Astrophysicists reveal a multi-million-year pause in the extension of the Earth's day.

Astrophysicists at the University of Toronto have solved a mystery: why Earth's day lengthens gradually under the influence of lunar tides, while the Earth's day lengthening pauses for more than a billion years. Their research shows that from about 2 billion years ago to 600 million years ago, sun-driven atmospheric tides counteracted the moon's influence, maintaining Earth's rotation rate and stabilizing the duration of the day at 19.5 hours.

Research by a team of astrophysicists suggests that from about 2 billion years ago to 600 million years ago, atmospheric tides driven by the Sun counteracted the moon's influence, keeping Earth's rotation rate steady and day length at a constant 19.5 hours. Source: Kevin M. Gill

Without the billion-year pause in slowing Earth's rotation, our current 24-hour day would stretch to more than 60 hours.

The study describing the results was recently published in the journal Science Advances. Using geological evidence and atmospheric research tools, scientists have shown that the tidal deadlock between the sun and the moon is the result of an accidental but hugely influential link between the temperature of the atmosphere and the speed of the Earth's rotation.

The authors of the paper include Norman Murray, A theoretical astrophysicist at the Canadian Institute for Theoretical Astrophysics (CITA) at the University of Toronto, Hanbo Wu, a graduate student at the Canadian Institute for Theoretical Astrophysics and the Department of Physics at the University of Toronto, and David A. Christine Menou, Department of Astronomy and Astrophysics, Dunlap, and Department of Physical and Environmental Sciences, University of Toronto, Garborough; Jeremy Laconte of the Astrophysics Laboratory in Bordeaux and a former CITA postdoctoral fellow; And Christopher Lee of the physics Department at the University of Toronto.

Power spectrum of the Earth's atmosphere. The X-axis is the wavelength, for example, 5 means a wavelength propagating from west to east that is one-fifth of the Earth's circumference, and -5 means a wavelength propagating from east to west that is one-fifth of the Earth's circumference. The Y-axis is frequency, in units of one cycle per day, for example 2 means two cycles per day, or 12 hours. Thin brown horizontal lines indicate one, two, three etc cycles per day (24 hour, 12 hour, 8 hour cycles, and so on). Source: Sakazaki & Hamilton

When the moon first formed about 4.5 billion years ago, a day lasted less than 10 hours. But since then, the moon's gravitational pull on Earth has slowed the Earth's rotation, causing the days to get longer and longer. Today, it is still lengthening at a rate of about 1.7 milliseconds per century.

The moon slows Earth's rotation by pulling on its oceans, creating tidal bulges on either side of the planet that we experience as high and low tides. The moon's gravitational pull on these bulges, combined with the friction between the tides and the seafloor, acts as a brake on our spinning planet.

"Sunlight also creates atmospheric tides with the same type of bulge," Murray said. The sun's gravity pulls on these atmospheric bulges, creating torque on the Earth. But instead of slowing down the Earth's rotation like the moon does, it speeds it up."

For most of Earth's geological history, lunar tides outperformed solar tides by about ten times; As a result, the Earth's rotation slows down and the days lengthen. But about 2 billion years ago, the atmospheric bulge was larger because the atmosphere was warmer then and because the atmosphere's natural resonance - the frequency at which waves move through the atmosphere - matched the length of the day.

Like a clock, the atmosphere's resonant frequency is determined by a variety of factors, including temperature. In other words, the speed at which waves - such as those produced by the massive eruption of Krakatoa in Indonesia in 1883 - pass through the atmosphere is determined by its temperature. The same principle explains why a clock always makes the same sound at a constant temperature.

Murray and his collaborators relied on geological evidence in their study, such as these samples from tidal estuaries, which reveal cycles of spring and low tides. Source: G.E. Williams

For most of Earth's history, atmospheric resonance has been out of sync with the Earth's rotation rate. Today, it takes 22.8 hours for two atmospheric "tides" to circle the Earth; Because the resonance is out of sync with Earth's 24-hour rotation period, atmospheric tides are relatively small.

But during the billion years studied, the atmosphere was warmer and the resonance period was about 10 hours. In addition, at the time of that epoch, the Earth's rotation was slowed by the moon to 20 hours.

As the atmospheric resonance and day length become even -10 and 20 hours - the atmospheric tides are strengthened, the bulge grows larger, and the tidal pull of the sun becomes strong enough to counteract the lunar tides.

"It's like pushing a child on a swing," Murray said. If your thrust is out of sync with the swing's cycle, the swing won't go very high. But if they are synchronized, and you are pushing the swing exactly when it stops at one end of its journey, then the push will increase the swing's power and it will swing farther and higher. This is what atmospheric resonance and tides do."

In addition to geological evidence, Murray and his colleagues used global Atmospheric circulation models (GCMs) to predict atmospheric temperatures during this period to arrive at their findings. Global atmospheric circulation models are the same ones climatologists use to study global warming. Murray thinks it's a big takeaway that these

models worked so well in the team's study.

"I've talked to climate change skeptics who don't believe the global circulation models that tell us we're in a climate crisis," Murray said. I told them we used these global circulation models in our research and they were correct."

Despite its distance from geological history, the results add a new perspective to the climate crisis. Since atmospheric resonance changes with temperature, Murray notes that our currently warming atmosphere could have an effect on this tidal imbalance.

"As the planet warms, the resonance frequency rises as well - the atmosphere gets further and further away from the resonance. As a result, the torque from the sun is reduced and, as a result, the length of the day is lengthened, faster than it would otherwise be."

Astrophysicists reveal a multi-million-year pause in the extension of the Earth's day.

Astrophysicists at the University of Toronto have solved a mystery: why Earth's day lengthens gradually under the influence of lunar tides, while the Earth's day lengthening pauses for more than a billion years. Their research shows that from about 2 billion years ago to 600 million years ago, sun-driven atmospheric tides counteracted the moon's influence, maintaining Earth's rotation rate and stabilizing the duration of the day at 19.5 hours.

Research by a team of astrophysicists suggests that from about 2 billion years ago to 600 million years ago, atmospheric tides driven by the Sun counteracted the moon's influence, keeping Earth's rotation rate steady and day length at a constant 19.5 hours. Source: Kevin M. Gill

Without the billion-year pause in slowing Earth's rotation, our current 24-hour day would stretch to more than 60 hours.

The study describing the results was recently published in the journal Science Advances. Using geological evidence and atmospheric research tools, scientists have shown that the tidal deadlock between the sun and the moon is the result of an accidental but hugely influential link between the temperature of the atmosphere and the speed of the Earth's rotation.

The authors of the paper include Norman Murray, A theoretical astrophysicist at the Canadian Institute for Theoretical Astrophysics (CITA) at the University of Toronto, Hanbo Wu, a graduate student at the Canadian Institute for Theoretical Astrophysics and the Department of Physics at the University of Toronto, and David A. Christine Menou, Department of Astronomy and Astrophysics, Dunlap, and Department of Physical and Environmental Sciences, University of Toronto, Garborough; Jeremy Laconte of the Astrophysics Laboratory in Bordeaux and a former CITA postdoctoral fellow; And Christopher Lee of the physics Department at the University of Toronto.

Power spectrum of the Earth's atmosphere. The X-axis is the wavelength, for example, 5 means a wavelength propagating from west to east that is one-fifth of the Earth's circumference, and -5 means a wavelength propagating from east to west that is one-fifth of the Earth's circumference. The Y-axis is frequency, in units of one cycle per day, for example 2 means two cycles per day, or 12 hours. Thin brown horizontal lines indicate one, two, three etc cycles per day (24 hour, 12 hour, 8 hour cycles, and so on). Source: Sakazaki & Hamilton

When the moon first formed about 4.5 billion years ago, a day lasted less than 10 hours. But since then, the moon's gravitational pull on Earth has slowed the Earth's rotation, causing the days to get longer and longer. Today, it is still lengthening at a rate of about 1.7 milliseconds per century.

The moon slows Earth's rotation by pulling on its oceans, creating tidal bulges on either side of the planet that we experience as high and low tides. The moon's gravitational pull on these bulges, combined with the friction between the tides and the seafloor, acts as a brake on our spinning planet.

"Sunlight also creates atmospheric tides with the same type of bulge," Murray said. The sun's gravity pulls on these atmospheric bulges, creating torque on the Earth. But instead of slowing down the Earth's rotation like the moon does, it speeds it up."

For most of Earth's geological history, lunar tides outperformed solar tides by about ten times; As a result, the Earth's rotation slows down and the days lengthen. But about 2 billion years ago, the atmospheric bulge was larger because the atmosphere was warmer then and because the atmosphere's natural resonance - the frequency at which waves move through the atmosphere - matched the length of the day.

Like a clock, the atmosphere's resonant frequency is determined by a variety of factors, including temperature. In other words, the speed at which waves - such as those produced by the massive eruption of Krakatoa in Indonesia in 1883 - pass through the atmosphere is determined by its temperature. The same principle explains why a clock always makes the same sound at a constant temperature.

Murray and his collaborators relied on geological evidence in their study, such as these samples from tidal estuaries, which reveal cycles of spring and low tides. Source: G.E. Williams

For most of Earth's history, atmospheric resonance has been out of sync with the Earth's rotation rate. Today, it takes 22.8 hours for two atmospheric "tides" to circle the Earth; Because the resonance is out of sync with Earth's 24-hour rotation period, atmospheric tides are relatively small.

But during the billion years studied, the atmosphere was warmer and the resonance period was about 10 hours. In addition, at the time of that epoch, the Earth's rotation was slowed by the moon to 20 hours.

As the atmospheric resonance and day length become even -10 and 20 hours - the atmospheric tides are strengthened, the bulge grows larger, and the tidal pull of the sun becomes strong enough to counteract the lunar tides.

"It's like pushing a child on a swing," Murray said. If your thrust is out of sync with the swing's cycle, the swing won't go very high. But if they are synchronized, and you are pushing the swing exactly when it stops at one end of its journey, then the push will increase the swing's power and it will swing farther and higher. This is what atmospheric resonance and tides do."

In addition to geological evidence, Murray and his colleagues used global Atmospheric circulation models (GCMs) to predict atmospheric temperatures during this period to arrive at their findings. Global atmospheric circulation models are the same ones climatologists use to study global warming. Murray thinks it's a big takeaway that these

models worked so well in the team's study.

"I've talked to climate change skeptics who don't believe the global circulation models that tell us we're in a climate crisis," Murray said. I told them we used these global circulation models in our research and they were correct."

Despite its distance from geological history, the results add a new perspective to the climate crisis. Since atmospheric resonance changes with temperature, Murray notes that our currently warming atmosphere could have an effect on this tidal imbalance.

"As the planet warms, the resonance frequency rises as well - the atmosphere gets further and further away from the resonance. As a result, the torque from the sun is reduced and, as a result, the length of the day is lengthened, faster than it would otherwise be."