Most of us wish we had more than 24 hours in a day to get everything done and really breathe. What if each day gives us more than double that time? If it hadn’t been for a phenomenon that put the lengthening of Earth’s days on hold billions of years ago, it probably would have happened.
Earth has not always had 24-hour days. There were less than 10 hours in a day when the moon first formed about 4.5 billion years ago, but they’ve gotten longer as lunar tidal forces gradually slowed Earth’s rotation. But there was a long period when the days did not grow at all. Astrophysicists have now discovered that days from 2 billion to 600 million years ago lasted about 19.5 hours because different tidal forces cancel each other out and keep the Earth spinning at the same speed for over a billion years. If that had never happened, our current time would be over 65 hours.
“The fact that the day lasts 24 hours … is not a coincidence,” the research team said in a study recently published in Science Advances.
Give it a spin
So how do tidal forces from the sun and moon affect Earth’s rotation? Tidal forces on the moon are generated by the gravitational pull of the moon. This is why the side of our planet closest to the moon and the side farthest will bulge out and the oceans will experience flooding (bulges affect the land but are not visible to the naked eye) . The gravity of the moon pulls on these bulges and so they resist the rotation of the earth. The locations 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 couple is the solar tidal couple, and it works similarly to that of the moon, causing very small changes in the ocean’s tides, so it slows down the Earth’s rotation.
The second type is the thermal tidal couple. As sunlight heats the atmosphere, it causes it to expand, creating another handle for the sun’s gravity to interact with. This influence pushes the Earth to spin faster. Although the Sun’s gravity is stronger, our star is 390 times farther from Earth than the Moon, so lunar tides generate twice as much force. As a result, the days are getting a little longer.
A period of stagnation
Two billion years ago, all that changed. Earth’s atmosphere was warmer. This affected the thermal waves that sunlight created in the atmosphere, with higher temperatures meaning higher wave speeds. The frequency at which those waves travel through the atmosphere created an atmospheric resonance, accentuating their effect. Over a billion-year period, that resonance and the length of the day would remain in sync, with atmospheric waves resonating every time the Earth completed about half a revolution.
Because Earth’s rotational period was almost exactly double its resonance period, the atmospheric tides caused by the sun grew stronger, giving the sun’s gravity more mass to work with. The result was a force roughly counteracting that of the lunar tides. The Earth ultimately did not move slower or faster. Days wouldn’t get longer until 600 million years ago – a billion years after the resonance began.
The team that conducted the study confirmed the result of their computer models by examining geological evidence of ebb and flow from extremely ancient rock formations. “The long duration and relatively recent occurrence of this resonance 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 push the resonance even more out of sync with rotation and lengthen days? It’s happening now. The more resonance and rotation are more in sync, the less solar tidal forces are able to counteract the lunar tidal forces that slowly lengthen Earth’s days over eons. Maybe we could all use a few extra hours a day, but not at the expense of our planet.
Science Advances, 2023. DOI: 10.1126/sciadv.add2499 (About DOIs).
Elizabeth Rayne is a creature that writes. Her work has appeared on SYFY WIRE, Space.com, Live Science, Grunge, Den of Geek, and Forbidden Futures. When she’s not writing, she’s altering, drawing, or cosplaying as a character she’s never heard of before. Follow her on Twitter @quothravenrayne.