The gravitational interactions that have helped us dodge 60-hour days

Most of us wish we had more than 24 hours in a day to get everything done and actually breathe. What if each day gave us more than double that time? If it wasn’t for a phenomenon that put the lengthening of Earth’s days on pause billions of years ago, that would have probably happened.

Earth has not always had 24-hour days. There were fewer than 10 hours in a day when the Moon first came into being around 4.5 billion years ago, but they have grown longer as lunar tidal forces gradually slowed Earth’s rotation. But there was a long period when days didn’t grow at all. Astrophysicists have now found that, from 2 billion to 600 million years ago, days were about 19.5 hours because several tidal forces canceled each other out and kept Earth rotating at the same speed for over a billion years. If that had never happened, our present days might be over 65 hours long.

“The fact that the day is 24 hours long…is not a coincidence,” the research team said in a study recently published in Science Advances.

Giving it a spin

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

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 operates the same way as the Moon’s, causing very small changes in ocean tides, so it slows down Earth’s spin.

The second type is the thermal tidal torque. As sunlight heats the atmosphere, it causes it to expand, creating another handle that the Sun’s gravity can interact with. This influence pushes Earth to rotate faster.  Although the Sun’s gravity is more powerful, our star is 390 times further from Earth than the Moon, so lunar tides generate twice the force. As a result, days continue to grow slightly longer.

A period of stasis

Two billion years ago, that all changed. Earth’s atmosphere was warmer. This affected the thermal waves that sunlight created in the atmosphere, with higher temperatures meaning higher wave velocities. The frequency at which those waves travel through the atmosphere created an atmospheric resonance, accentuating their effect. For a stretch of a billion years, that resonance and the length of the day would stay in sync, with atmospheric waves resonating every time the Earth completed about half a rotation.

Because the rotational period of Earth was almost exactly double that of the resonance period, the atmospheric tides caused by the Sun became stronger, given the Sun’s gravity more mass to work with. The result was a torque that roughly countered the one from the lunar tides. Earth ended up moving neither slower or faster. Days would not grow longer again until 600 million years ago—a billion years after the resonance started.

The team carrying out the study confirmed the result of their computational 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 is currently 24 hours long,” the astrophysicists also said in the study.

Could rising temperatures due to global warming throw resonance even more out of sync with rotation and lengthen days? It’s happening right now. The more out of sync resonance and rotation are, the less solar tidal forces are able to counter the lunar tidal forces that have slowly extended days on Earth over eons. Maybe we could all use a few extra hours in the day, but not at the expense of our planet.

Science Advances, 2023.  DOI: 10.1126/sciadv.add2499 (About DOIs).

Elizabeth Rayne is a creature who writes. Her work has appeared on SYFY WIRE, Space.com, Live Science, Grunge, Den of Geek, and Forbidden Futures. When not writing, she is either shapeshifting, drawing, or cosplaying as a character nobody has ever heard of. Follow her on Twitter @quothravenrayne.

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