This is good news for some, bad news for others: the typical day on Earth will not remain 24 hours forever. Indeed, scientists have noted that the rotation of our planet is slowing down and that an Earth day could one day last 25 hours. This process is so slow that it is imperceptible on the scale of a human life, you are not likely to work another hour right away.
This slowdown is mainly explained by the gravitational interaction between the Earth and the Moon. The same forces that cause tides also act as a tiny brake on Earth’s rotation, lengthening the length of days extremely gradually, summarizes an article in Ecoticias.
As you probably know, the length of 24 hours is actually a solar day, based on the position of the Sun in the sky. But if we measure the rotation of the Earth relative to distant stars, we obtain a sidereal day, slightly shorter by about 23 hours 56 minutes 4 seconds. The solar day is about 4 minutes longer because the Earth continues to revolve around the Sun as it rotates.
The Moon as a natural brake
But it doesn’t stop there, that would be too simple: even the 24-hour solar day is not perfectly constant. It varies slightly and, over very long periods, tends to lengthen. This phenomenon, although minimal, can be measured with extremely precise modern instruments.
The main cause of this slowdown is the effect of the tides. Under the influence of lunar gravity, the oceans form bulges that are not perfectly aligned with the Moon. Due to friction with the seabed, this asymmetry dissipates energy and slows down the rotation of the Earth, while gradually moving the Moon further away.
To understand better, we can imagine a rotating office chair that we brake slightly with our foot: it continues to rotate, but gradually slows down. We are experiencing a similar process, but on a planetary scale and over millions of years.
Scientists are able to detect this tiny slowdown using very precise atomic clocks, combined with astronomical observations and the study of historical records, such as ancient eclipses. To maintain consistency between official time and the Earth’s actual rotation, leap seconds are sometimes added to clocks.
So when will we reach 25 hour days? The most current estimates suggest a time frame of around 200 million years, provided that the Earth-Moon system continues to evolve in the same way.
