Anomaly

Anomaly, an astronomical term connected with planetary motion, possessing three distinct applications: (1) the true anomaly of a planet at any instant is the angle its radius vector has swept out during the time since its last perihelion, or position of least distance from the sun; (2) the mean anomaly is the angle the radius vector would have traversed during this time, had the planet moved with its average speed instead of varying its rate of motion at different parts of the orbit; (3) the excentric anomaly, the angle subtended at the centre of the orbit, by the corresponding arc of the auxiliary circle.