Why a day is 24 hours long: Astrophysicists reveal why Earth’s day lasts 19.5 hours The clock is more than a billion years old

Why a day is 24 hours long: Astrophysicists reveal why Earth's day lasts 19.5 hours The clock is more than a billion years old

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A team of astrophysicists has shown that from about two billion years ago until 600 million years ago, atmospheric water driven by the Sun counteracted the effects of the Moon, keeping the Earth’s rotation rate constant and the length of the day at a constant 19.5 hours. Credit: Kevin M. Gill

A team of astrophysicists at the University of Toronto (U of T) has revealed that the slow and steady stretching of Earth’s days caused by the tidal pull of the Moon has stopped for more than a billion years.

They show that from about two billion years ago until 600 million years ago, atmospheric water driven by the Sun counteracted the effect of the Moon, keeping the Earth’s rotation rate constant and the length of the day at a constant 19.5 hours.

Without the billion-year pause in our planet’s slow rotation, our current 24-hour day would be longer than 60 hours.

The study describing the results, “Why the day is 24 hours long; the history of the Earth’s atmospheric heat flux, composition, and average temperature,” was published today in the journal Advanced science. Drawing on geological evidence and using atmospheric research tools, scientists show that the tidal stalemate between the sun and the moon is the result of an accidental, but significant consequence of the connection between the temperature of the atmosphere and the rate of rotation of the earth.

Authors of the paper include Norman Murray, a theoretical physicist with the U of T’s Canadian Institute for Theoretical Astrophysics (CITA); Graduate student Hanbo Wu, CITA and Department of Physics, U of T; Kristen Menou, David A. Dunlap Department of Astronomy & Astrophysics and Department of Physical & Environmental Sciences, University of Toronto Scarborough; Jeremy Laconte, Laboratoire d’astrophysique de Bordeaux and other former CITA postdoctoral fellows; and Christopher Lee, Department of Physics, U of T.

When the moon first appeared about 4.5 billion years ago, the day was less than 10 hours long. But since then, the moon’s gravitational pull on Earth has slowed the planet’s rotation, resulting in longer days. Today, it continues to lengthen at a rate of about 1.7 milliseconds every century.






The energy range of the Earth’s atmosphere. The x-axis is the wavelength, ie 5 is the wavelength of one-fifth of the Earth’s circumference for waves traveling from west to east, and -5 means the same, but for waves traveling from east to west. The y-axis is frequency, in cycles per day, for example 2 means two cycles per day, or 12 hours. The thin horizontal brown lines show the solar forcing at one, two, three, etc. cycles per day (24 hours, 12 hours, 8 hours, etc.). Credit: Sakazaki & Hamilton

The Moon slows the planet’s rotation by pulling on Earth’s oceans, causing tides to fall on opposite sides of the planet where we experience high and low tides. The pull of the Moon’s gravity on those shoulders, combined with the tension between the currents and the ocean floor, act like brakes on our spinning planet.

“The sun also creates the same puffy atmosphere,” Murray said. “The Sun’s gravity pulls on these atmospheric sheets, creating a torque on the Earth, but instead of slowing the Earth’s rotation like the Moon, it speeds it up.”

For most of Earth’s geologic history, lunar tides have overpowered solar tides by approximately a factor of ten; As a result, the speed of the Earth’s rotation slows down and the days get longer.

But about two billion years ago, the anus became larger because the atmosphere warmed and because of its natural resonance—the frequency at which waves travel through it matches the length of the day.

The atmosphere, like a bell, resonates at a frequency determined by various factors, including temperature. In other words, waves – like the massive eruption of the Krakatoa volcano in Indonesia in 1883 – travel through it at a speed determined by its temperature. The same principle explains why a bell always produces the same note if its temperature is constant.

Throughout most of Earth’s history atmospheric echoes have been inconsistent with the planet’s rotation rate. Currently, each two layers of the atmosphere “high tide” takes 22.8 hours to travel around the world; Because the resonance and the Earth’s 24-hour rotation period are not in sync, the atmospheric tide is relatively small.






Murray and his collaborators relied on geological evidence in their study, like these samples from a tidal estuary that reveal the cycle of spring and neap tides. Credit: GE Williams

But during the study period of billions of years, the atmosphere warmed up for about 10 hours. Also, at the advent of that era, the Earth’s rotation, slowed by the Moon, reached 20 hour.

When atmospheric reflectance and day length become a factor—ten and twenty—atmospheric water is enriched, bubbles grow, and the Sun’s pull is strong enough to counteract the Moon’s water.

“It’s like pushing a child in a ring,” Murray said. “If the thrust and the timing of your swing are not consistent, it won’t go very high. But, if they are aligned and you’re pushing like the swing stops at the end of its travel, the thrust will increase the momentum of the swing and it will go forward and higher. That’s what happens with the atmosphere and the current.”

Along with geological evidence, Murray and colleagues reached their results using global atmospheric circulation models (GCMs) to predict atmospheric temperatures during this period. GCMs are the same models used by climate scientists to study global warming. According to Murray, the fact that they worked so well in the team’s research is a timely lesson.

“I’ve talked to climate change skeptics who don’t believe in global circulation models that tell us we’re in a climate crisis,” Murray said. “And I told them: We used these global circulation models in our research, and they got it right. They worked.”

Despite being remote in geological history, the results add additional perspective to the climate crisis. Because atmospheric reflectivity changes with temperature, Murray points out that our current warming atmosphere could have consequences for this flood imbalance.

“As we increase the temperature of the Earth with global warming, we also make the resonant frequency move higher – we move our atmosphere further away from the resonance, therefore, there is less torque from the sun, therefore, the length of the day is. The longer it is, the faster it would be otherwise.”

Additional information:
Hanbo Wu et al, the temperature tide of the Earth’s atmosphere, the length of the day, and the history of the composition of The atmosphere and the average temperature of the world. Advanced science (2023). DOI: 10.1126/sciadv.add2499. www.science.org/doi/10.1126/sciadv.add2499

Journal information:
Advanced science

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