An Etymological Dictionary of Astronomy and Astrophysics

Any of the six parameters 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) and give its position at any instant.

Two of them define the size and the form of the orbit: → semi-major axis (a) and → eccentricity (e).

Three angular values determine the orbit position in space: the → inclination (i) of the object’s → orbital plane to the reference plane (such as the → ecliptic), the → longitude of ascending node (Ω), and the → argument of periapsis (ω). And finally the sixth element is the → time of periapsis passage which allows calculating the body’s position along the orbit at any instant.

See also: → orbital; → element.