UT1 – Universal Time 1

UT1 (Universal Time 1) is a measure of the Earth’s actual rotation angle relative to the Sun, reflecting the true length of the day. Unlike atomic time (UTC), UT1 varies slightly due to irregularities in Earth’s rotation caused by tides, core-mantle interactions, and other geophysical processes. UT1 is exclusively determined from VLBI (Very Long Baseline Interferometry) observations, which provide the most precise measurements of the Earth’s orientation in space. Accurate knowledge of UT1 is essential for navigation, satellite tracking, and astronomy, as it links civil time to the real rotation of the Earth.

More Info: https://geodesy.science/item/earth-orientation-parameter