Longitude of the periapsis
Encyclopedia
In astrodynamics
, 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. However, the angle ω is less ambiguously known as the argument of periapsis
.
can be calculated from longitude of ascending node 
and argument of periapsis
:
which are derived from orbital state vectors
.
Astrodynamics
Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to the practical problems concerning the motion of rockets and other spacecraft. The motion of these objects is usually calculated from Newton's laws of motion and Newton's law of universal gravitation. It...
, the longitude of the periapsis (symbolized ϖ) of an orbiting body is the longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
(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
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
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
Plane of reference
A term used in celestial mechanics, the plane of reference is the plane by means of which orbital elements are defined. The two main orbital elements that are measured with respect to the plane of reference are the inclination and the longitude of the ascending node.Depending on the type of body...
and the rest being measured in the plane of the orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
. Likewise, any angle derived from the longitude of periapsis (e.g. mean longitude
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...
and true longitude
True longitude
In astrodynamics true longitude is the longitude at which an orbiting body could actually be found if its inclination were zero. Together with the inclination and the ascending node, the true longitude can tell us the precise direction from the central object at which the body would be located at...
) 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. However, the angle ω is less ambiguously known as the argument of periapsis
Argument of periapsis
The argument of periapsis , symbolized as ω, is one of the orbital elements of an orbiting body...
.
Calculation from state vectors
can be calculated from longitude of ascending node and argument of periapsisArgument of periapsis
The argument of periapsis , symbolized as ω, is one of the orbital elements of an orbiting body...
:which are derived from orbital state vectors
Orbital state vectors
In astrodynamics or celestial dynamics orbital state vectors are vectors of position and velocity that together with their time uniquely determine the state of an orbiting body....
.
External links
- Determination of the Earth's Orbital Parameters Past and future longitude of perihelion for Earth.