Keplerian (Orbital) Elements

Orbital elements are the parameters required to uniquely identify a specific orbit and the position of a body along the orbit at a specific time (epoch).

Keplerian elements are the six traditional orbital elements, as described below.

Eccentricity

The eccentricity of an orbit defines the shape of the ellipse, describing how much it is elongated compared to a circle. An eccentricity of 0 means the orbit is a perfect circle.

Semi-Major Axis

The sum of the periapsis and apoapsis distances divided by two. For classic two-body orbits, the semi-major axis is the distance between the centers of the bodies, not the distance of the bodies from the center of mass.

Inclination

The inclination is the vertical tilt of the ellipse with respect to the reference plane, measured at the ascending node (a point where the orbit intersects the plane of reference) in degrees.

Longitude of The Ascending Node

Horizontally orients the ascending node of the ellipse (a point where the orbit intersects the plane of reference) with respect to the reference direction.

Argument of Periapsis

Defines the orientation of the ellipse in the orbital plane, as an angle measured from the ascending node to the periapsis (the closest point the satellite object comes to the primary object around which it orbits).

True Anomaly

The true anomaly defines the position of the orbiting body along the ellipse at a specific time (the "epoch").

Further Reading

Orbital elementsWikipedia