Earth's rotation, day by day

The Earth does not spin quite evenly

A solar day lasts 86,400 seconds by definition, but the Earth's actual rotation departs from it by one to three milliseconds, and that departure changes from one day to the next. The IERS measures it daily by radio interferometry on quasars and by satellites, and publishes it in its Bulletin A as the length of day, LOD. A positive value means a day longer than the reference, a negative value a shorter day.

Tides slow the Earth over the long term, but on the scale of months and years the winds, ocean currents, melting ice and motions of the core dominate: the curve oscillates with the seasons and with the fourteen-month wobble of the axis. Since 2020 the Earth has been spinning slightly faster than before, and the shortest days ever measured date from 2024 and 2025.

When the accumulated offset approaches 0.9 seconds, a leap second is added to universal time; if the acceleration continues, one will someday have to be removed, which has never been done.