UT1

UT1, or Universal Time 1, is a time standard that tracks the rotation of the Earth relative to the distant stars. It is a non-uniform time scale derived from astronomical observations and represents the angle of the Earth's rotation with respect to the International Celestial Reference Frame. UT1 changes as the Earth spins unevenly due to geophysical processes such as winds, tides, and core-mantle interactions, causing the length of the day to vary by milliseconds.

UT1 is not realized by a single clock but by a convention that links the observed rotation to a time scale. The difference between UT1 and Coordinated Universal Time (UTC) is expressed as DUT1. By convention, UTC is maintained to stay within 0.9 seconds of UT1 by adding leap seconds when necessary. This ensures civil time remains close to the Earth's rotation.

UT1 is realized from measurements of Earth's rotation obtained through very long baseline interferometry (VLBI), satellite laser ranging, and other geodetic techniques, along with monitoring of polar motion. Observatories and the International Earth Rotation and Reference Systems Service (IERS) publish UT1-UTC and DUT1 values daily, enabling users to convert between UT1 and UTC.

UT1 is used in astronomy to provide a reference for transforming between celestial coordinates and terrestrial coordinates, for ephemerides, and for celestial navigation. It is also used in spacecraft tracking and satellite orbit determinations when precise Earth orientation is required.

Historically, UT1 evolved from earlier forms of universal time tied to the rotation of the Earth. It remains defined in practice by the rotation of the Earth and the IERS conventions.