Polar motion

Polar motion is the movement of Earth’s rotation axis relative to its crust. Its changing position reflects shifts in the planet’s mass distribution and angular momentum.

Earth's rotation pole: The point where Earth’s instantaneous rotation axis intersects its surface. Polar motion tracks this point as it moves across Earth’s crust.
Earth's rotation axis: The imaginary line around which Earth rotates. Polar motion describes changes in this axis’s orientation relative to the solid Earth.
Earth orientation parameters: Measured quantities describing Earth’s rotation and orientation relative to terrestrial and celestial reference systems. Polar motion is one of the parameters used to specify Earth’s orientation.
Length of day: The duration of one rotation of Earth relative to the Sun or distant space. Angular-momentum changes that accompany polar motion can also alter the length of day.
International Latitude Service: An international observing network established in 1899 to measure changes in latitude caused by polar motion. Its coordinated measurements established a long record of the moving pole.
Chandler wobble: A roughly 433-day free oscillation of Earth’s rotation pole. This natural wobble is a major periodic component of polar motion.
Terrestrial reference frame: A coordinate system fixed to Earth’s surface and used to locate positions on the planet. The rotation pole’s movement is defined relative to a frame fixed to Earth.
International Earth Rotation and Reference Systems Service: An international organization that provides data and standards for Earth orientation and reference systems. It monitors and publishes measurements of the changing rotation pole.
Earth's nutation: Small periodic variations in the orientation of Earth’s rotation axis in space. Nutation concerns axis orientation in space, while polar motion concerns its position relative to the crust.
Polar motion time series: A chronological record of measured positions of Earth’s rotation pole relative to its crust. The series displays the combined Chandler, annual, and irregular components of polar motion.

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