dUT1

DUT1 (Difference Between UT1 and UTC) is a key Earth Orientation Parameter that measures the difference between Universal Time 1 (UT1), which is based on Earth’s actual rotation, and Coordinated Universal Time (UTC), which is based on atomic clocks. This difference arises because Earth’s rotation is irregular and gradually slowing due to tidal forces and other factors. DUT1 is determined exclusively through Very Long Baseline Interferometry (VLBI) measurements, providing the most accurate estimates of Earth’s rotation. To keep UTC within 0.9 seconds of UT1, leap seconds are occasionally added to UTC, and DUT1 is crucial for applications requiring precise timekeeping, such as satellite navigation and astronomy.

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