How the day has 24 hours

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The moon constantly slows down the rotation of the earth – but the sun counteracts this. According to a recent study, this interaction is responsible for the constant length of the day. But global warming could also influence it.

The tidal mountains of the oceans caused by the moon act like brake shoes and slow down the Earth’s rotation. If the rotation of our planet depended solely on the Earth’s satellite, a day would now have to be 60 hours long. However, the sun has brought the slowing down of the Earth’s rotation to a standstill for around one and a half billion years, thus ensuring the current day length of 24 hours. This is the conclusion reached by a research team from Canada and France based on geological studies of tidal deposits and with the help of climate models. However, the scientists conclude in the journal “Science Advances” that global warming could intensify the slowdown in the future.

Days used to be significantly shorter
The young Earth rotated considerably faster 4.5 billion years ago than it does today. Back then, a day was significantly shorter than ten hours. At that time, the newly formed moon orbited the Earth in a much narrower orbit and the tides were therefore much stronger than they are today. As the tidal mountains acted as a brake, the Earth’s rotation slowed steadily – until around two billion years ago. As the studies by Norman Murray from the University of Toronto in Canada and his team show, this process came to a standstill at that time – the length of the day remained constant at around 19.5 hours for 1.4 billion years. Only then did it continue to increase until today.

Atmospheric tides
With the help of climate models, such as those used to predict current global warming, the researchers have now discovered the cause of the standstill. “The sun’s radiation also causes tides in the Earth’s atmosphere,” explains Murray. In contrast to the lunar tides, these atmospheric tides accelerate the Earth’s rotation, but are considerably smaller in comparison and therefore usually of little significance. But not always.

This is because the Earth’s atmosphere can vibrate like a bell. The oscillation depends on the temperature of the atmosphere. Two billion years ago, the atmosphere was warmer than it is today – and a “resonance” occurred: the oscillation of the atmosphere suddenly coincided with the rotation period – and thus also with the tides caused by solar radiation. The resonance caused the solar tides to build up and their influence on the Earth’s rotation became so strong that it compensated for the deceleration caused by the moon.

Murray compares the phenomenon to a child’s swing: “If you give the child a push regardless of the movement of the swing, the swing doesn’t go very high. However, if you push in the same rhythm as the swing, i.e. in resonance, the swing moves higher and higher. Similarly, atmospheric resonance has rocked the tides of the sun.”

Warming extends days
The study by Murray and Co. not only shows why the day on Earth is now 24 hours long. It also offers a glimpse of the Earth’s future. The oscillation of the Earth’s atmosphere today lasts 22.8 hours – so although it does not resonate with the length of the day, it is not too far off either.

“However, if the temperature of the atmosphere continues to increase due to global warming, this difference will become greater,” says Murray. “This further reduces the influence of the sun on the Earth’s rotation – and the length of the day increases faster than without the warming.” However, the development is not alarming: the length of the day is currently decreasing by 1.7 thousandths of a second per century – even a significantly greater decrease would be insignificant in human timescales.