August 12, 2022 Shortest day in modern times 08/12/2022 As noted in social media and the press (for example, here), the Earth recently experienced its shortest day in modern history. The length of day (LOD) of June 29, 2022 was recorded with 1.59 ms shorter than a standard 24-hour day (86,400 SI seconds). While this is a record in the era of atomic clocks (since the 1960s), it is nothing out of the extraordinary. LOD fluctuates over time and there were local minima in recent years as well. What is surprising is the fact that the Earth was speeding up a bit. In general terms, the Earth is slowing down mostly due to tidal friction caused by the moon: tidal bulges slow down the rotation of the Earth. For that, leap seconds are inserted to account for the slowed rotation. The currently observed increased rotational speed may be attributed to different geophysical phenomena, by themselves or in combination, including post-glacial rebound effects following the melting of ice masses particularly in the polar regions (including both recent ice mass loss as well as the long-term loss since the Pleistocene), climatological impacts on global hydrology due to recent consecutive La Nina events and changes in the parameters of the Earth's Chandler wobble (see articles here and here). On geological time scales (millions of years), however, the Earth has seen much shorter days (minutes to hours). Also, pre-atomic clock era LOD derivations using telescopic data from lunar occultations of stars point at around 2 ms shorter days in the 1870s, though with a much larger uncertainty. Hence, June 29, 2022 was not the shortest day ever in the history of the Earth but the shortest day in recent modern times. Monitoring these changes in the Earth's rotation is a critical function of the NASA Space Geodesy Project and its international partners as the changes are often unpredictable and provide key information about the geophysical changes affecting the Earth system (solid Earth, hydrosphere and atmosphere) and support precise positioning, satellite navigation, and the definition and maintenance of the Terrestrial Reference Frame. The time history of LOD from space geodesy measurements since 1975 are available from the IERS here.
