Why we could have 60 hour days – but don’t

The tidal mountains of the seas created by the moon act as brake shoes and slow down the rotation of the earth. If the rotation of our planet depends only on the Earth’s satellite, a day should now be 60 hours long.

But the sun stopped the slowing down of Earth’s rotation for about a billion and a half years, which gives us today’s 24-hour day length. Researchers from Canada and France came to this conclusion through geological studies of tidal deposits and using climate models. However, the scientists in the journal Science Advances concluded that global warming could amplify the slowdown going forward.

4.5 billion years ago, the young Earth rotated much faster than it does now. A day was then significantly shorter than ten hours. At that time, the newly formed moon was still in a much narrower orbit around the Earth, and the tides were correspondingly much stronger than they are now.

Because the flood mountains act as a brake, the Earth’s rotation slowed steadily – until about two billion years ago. As the studies by Norman Murray of the University of Toronto in Canada and his colleagues show, this process came to a halt – day length remained constant at around 19.5 hours for 1.4 billion years. Only then did it continue to increase until today.

Using climate models, such as those used to predict current global warming, the researchers have now pinpointed the cause of the halt. “The radiation from the sun also causes tides in the Earth’s atmosphere,” Murray explains. Unlike lunar tides, these atmospheric tides accelerate the Earth’s rotation, but are considerably smaller by comparison and are therefore usually of little consequence. However, as the researchers showed, not always.

Because the Earth’s atmosphere can vibrate like a bubble. The oscillation depends on the temperature of the atmosphere. Two billion years ago, the atmosphere was warmer than it is now – and a “resonance” occurred: the vibration of the atmosphere suddenly coincided with the rotational period – and thus also with the tides caused by solar radiation. The resonance increased the solar tides, and their influence on the Earth’s rotation became so strong that it compensated for the lunar delay.

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

Murray and his colleagues’ research not only shows why the day on Earth today lasts 24 hours. It also offers a glimpse into Earth’s future. Earth’s atmosphere oscillation lasts 22.8 hours today – so it’s not in resonance with the length of the day, but not too far out either.

“However, as the temperature of the atmosphere continues to rise due to global warming, this difference will widen,” Murray said. “As a result, the sun’s influence on the Earth’s rotation continues to decrease – 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 larger decrease would be meaningless in human terms. (sda/dpa)