An epoch being a specific date and time. See "Mean Anomaly".
J2000
A commonly used epoch starting at January 1st 2000.
Orbital Anomalies
Auxiliary Circle
A circle drawn centered on the center of the orbit and radius equal to the semi-major axis of the orbit.
True Anomaly (ν)
The angular position of an orbiting body on its orbital path. The angle is measured from the periapsis to the central body to the orbiting body. Due to Kepler's second law it is difficult to find past or future true anomalies. The mean and eccentric anomalies can be used to compensate for this.
Mean Anomaly (M)
Unlike the true anomaly the mean anomaly is a function of time as well as position. It is required due to Kepler's second law. It increases linearly from 0 through 2π radians. Simply put it is the time since last periapsis measured as fractions of a circle.
Eccentric Anomaly (E)
The eccentric anomaly gives the angular position from the center of the orbital ellipse to the position of the orbiting body on the auxiliary circle. The eccentric anomaly is used to convert from true anomaly to mean anomaly and vice versa.
