What is Polar Motion?
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 90oW 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).