The anomalistic revolution of a planet or other heavenly body is the revolution between two consecutive passages through the pericentre.
One characteristic of astronomy which tends to make its progress slow and continuous arises out of the general fact that, except in the case of motions to or from us, which can be determined by a single observation with the spectroscope, the motion of a heavenly body can be determined only by comparing its position at two different epochs.
The position of a heavenly body is then defined by its direction and distance from the supposed observer.
Now let P be any point in space, say the centre of a heavenly body.
It is readily seen that the position of a heavenly body is completely defined when these co-ordinates are given.
