In fact, the Moon slows the planet’s rotation by pulling on Earth’s oceans, creating tidal bulges on opposite sides of the planet, which we experience as high and low tides. Thus, the gravitational pull of the Moon, combined with the friction caused between the tides and the ocean floor, acts as a kind of brake on our rotating planet.
However, the Moon is not the only agent involved in the speed of rotation of the Earth and, therefore, in the length of its days. “The Sun also produces an atmospheric tide with the same type of bulges,” says Norman Murray, theoretical astrophysicist at the Canadian Institute for Theoretical Astrophysics, lead author of a recently published paper in the journal Science Advances titled Why the day lasts 24 hours: the story of Earth’s atmospheric thermal tide, composition and average temperature . “The Sun’s gravity pulls on these atmospheric bulges, producing torque on Earth. But instead of slowing the Earth’s rotation like the Moon, it speeds it up,” he adds.
TEMPERATURE AND ROTATION: RESET EARTH CLOCK
For most of the geological history of our planet, lunar tides passed the solar tides by a factor of ten, causing the Earth’s rotation to slow down and the days to lengthen.
The study by Murray and colleagues, however, estimates that around 2 billion to 600 million years ago, an atmospheric tide driven by the Sun counteracted the effect of the Moon, keeping the Earth’s rotation rate constant. and day length constant at 19.5 hours. .
“About 2 billion years ago, atmospheric bulges were larger because the atmosphere was warmer and because its natural resonance, i.e. the frequency at which waves pass through it, coincided with the length of the day”, explain the scientists. “The atmosphere, like a bell, rings at a frequency determined by many factors, including temperature.”
In other words, the waves – like those generated by the massive eruption of the Krakatoa volcano in Indonesia in 1883 – travel through the Earth at a speed determined by its temperature. It is the same principle that explains the fact that a bell always produces the same note if its temperature is constant.
But during this 1,000-year study period, Earth’s atmosphere was warmer and resonated with a period of about 10 hours. Moreover, at that time, the rotation of the Earth, slowed down by the Moon, reached 20 hours.
When Atmospheric resonance and day length became even factors (10 and 20), the atmospheric tide strengthened, the bulges became larger, and the pull of the solar tides became strong enough to counter the lunar tides. Murray explains the phenomenon with a metaphor: “It’s like pushing a child on a swing – if your push and the timing of the swing aren’t in sync, the child won’t get very far. But if they’re in sync and the push is given the moment the swing stops at one end of its path, this push will increase the inertia of the swing itself, going higher and higher. with atmospheric resonance and tides on Earth”.
CAN CLIMATE CHANGE AFFECT THE LENGTH OF DAYS?
Along with the geological evidence, Murray and his colleagues compared their results with global atmospheric circulation models (GCM) to predict the temperature of the atmosphere during this period. GCMs are the same models that climatologists use to study global warming.
Scientists have also found a possible link between the length of days and the climatic history of our planet. As atmospheric resonance changes with temperature, Murray points out that the current warming of our atmosphere could have consequences on this tidal imbalance and therefore on the speed of rotation of the Earth.
“As we increase the Earth’s temperature with global warming, we also increase the resonant frequency,” says Murray. “As a result, it is possible that over the centuries, the days tend to get longer,” he concludes.
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