celestial mechanics Celestial mechanics is the branch of astronomy that deals with the motions of objects in outer space. Historically, celestial mechanics applies principles of physics (classical mechanics) to astronomical objects, such as stars and planets, to ...

true longitude is the

ecliptic longitude The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System b ...

at which an orbiting body could actually be found if its

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

were zero. Together with the inclination and the

ascending node An orbital node is either of the two points where an orbit intersects a plane of reference to which it is inclined. A non-inclined orbit, which is contained in the reference plane, has no nodes. Planes of reference Common planes of refere ...

, the true longitude can tell us the precise direction from the central object at which the body would be located at a particular time.

Calculation

The true longitude can be calculated as follows: : where: * is the orbit's

true anomaly In celestial mechanics, true anomaly is an angular parameter that defines the position of a body moving along a Keplerian orbit. It is the angle between the direction of periapsis and the current position of the body, as seen from the main foc ...

, * is the longitude of orbit's periapsis, ** is the argument of periapsis, and ** is the longitude of the orbit's ascending node,

References

Orbits {{Astronomy-stub