We have known for some time now that the Earth’s days are getting much longer and the Moon is moving away from us at a rapid pace.
But there was a time when this day’s rise was put on hold. From about 2 billion years ago up to 600 million years ago, the day sat at a relatively constant 19.5 hours long.
Now, scientists have discovered the reason: The Sun exerts its own influence on the planet’s atmosphere to counteract the pulling effect of the Moon’s tidal grip.
If this doesn’t work, Earth’s day would currently be more than 60 hours long – and we should take this into account when developing models for the impact of climate change, say a team of Led by astrophysicists Hanbo Wu and Norman Murray from the University of Toronto i. Canada.
Back when the Moon first formed about 4.5 billion years ago, Earth’s day length – defined by the speed at which it spins on its axis – was much shorter. Estimates put it as low as just a a few times. Geological records show that the day has lengthened over time. This is because the Moon is gradually receding at a rate of approx 3.78 centimeters (1.49 inches) per year.
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The gravitational influence of the Moon is very controlling tides of the Earth’s oceans. As it moves around the Earth, the moon’s pull on the water creates ocean bulges on either side of the planet. The Moon’s gravity pulls on those bulges to exert a braking effect, slowing the Earth’s rotation. Scientists often compare the effect to a spinning skater stretching their arms to slow down. It adds about 1.7 milliseconds to the Earth’s day every century.
But the ocean is not the only fluid that surrounds the Earth. A gas is also a fluidand the Earth has many.
“Sunlight also produces the atmospheric tide with the same kind of bulges,” Murray explains. “The Sun’s gravity pulls on these atmospheric bulges, producing a torque on the Earth. But instead of slowing the Earth’s rotation like the Moon, it speeds it up.”
Since the influence of the Moon has been much stronger than that of the Sun for most of Earth’s history, the slowdown is in the lead. But there was the aforementioned period where the days remained constant. The team discovered this because of the atmosphere: It was warmer and, therefore, bigger.
But a property called resonance also played a role. This is when waves travel through the atmosphere at a specific height and speed to produce synchronization. The velocity of these waves is determined in part by temperature.
During this stable period in the Earth’s day length timeline, the team found through computer modeling that the temperature produced waves that synchronized with the length of the day. The day was about 20 hours; the resonance, the time it takes for a bulge to travel around the planet, was about 10 times.
The speed-up effect of this atmospheric traffic on the Earth’s rotation rate neatly counteracted the slowing-down effect of the waning Moon.
“It’s like pushing a baby on a swing,” Murray says. “If your pressure and the period of the swing are out of sync, it will not go very high. But, if they are in sync and you are pushing just as the swing stops at one of its travels, the pressure will put with the momentum of the swing, and it will go further and higher. That’s what happened with atmospheric resonance and tide.”
The research is published in Scientific Progress.