New research from the University of Toronto shows that from approximately 2 billion until 600 million years ago, an atmospheric tide driven by the Sun countered the effect of the Moon, keeping Earth’s rotational rate steady and the length of day at a constant 19.5 hours; without this billion-year pause in the slowing of our planet’s rotation, our current 24-hour day would stretch to over 60 hours.
When the Moon first formed some 4.5 billion years ago, the day was less than 10 hours long.
But since then, the lunar gravitational pull on the Earth has been slowing our planet’s rotation, resulting in an increasingly longer day.
Today, it continues to lengthen at a rate of some 1.7 milliseconds every century.
The Moon slows the planet’s rotation by pulling on Earth’s oceans, creating tidal bulges on opposite sides of the planet that we experience as high and low tides.
The gravitational pull of the Moon on those bulges, plus the friction between the tides and the ocean floor, acts like a brake on our spinning planet.
“Sunlight also produces an atmospheric tide with the same type of bulges,” said University of Toronto theoretical astrophysicist Norman Murray.
“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.”
For most of Earth’s geological history, the lunar tides have overpowered the solar tides by about a factor of ten; hence, the Earth’s slowing rotational speed and lengthening days.
But some two billion years ago, the atmospheric bulges were larger because the atmosphere was warmer and because its natural resonance — the frequency at which waves move through it — matched the length of day.
The atmosphere, like a bell, resonates at a frequency determined by various factors, including temperature. In other words, waves travel through it at a velocity determined by its temperature. The same principle explains why a bell…
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