Orbital Reference System -- from Eric Weisstein's World of Physics

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Orbital Reference System

A coordinate system with the origin at the attracting focus, the X-axis towards the periapsis, and the Y-axis in the direction for which the true anomaly is 90^-1. It is given by

where r is the orbital radius, v is the true anomaly, a is the semimajor axis Eric Weisstein's World of Math, e is the eccentricity, E is the eccentric anomaly, h is the specific angular momentum, and n is the mean motion.