Earth rotation is gradually slowing down – is it?

Abstract

It is an accepted fact that the speed of Earth rotation is decreasing. However, if we look at length of day time series based on the observations of modern space geodetic techniques, which cover only the last 50 years, we see an average decrease in length of day, equivalent to an acceleration of Earth rotation. This article emphasizes the importance of the time span when investigating trends. It explains why it is still valid that Earth rotation is gradually slowing down, although it has been speeding up in recent decades. Furthermore, we discuss the relations between length of day and universal time and the occurrence of leap seconds. We finally provide current results of very long baseline interferometry analyses. These show that the time determined by Earth rotation has gained 90 ms on atomic time from January 2022 to January 2023.