In astrodynamics, the longitude of the periapsis (symbolized $\backslash varpi$) 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 may be used to refer to ω, the angle between the ascending node and the periapsis; this angle is less ambiguously known as the argument of periapsis.

Calculation from state vectors

$\backslash varpi$ can be calculated from longitude of ascending node $\backslash Omega$ and argument of periapsis $\backslash omega$:
$\backslash varpi\; =\; \backslash Omega\; +\; \backslash omega$
which are derived from orbital state vectors.