Orbital elements

Summary

Orbital elements are six numbers that fix a Kepler orbit and the position of the body on it. Two give the size and shape of the conic and three give its orientation in space. The last one gives the position along the orbit at a reference time. They replace the position and velocity vectors by quantities that are constant for an unperturbed orbit. This makes them the natural coordinates for describing slow changes caused by perturbations. See the definition.