We define a day as 86,400 seconds, or 24 hours – the time it takes for Earth to rotate once. However, the Earth doesn’t rotate perfectly uniformly. Usually, the Earth’s rotation is actually slowing down so that the length of the day increases by about 1.8 milliseconds per century, on average. This means that 600 million years ago a day lasted only 21 hours.
The variation in day length is due to several factors, including the tidal effects of the Moon and Sun, core-mantle coupling inside the Earth, and the overall distribution of mass on the planet. Seismic activity, glaciation, the weather, the oceans and the Earth’s magnetic field may also affect the length of the day.
In 2020 scientists made a startling discovery. They found that, instead of slowing down, the Earth has started to spin faster. It is now spinning faster than at any time in the last 50 years. In fact, the shortest 28 days on record all occurred during 2020.
As yet, scientists are not entirely sure what is causing this increase in Earth’s rotation rate, but some have suggested it could be due to the melting of glaciers during the 20th Century, or the accumulation of large quantities of water in northern hemisphere reservoirs. However, experts predict that this speeding up is a temporary effect and the Earth will start slowing down again in the future.
But, for now, should we be worried? Although it will have no effect on our daily lives, there could be serious implications for technology such as GPS satellites, smartphones, computers and communication networks, all of which rely on extremely accurate timing systems. But such problems are ultimately surmountable, perhaps simply by subtracting a leap second rather than adding one.
So no, we shouldn’t be worried – unless the shortening of the day is attributable to human activity.