Conventional International Origin

Conventional International Origin (CIO) is a conventionally defined reference axis of the pole's average location on the Earth's surface over the year 1900–1905.^[1]

Polar motion is the movement of Earth's rotation axis across its surface. The axis of the Earth's rotation tends, as the axis of a gyroscope, to maintain its orientation to inertial space. The Conventional International Origin is used to measure this movement.