The Gravitational Interactions That Helped Us Dodge 60-Hour Days

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Most of us wish we had more than 24 hours a day to do everything and breathe. What if each day gave us more than double that time? If it hadn't been for a phenomenon that interrupted the lengthening of days on Earth billions of years ago, this probably would have happened.

Earth hasn't always had 24-hour days. There were less than 10 hours in a day when the Moon appeared about 4.5 billion years ago, but they got longer as lunar tidal forces gradually slowed Earth's rotation. But there was a long period when the days did not increase at all. Astrophysicists have now discovered that from 2 billion to 600 million years ago, days lasted around 19.5 hours because several tidal forces canceled each other out and kept the Earth spinning at the same speed for over a billion. of years. If this had never happened, our current days could last more than 65 hours.

"The fact that the day lasts 24 hours...is no coincidence," the research team said in a study recently published in Science Advances.

So how do the tidal forces of the Sun and Moon affect the Earth's rotation? Lunar tidal forces are generated by the gravitational pull of the Moon. This is why the face of our planet closest to the Moon and the farthest face will bulge and the oceans will experience a high tide (the bulges affect the land but are imperceptible to the naked eye). The Moon's gravity pulls on these bulges and they resist the Earth's rotation. The sites of these bulges change as the Earth rotates, creating friction that also slows that rotation.

There are two types of solar tides that produce torque, a twisting force that affects rotation. The first type of solar torque is the solar tidal torque, and it works similarly to that of the Moon, causing very small changes in the ocean tides, so it slows the Earth's rotation.

The second type is the thermal tidal couple. When sunlight heats the atmosphere, it causes it to expand, creating another handful that the Sun's gravity can interact with. This influence causes the Earth to spin faster. Although the Sun's gravity is stronger, our star is 390 times farther from Earth than the Moon, so the lunar tides generate twice as much force. As a result, the days continue to lengthen slightly.

Two billion years ago, everything changed. Earth's atmosphere was warmer. This affected heat waves created by sunlight in the atmosphere, with higher temperatures meaning higher wave speeds. The frequency at which these waves pass through the atmosphere creates atmospheric resonance, accentuating their effect. For a billion years, this resonance and the length of day would stay in sync, with atmospheric waves resonating every time the Earth made about half a rotation.

Because the Earth's rotation period was almost exactly twice that of the resonance period, the atmospheric tides caused by the Sun became stronger, giving the Sun's gravity more mass to work with. The result was a torque that roughly countered that of the lunar tides. The Earth ended up moving neither slower nor faster. The days wouldn't get longer until 600 million years ago, a billion years after resonance began.

The study team confirmed the result of their computer models by examining geological evidence of high and low tides from extremely ancient rock formations. "The long duration and relatively recent occurrence of this resonant state may be responsible for the fact that the day currently lasts 24 hours," the astrophysicists also said in the study.

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