You are here: Home USNO Earth Orientation EO Information Center Frequently Asked Questions What is Polar Motion?

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).

pole.png

 

USNO Master Clock Time
Javascript must be Enabled