The length of a day on Earth is getting longer. As the tidal pull of the moon tugs on our oceans and slows our rotation over deep time, the length of a day on Earth should reach 25 hours in about 200 million years.
However, it would already be 60-hours long if it wasn’t for a mysterious pause that ended about 600 million years ago, according to a new paper published in the journal Science Advances.
There’s the small matter of Earth currently spinning faster than it should be, but over deep time days are getting longer as the moon gradually drifts away at 3.8 cm per year. The length of a day on Earth is getting longer each century by around 1.7 milliseconds.
The research shows that Earth rotated at a steady rate for 1.4 billion years, with the resulting length of a day remaining unchanged at 19.5 hours. When the moon first formed 4.5 billion years ago—and was much closer to Earth—a day on Earth lasted about 10 hours.
The cause of the pause, say the scientists, was a natural resonance between the effect of the moon—the most important factor in determining Earth’s rotational rate—and the tidal influence of the sun, which speeds-up Earth’s spin. These thermal tides in Earth’s atmosphere are driven by sunlight.
While the moon tugs on our oceans, causing bulges—tides—that creates friction that slows us down, “sunlight also produces an atmospheric tide with the same type of bulges,” said Norman Murray, a theoretical astrophysicist with University of Toronto’s Canadian Institute for Theoretical Astrophysics (CITA). “The sun’s gravity pulls on these atmospheric bulges, producing a torque on the Earth. But instead of slowing down Earth’s rotation like the moon, it speeds it up.”
The natural resonance the researchers found billions of years ago was caused, they think, by the planet’s warmer atmosphere amplifying the sun’s atmospheric tide.
“It’s like pushing a child on a swing,” said Murray. “If your push and the period of the swing are out of sync, it’s not going to go very high. But, if they’re in sync and you’re pushing just as the swing stops at one end of its travel, the push will add to the momentum of the swing and it will go further and higher.”
A warming of Earth’s temperature—as is happening right now—will move Earth farther from its atmospheric resonance, meaning increased imbalance. “As we increase Earth’s temperature with global warming, we’re also making the resonant frequency move higher—we’re moving our atmosphere farther away from resonance,” said Murray. “As a result, there’s less torque from the sun and therefore, the length the day is going to get longer, sooner than it would otherwise.”
Wishing you clear skies and wide eyes.
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