PROSTHAPHERESIS

, in Astronomy, the difference between the true and mean motion, or between the true and mean place, of a planet, or between the true and equated anomaly; called also Equation of the Orbit, or Equation of the Centre, or simply the Equation; and it is equal to the angle formed at the planet, and subtended by the excentricity of its orbit.

Thus, if S be the sun, and P the place of a planet in its orbit APB, whose centre is C. Then the mean anomaly is the [angle] ACP, and the true anomaly the [angle] ASP, the difference of which is the [angle] CPS, which is the Prosthapheresis; which is so called, because it is sometimes to be added to, and sometimes to be subtracted from the mean motion, to give the true one; as is evident from the sigure.