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# Longitude of the periapsis

In celestial mechanics, the longitude of the periapsis, also called longitude of the pericenter, 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 orbit inclination were zero. 

## Apsis

An apsis (ἁψίς; plural apsides, Greek: ἁψῖδες) is an extreme point in the orbit of an object.

## Argument of periapsis

The argument of periapsis (also called argument of perifocus or argument of pericenter), symbolized as ω, is one of the orbital elements of an orbiting body.

## Celestial mechanics

Celestial mechanics is the branch of astronomy that deals with the motions of celestial objects.

## Longitude

Longitude, is a geographic coordinate that specifies the east-west position of a point on the Earth's surface.

## Longitude of the ascending node

The longitude of the ascending node (☊ or Ω) is one of the orbital elements used to specify the orbit of an object in space.

## Mean longitude

Mean longitude is the ecliptic longitude at which an orbiting body could be found if its orbit were circular and free of perturbations.

## Orbit

In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet.

## Orbital inclination

Orbital inclination measures the tilt of an object's orbit around a celestial body.

## Orbital state vectors

In astrodynamics and celestial dynamics, the orbital state vectors (sometimes state vectors) of an orbit are cartesian vectors of position (\mathbf) and velocity (\mathbf) that together with their time (epoch) (t\) uniquely determine the trajectory of the orbiting body in space.

## Plane of reference

In celestial mechanics, the plane of reference (or reference plane) is the plane used to define orbital elements (positions).

## True longitude

In celestial mechanics true longitude is the ecliptic longitude at which an orbiting body could actually be found if its inclination were zero.

## References

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