According to a group of astrophysicists at the University of Toronto (U of T), the slow and steady lengthening of Earth’s day caused by the tides of the Moon has been halted for more than a billion years.
They show that from about two billion years ago until 600 million years ago, atmospheric tides driven by solar radiation counteracted the effect of the Moon, keeping the Earth’s rotation rate and the length of the 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 more than 60 hours.
A study describing the result: “Why is there 24 hours in a day; history of Earth’s atmospheric heat flux, composition, and mean temperature,” was published today in the journal Science Advances. Based on geological evidence and using atmospheric research tools, scientists show that the deadlock between the Sun and the Moon is the result of a random but enormously consequential connection between the temperature of the atmosphere and the speed of the Earth’s rotation.
The paper’s authors include Norman Murray, a theoretical astrophysicist at U 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 and Astrophysics and Department of Physical and Environmental Sciences, University of Toronto Scarborough; Jeremy Laconte, Laboratoire d’astrophysique de Bordeaux and former CITA postdoctoral fellow; and Christopher Lee, Department of Physics, U of T.
When the Moon first formed some 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 our planet’s rotation, resulting in an increasingly long day. Today, it continues to lengthen at a rate of about 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 tides. The Moon’s gravitational force on these bulges plus the friction between the tides and the ocean floor acts as a brake on our spinning planet.
“Sunlight also creates an atmospheric tide with the same type of bulge,” says Murray. “The Sun’s gravity pulls on these atmospheric bulges and creates 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, the lunar tides exceeded the solar tides by about ten times; thus slowing down the speed of the Earth’s rotation and lengthening the days.
But about two billion years ago, atmospheric bulges were larger because the atmosphere was warmer and because its natural resonance—the frequency at which the waves travel—matched the length of the day.
The atmosphere, like a bell, resonates at a frequency determined by various factors, including temperature. In other words, the waves – similar to those generated by the huge 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 emits the same tone if its temperature is constant.
For most of Earth’s history, this atmospheric resonance was out of sync with the planet’s rotation rate. Today, each of the two atmospheric “tides” takes 22.8 hours to travel around the world; because this resonance and the Earth’s 24-hour rotation period are not synchronized, the atmospheric tide is relatively small.
But during the billion-year period observed, the atmosphere was warmer and resonated with a period of about 10 hours. Also at the arrival of this epoch, the rotation of the Earth, slowed down by the Moon, reached 20 hours.
When atmospheric resonance and day length became equal factors—ten and 20—the atmospheric tide was strengthened, the bulges enlarged, and the tidal force of the Sun was strong enough to counter the lunar tide.
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