.
Calculation from state vectors
can be calculated from
longitude of ascending node 
and
argument of periapsis 
:
which are derived from
orbital state vectors.
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