It is not a mystery for anyone, the earth takes a tour of itself in twenty-four hours. What we know less is that this is not always the case: our planet does not run all the time at the same speed, with variations and a “peak” in summer. In 2025, this phenomenon occurred on July 9, the day when the earth made a complete turn on itself in about 1.3 millisecond less than twenty-four hours. This slight change should reproduce on July 22 and August 5.
If the phenomenon can surprise, “it is not extraordinary and permanently performs,” comments Christian Bizouard, astronomer and research director at the time space (LTE) laboratory of the Paris Observatory. “Every year there are ups and downs in the speed of rotation of the earth, and the maximums occur during the summer,” said the specialist. This year, the shortest day should be on August 5, which will last approximately 1.5 millisecond less than the 86,400 seconds-twenty-four hours-of the reference atomic time.
This variation, “too small to have an effect”, will be less important than that recorded on July 5, 2024, where the earth has put 1.66 millisecond less to take a tour of itself, breaking a seventy-year-old record.
The effect of winds and tides
This maximum speed of the earth in summer is explained by several phenomena, which combine. The first is a set of atmospheric factors, the main one being the effect of the winds. “There is a modulation in the force of the winds which produces this effect on the rotation of the earth, it is an effect that we understand very well,” explains Christian Bizouard.
These variations are accentuated by the effect of the tides, which depends on the elevation of the moon compared to the plane of the equator. “Whatever the position of the moon, it produces a deformation of our planet,” explains the astronomer. But this deformation can be more or less distant from the axis of rotation of the earth “, which influences his speed in the manner of” a skater who turns on himself slower when he spreads his arms and faster when he brings them back along his body “.
Thus, “when the moon is a little below the plane of the equator, the deformation it causes is closer to the axis of rotation of the earth, therefore its speed of rotation increases”. If this tidal effect occurs all year round every thirteen days, it is combined in summer at a minimum speed of rotation linked to atmospheric factors, leading to the shortest days.
A historically changing speed of rotation
These occasional variations are part of a long history of changes in the speed of rotation of the earth. For example, “the duration of the day seems to have gone from 6 milliseconds less than twenty-four hours in 1660 to approximately 4 seconds in 1910,” said the naval observatory of the United States in 2022. Much stronger differences “there are seventy million years”, where dinosaurs lived “23 hours 30 days”. Even further, “fossilized corals of 430 million years ago indicate that the days […] lasted around 9 pm, “said the American service.
More recently, “the duration of the day has increased by 60 milliseconds on average since 2000 before Jesus Christ”, a progressive increase of 2 milliseconds per century, notes Christian Bizouard. This acceleration of the speed of rotation of the blue planet is explained “by the friction caused by the tides, which make the earth gradually lose its energy”, develops the specialist.
The earth turns faster and faster
This very long -term slowdown does not prevent acceleration over several decades. We thus note a tendency to increase the speed of rotation of the earth since the 1970s. This phenomenon, considered to be “remarkable” by the director of the LTE, is particularly visible in the last five years, with a duration of the day which “sometimes records less than twenty-four hours”, according to the American naval observatory.
Our file on astronomy
Variations that scientists “cannot explain” at present, specifies Christian Bizouard, who evokes “the great mystery of the rotation of the earth”. What is certain is that the days never really make twenty-four hours … especially since “such days do not exist in nature and are a human invention”, the rotation of the earth in relation to the sun being variable and presenting irregularities of the order of the second. A very small second which has great importance.
