An orbital element is any one of six elements needed to specify the orbit of an object around a primary body, such as a planet around the Sun or a satellite around the Earth.

 

The semi-major axis (a) and the eccentricity (e) give the size and shape of the orbit, and the inclination (i ) of the object's orbital plane to the ecliptic, the longitude of the ascending node, and the longitude of perihelion specify the orientation of the orbit in space. A sixth quantity determines the position of the object along the orbit at any given time. This can be the time of perihelion passage (T) or the longitude at the epoch.

 

To determine the orbit of a binary star system in which the mass is not known, a seventh element, the period, must be established.

 

The mean orbital elements are the orbital elements of a reference orbit chosen to approximate a real orbit that is perturbed by the presence of another object.