Geometric precession

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Geometric precession is the term used to describe the Earth's changing orientation to inertial space not caused by local forces.

Local forces, principally the gravity of the Sun and moon acting upon the oblate Earth, are thought to be the principal factors reorienting the Earth’s axis relative to the Sun, moon and objects within the solar system (gyroscopic precession). At the same time the solar system is orbiting the galactic center with a period of about 200 to 240 million years. Consequently, the Earth’s orientation relative to objects outside the galaxy, which is typically used to measure changes in the Earth’s orientation, will also be affected by this motion without any additional local force acting directly upon the Earth. This movement of the larger reference frame, a solar system that curves through space, results in a geometric reorientation of the Earth relative to inertial space as measured from Earth. It is a component in the total amount of observable precession, but not included in the calculation of local dynamics.

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