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Earth rotation is leading!

On 26 August 2023, universal time determined by Earth rotation (UT1) overtook the coordinated universal time (UTC), defined by atomic clocks. UT1 led UTC by almost seven milliseconds on 10 September 2023.

Time series of UT1-UTC from Very Long Baseline Interferometry from 18 August to 11 September 2023. The curved line crosses the zero line from minus to plus on 26 August. The values range from -5 milliseconds to +7 milliseconds.

© Vienna Center for VLBI (SBoehm)

What is unique about that?

Since 1972, UTC, the coordinated universal time based on the regular beat of atomic clocks, is adjusted to the variable Earth rotation time scale UT1 by leap seconds before the amount of the difference UT1-UTC exceeds 0.9 seconds. Since the actual length of a solar day was a few milliseconds more than 86400 seconds for many decades, the parameter UT1-UTC always drifted from positive to negative until it was lifted back into positive range by an added leap second. The last leap second was added at the end of 2016. Since mid-2020, solar days have been shorter than 86400 seconds on average. This has led to an unprecedented upward trend in UT1-UTC since the introduction of leap seconds, which continues today. The UT1-UTC difference thus crossed the zero line and entered the positive range on 26 August without a leap second. This means the "Earth clock" has been fast since then.

How do we know?

In parallel to other groups worldwide, the research unit Higher Geodesy of TU Wien, in cooperation with the Federal Office of Metrology and Surveying (BEV), regularly analyses measurement data from Very Long Baseline Interferometry (VLBI), which are provided by the International VLBI Service for Geodesy and Astrometry. From rapidly available measurement sessions, the so-called intensive sessions, quasi-daily values of UT1-UTC can be determined via highly accurate positions of very distant radio sources (fixed points in the sky) and stations on Earth. The currently available intensive sessions since 18 August 2023 were analyzed with the Vienna VLBI and Satellite Software (VieVS). The results for the parameter UT1-UTC are shown in the figure. For more information and images on our VLBI activities, please visit the Vienna Center for VLBI website, opens an external URL in a new window.

Shouldn't Earth's rotation be slowing down?

Yes, over very long periods, the Earth's rotational speed is continuously decreasing, and the length of the day is increasing, about 1.8 milliseconds per century. The rotation is slowed down primarily by the friction of the water masses of the tides sloshing back and forth on ocean floors and the continental shelves. In addition, the speed of rotation is subject to periodic and irregular fluctuations due to changes in the atmosphere and oceans and processes in the Earth's interior. Interactions between the Earth's core and mantle produce irregular decadal fluctuations, to which the increase in the Earth's rotation speed observed for the last 50 years is also attributed. A more detailed treatment of the historical development of the length of day and the leap second topic is given in issue 2/2023 of the Austrian Journal of Surveying and Geoinformation, opens a file in a new window.