Earth’s current day-night cycle should be 60 hours instead of 24, but something prevented it

When the Moon was formed, about 4.5 billion years ago, the day-night cycle on Earth lasted less than 10 hours instead of the current 24. But ever since, the Moon’s gravitational pull on Earth has slowed our planet’s rotation, resulting in increasingly longer days. Today, it continues to lengthen, albeit at a rate of just 1.7 thousandths of a second every century.

However, the slowing of the Earth’s rotation should have gone much further in the past. Calculations indicate that, without anything to prevent it, the rotation period would have lengthened to the point that today the Earth’s day-night cycle should be no less than 60 hours instead of 24.

Astrophysicists have discovered a phenomenon that bucked that trend for a long period of Earth’s history, and that explains why the day-night cycle is currently just 24 hours.

The team of Hanbo Wu and Norman Murray, from the Canadian Institute for Theoretical Astrophysics, attached to the University of Toronto in Canada, has determined that from about 2 billion years ago to 600 million years ago, an atmospheric tide driven by the Sun counteracted the effect. of the Moon, keeping the Earth’s rotation speed constant and the length of the day at 19.5 hours.

Without this 1.4 billion-year pause in the slowdown of our planet’s rotation, instead of our current 24-hour day we would have one of just over 60 hours.

The earth. (Photo: NASA JPL)

The Moon slows Earth’s rotation by pulling on the oceans, creating tidal bulges on opposite sides of the planet that we experience as high and low tides. The Moon’s gravitational pull on these bulges, plus friction between the tides and the ocean floor, acts as a brake on our planet’s rotation.

Sunlight also produces an atmospheric tide with the same type of bulges. But instead of slowing down the Earth’s rotation like the Moon does, it speeds it up.

For most of Earth’s geological history, lunar tides have exceeded solar ones by a factor of about ten; hence the decrease in the speed of rotation of the Earth and the lengthening of the days.

However, around 2 billion years ago, atmospheric bulges were larger because the atmosphere was warmer and because their natural resonance (the frequency at which these air masses or waves move through it) coincided with the duration of the day.

The atmosphere, like a bell, resonates at a frequency determined by various factors, including temperature.

For most of Earth’s history, that atmospheric resonance has been out of sync with the planet’s rate of rotation. Today, each of the two atmospheric “high tides” takes 22.8 hours to circumnavigate the globe; Because that resonance and the Earth’s 24-hour rotation period are out of sync, the atmospheric tide is relatively small.

But during the aforementioned 1.4 billion year period, the atmosphere was warmer and resonated with a period of about 10 hours. In addition, when that time arrived, the rotation of the Earth, slowed down by the Moon, reached 20 hours.

When atmospheric resonance and day length became even factors (10 and 20), the atmospheric tide strengthened, the bulges grew larger, and the tidal pull from the Sun became strong enough to counteract the lunar tide.

“It’s like pushing a kid on a swing,” says Murray. “If your push and swing period aren’t in sync, it’s not going to go very high. But if they’re in sync and you push just as the swing stops at one end of its travel, the push will increase the momentum of the swing and it will go farther.” and higher. That’s what happened with atmospheric resonance and the tide.”

The study is titled “Why the day is 24 hours long; the history of Earth’s atmospheric thermal tide, composition, and mean temperature“. And it has been published in the academic journal Science Advances. (Fountain: NCYT by Amazings)