The angles which characterize the direction of the rotational pole
within the Earth are called the polar coordinates, x and y. Variation in
these coordinates is called polar motion. The polar coordinates measure
the position of the Earth's instantaneous pole of rotation in a reference
frame which is defined by the adopted locations of terrestrial
observatories. The coordinate x is measured along the 0o
(Greenwich) meridian while the coordinate y is measured along the
W meridian. These two coordinates determine the directions
on a plane onto which the polar motion is projected.
Polar motion consists largely of two motions, an annual elliptical
component and a Chandler circular component with a period of about 435
days. These two motions describe most of the spiral motion of the pole
as seen from the Earth (see the figure below).