The tidal mountains of the seas caused by the moon act like brake shoes and slow down the rotation of the earth. If the rotation of our planet depended solely on the Earth’s satellite, a day would now have to be 60 hours long. But the sun halted the slowing of the Earth’s rotation for about a billion and a half years, giving us today’s 24-hour day length.
This is the conclusion reached by researchers from Canada and France through geological studies of tidal deposits and with the help of climate models. However, the scientists in the journal “Science Advances” conclude that global warming could intensify the slowdown in the future.
Day length once at around 19.5 hours
The young earth spun much faster 4.5 billion years ago than it does today. A day was then significantly shorter than ten hours. At that time, the newly formed moon was still orbiting the earth in a much narrower orbit and the tides were correspondingly much stronger than today. Since 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 from the University of Toronto in Canada and his colleagues show, this process came to a standstill – the day length remained constant at around 19.5 hours for 1.4 billion years. Only then did it continue to increase until today.
With the help of climate models, such as those used to predict current global warming, the researchers have now tracked down the cause of the standstill. “The radiation from the sun 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 to these and are therefore usually of little importance. However, as the researchers show, not always.
Phenomenon like a children’s swing
Because the atmosphere of the earth 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 period of rotation – and thus also with the tides caused by solar radiation. Due to the resonance, the solar tides increased and their influence on the earth’s rotation became so strong that it compensated for the deceleration by the moon.
Murray compares the phenomenon to a child’s swing: “If you give the child a push independent of the movement of the swing, the swing will not get very high. However, if you push in the same rhythm as the swing, i.e. in resonance, the swing will move higher and higher. Similarly, atmospheric resonance has rocked the Sun’s tides.”
Day length is currently decreasing minimally
The study by Murray and his colleagues not only shows why the day on earth is 24 hours long today. It also offers a glimpse into the future of the earth. The oscillation of the earth’s atmosphere lasts 22.8 hours today – so it is not in resonance with the length of the day, but not too far off either.
“However, as global warming continues to increase the temperature of the atmosphere, this difference will increase,” says Murray. “As a result, the influence of the sun 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 worrying: 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.
