UT1UTC

UT1 (Universal Time 1) is a time scale based on the rotation of the Earth. It represents the mean solar time at the Greenwich meridian as realized by the planet’s actual rotation, and is used primarily in astronomy, astrometry, geodesy, and spacecraft tracking. UT1 is a physical time standard tied to Earth's rotation and can deviate in rate from atomic time scales due to irregularities in rotation and tidal effects, making it non-uniform over long periods.

UTC (Coordinated Universal Time) is the time scale maintained by highly stable atomic clocks, with occasional leap seconds added to keep UTC within a small margin of UT1. The difference between UT1 and UTC is expressed as DUT1 = UT1 − UTC and is monitored and published by the IERS (International Earth Rotation and Reference Systems Service). By design, the absolute value of this difference is kept within about 0.9 seconds.

UT1 is determined from astronomical observations, notably very-long-baseline interferometry (VLBI), which tracks Earth’s rotation and orientation relative to the International Celestial Reference Frame (ICRF). The resulting information feeds the Earth Orientation Parameters used to compute UT1 and related quantities such as GMST (Greenwich Mean Sidereal Time) and GAST (Greenwich Apparent Sidereal Time).

Practical use of UT1 lies in timing events and calculations that depend on the actual rotation of the Earth, such as astrometric measurements, satellite navigation, and certain astronomical observations. To convert between UTC and UT1, one adds the reported DUT1 value, acknowledging that UT1, as a rotation-based measure, can diverge from UTC by up to roughly a second.