UT0

UT0, or Universal Time 0, is a historical time-scale in astronomy and geodesy that describes Earth's rotation as observed with a reference tied to the Earth's crust, without correcting for polar motion. It represents the apparent solar time based on the Greenwich meridian as the Earth rotates, but it can drift due to changes in the position of the rotation axis.

In modern practice, UT0 is distinguished from UT1 and UT2. UT1 is UT0 corrected for polar motion, removing the slow wobble caused by the movement of the Earth's rotation axis relative to the crust. UT1 serves as the standard coordinate time scale for describing Earth's rotation in relation to distant celestial objects. UT2 is a further refinement that includes seasonal or annual variations in Earth's rotation, accounting for systematic fluctuations primarily from atmospheric and oceanic processes; it is not commonly used in current civil timekeeping.

Historically, UT0 was used before UT1 became the preferred standard for expressing universal time. As observational methods and models improved, UT1 became the conventional reference for astronomical timing, navigation, and timekeeping, with UT0 available mainly in historical datasets or specific technical contexts.

Origin and usage of UT0 are largely of archival and educational interest today. When discussing universal time, UT1 is typically the referenced scale, while UT0 provides context for how the modern scales evolved from earlier measurements of Earth's rotation.

See also: UT1, UT2, universal time, polar motion, Greenwich meridian.