In astronomy the word denotes the angular distance of a body from the **pericentre** of the orbit in which it is moving.

Starting from the **pericentre**, it is completed on the return to the **pericentre**.

If the **pericentre** is fixed, this is an actual revolution; but if it moves the anomalistic revolution is greater or less than a complete circumference.

From the properties of the ellipse, A is the **pericentre** or nearest point of the orbit to the centre of attraction and B the apocentre or most distant point.

To do this the actual speed in the orbit, and in a yet higher degree the angular speed around F, must be greatest at **pericentre**, and continually diminish till the apocentre is reached.

It follows that p must be greatest at **pericentre**, where its distance from F is least.

From the law of angular motion of the latter its radius vector will run ahead of PQ near A, PQ will overtake and pass it at apocentre, and the two will again coincide at **pericentre** when the revolution is completed.

A fifth element is the position of the **pericentre**, which may be expressed by its angular distance XFN from the ascending node.

The process by which the position of a planet at any time is determined from its elements may now be conceived as follows The epoch of passage through **pericentre** being given, let t be the interval of time between this epoch and that for which the position of the body is required.

The angle from the **pericentre** to the actual radius vector, and the length of the latter being found, the angular distance of the planet from the node in the plane of the orbit is found by adding to the true anomaly the distance from the node to the **pericentre**.

It arises from the ellipticity of the orbit, is zero at **pericentre** and apocentre, and reaches its greatest amount nearly midway between these points.