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Ocean tides caused by the Moon act like brake pads, slowing Earth’s rotation. If our planet’s rotation depended solely on Earth’s satellite, a day would have to be 60 hours long by now. But the sun has slowed Earth’s rotation to a halt for about a billion years, ensuring today’s 24-hour day length. Global warming, however, could intensify the slowdown in the future.
Researchers from Canada and France came to this conclusion through geological studies of tidal deposits and with the help of climate models. The analysis was published in the journal Science Advances.
The young Earth turned 4.5 billion years ago, considerably faster than today. At that time, a day was significantly shorter than ten hours. At that time, the freshly formed Moon still orbited the Earth on a much narrower path, and accordingly, the tides were much more substantial than today. With the tidal mountains acting as a brake, the Earth’s rotation slowed steadily – until about two billion years ago. As the investigations of Norman Murray of the University of Toronto in Canada and his colleagues show, this process came to a standstill at that time – the day length remained constant for 1.4 billion years at about 19.5 hours. 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 now tracked down the cause of the standstill. “The sun’s radiation additionally causes tides in the Earth’s atmospheric envelope,” Murray explains. Unlike the lunar tides, these atmospheric tides accelerate the Earth’s rotation, but they are considerably smaller in comparison and, therefore, mainly of no great significance; however, as the researchers show, they are not constant.
This is because the Earth’s air envelope can oscillate 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 air envelope was suddenly by the rotation period – and thus also with the tides caused by the solar radiation. The resonance caused the solar tides to swing up, and their influence on the Earth’s rotation became so strong that it offset the deceleration caused by the Moon.
Murray compares the phenomenon to a child’s swing: “If one gives the child a push regardless of the swing’s motion, the swing does not come up very high. However, if one pushes in the same rhythm as the swing, i.e., in resonance, the swing moves higher and higher. In much the same way, atmospheric resonance has rocked up the tides of the sun.”
However, the study by Murray and his colleagues shows why today’s day on Earth is 24 hours long. It also offers a glimpse of the Earth’s future. The oscillation of Earth’s atmosphere today lasts 22.8 hours – so while it doesn’t resonate with the length of the day, it’s not too far off either. “However, if global warming continues to increase the temperature of the atmosphere, that difference will increase,” Murray says. “As a result, the sun’s influence on Earth’s rotation continues to decrease – and day length increases faster than it would without the warming.” The trend is not worrisome, however: day length is increasing by 1.7 thousandths of a second per century – even a much more significant increase would be insignificant when viewed in human timescales.
( S E R V I C E – www.science.org/doi/10.1126/sciadv.add2499 )/picture: Image by Sergio Cerrato – Italia from Pixabay
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