The year 2021 is projected to be the shortest in decades . The last time an average day was less than 86,400 seconds in a full year was in 1937.
By calculations , an average day in 2021 will be 0.05 milliseconds less than 86,400 seconds. Throughout the year, the atomic clocks will have accumulated a lag of about 19 milliseconds.
The length of an average solar day can vary in milliseconds (1 millisecond equals 0.001 seconds). But the Earth’s rotational speed is constantly changing due to the complex motion of its molten core, oceans, and atmosphere, as well as other effects. Official measurements of the length of the day are carried out by the International Earth Rotation and Reference Systems Service (IERS).
To determine the actual length of a day, IERS scientists determine the exact speed of Earth’s rotation by measuring the precise times a fixed star passes a certain location in the sky each day. This measurement is expressed as Universal Time (UT1), a type of solar time.
Before 2020 began, for example, the shortest day since 1973 was July 5, 2005 , when the Earth’s rotation took 1.0516 milliseconds less than 86,400 seconds.
Yet in mid-2020, Earth broke that record no less than 28 times . The shortest day of all came on July 19, when the Earth completed its rotation in 1,4602 milliseconds less than 86,400 seconds. 2021, if the calculations are confirmed, will break this record.