The tidal pull of the Moon caused the slow and steady lengthening of Earth’s day

Astrophysicists reveal why Earth’s day was a constant 19.5 hours for over a billion years.

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The evolution of the spin of Earth, quantified by the length of the day, and the evolution of the orbital angular momentum of the Moon are tightly coupled by gravitational tides, both ocean tides and (much smaller) solid body tides on Earth. A new study found a possible explanation for why the day on earth is 24 hours long.

According to a group of astrophysicists, the slow and steady lengthening of the Earth’s day brought on by the Moon’s gravitational pull has been stopped for more than a billion years.

They demonstrate that from roughly two billion years ago until 600 million years ago, a solar-driven atmospheric tide balanced the influence of the Moon and kept the Earth’s rotating rate constant, maintaining a constant day length of 19.5 hours. Our 24-hour day would be longer- over 60 hours- if not for this billion-year break in slowing our planet’s rotation.

The scientists demonstrate that the tidal impasse between the sun and Moon was caused by the incidental but highly significant relationship between the temperature of the atmosphere and the rate of Earth’s rotation. They do this by drawing on geological evidence and employing atmospheric research tools.

Around 4.5 billion years ago, when the Moon formed initially, the day was only around 10 hours long. But ever longer days have resulted from the Earth’s rotation being slowed down by the Moon’s gravitational attraction since that time. It is currently extending at a pace of about 1.7 milliseconds each century.

By dragging on Earth’s oceans, the Moon slows the planet’s rotation, causing tidal bulges on the opposing sides of the planet that humans perceive as high and low tides. Our planet’s rotation is slowed down by the Moon’s gravitational pull on those bulges and friction caused by the tides and the ocean floor.

Norman Murray, a theoretical astrophysicist with the Faculty of Arts & Science’s Canadian Institute for Theoretical Astrophysics (CITA), said, “Sunlight also produces an atmospheric tide with the same type of bulges. The sun’s gravity pulls on these atmospheric bulges, producing a torque on the Earth. But instead of slowing Earth’s rotation like the Moon, it speeds it up.”

The lunar tides have dominated the solar tides during the majority of Earth’s geological history by a factor of around 10; as a result, the Earth’s rotating speed is slowing and the days are getting longer.

However, about two billion years ago, the atmosphere was warmer and had a natural resonance, or the frequency at which waves pass through it, that matched the length of day. As a result, the atmospheric bulges were greater.

Several variables, including temperature, control the frequency at which the atmosphere resonates. In other words, waves, like those produced by the massive Krakatoa eruption in Indonesia in 1883, move through it at a speed dependent on its temperature. The same theory explains why, if a bell’s temperature is constant, it will constantly ring out the same note.

That atmospheric resonance has not matched the rotating rate of the planet during the majority of Earth’s history. The Earth’s 24-hour rotational period and the two atmospheric “high tides” that occur today take 22.8 hours to traverse around the globe. As a result, the atmospheric tide is rather tiny.

However, the atmosphere was warmer and echoed for about 10 hours during the billion-year era under study. The Earth’s rotation, slowed by the Moon, had also reached 20 hours.

The atmospheric tide was reinforced, the bulges grew larger, and the sun’s tidal pull became powerful enough to counteract the lunar tide when the atmospheric resonance and length of the day became even factors—ten and twenty.

Murray and his colleagues used global atmospheric circulation models (GCMs) to forecast the atmosphere’s temperature during this time and geological evidence to arrive at their conclusion. The same models climatologists use to research global warming are known as GCMs. In Murray’s opinion, the fact that they performed so well during the team’s research is a valuable lesson.

Journal Reference:

  1. Hanbo Wu, Norman Murray, Kristen Menou, et al. Why the day is 24 hours long: The history of Earth’s atmospheric thermal tide, composition, and mean temperature. Science Advances. DOI: 10.1126/sciadv.add2499
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