The enigma of why Earth’s day, which was progressively growing longer due to the moon’s tidal pull, stalled in its lengthening for more than a billion years has been solved by astronomers at the University of Toronto (U of T).
According to their findings, solar-driven air tides counteracted the moon’s impact between around two billion and 600 million years ago, keeping Earth’s spin rate constant and fixing the length of the day at 19.5 hours.
Our 24-hour day would be longer if not for this billion-year break in the slowing of our planet’s rotation.
Recent publication in the journal Science Advances of the study outlining the outcome. The scientists demonstrate that the tidal impasse between the sun and moon was caused by the accidental but extremely 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.
The Canadian Institute for Theoretical Astrophysics (CITA) at the University of Toronto (U of T) and its theoretical astrophysicist Norman Murray, graduate student Hanbo Wu from the CITA and the Department of Physics, Kristen Menou from the David A. Dunlap Department of Astronomy & Astrophysics and the Department of Physical & Environmental Sciences at the University of Toronto Scarborough, Jeremy Laconte from the Laboratoire d’astrophysique de Bordeaux, a former
Around 4.5 billion years ago, when the moon originally formed, the day was only around 10 hours long. But ever longer days have been the result of 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 as well as friction caused by the tides and the ocean floor.
“Sunlight also produces an atmospheric tide with the same type of bulges,” claims Murray. “These atmospheric bulges are pulled by the gravity of the sun, which causes a torque on the Earth. However, unlike the moon, it speeds up Earth’s rotation instead of slowing it down.
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.
The frequency at which the atmosphere resonates is controlled by a number of variables, including temperature. In other words, waves, like those produced by the massive Krakatoa eruption in Indonesia in 1883, move through it at a speed that is 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 planet’s rotating pace during the majority of Earth’s existence. The Earth’s 24-hour rotational period and the two atmospheric “high tides” that occur today each 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 with a duration of about 10 hours during the billion-year era under study. The Earth’s rotation, which was slowed by the moon, had also reached 20 hours at that time.
The atmospheric tide was strengthened, the bulges grew greater, and the sun’s tidal pull became powerful enough to counteract the lunar tide when the atmospheric resonance and length of day became even factors—ten and twenty.
Murray compares it to pushing a kid on a swing. “It won’t go very high if your push and the length of the swing are out of time. However, if they are in time and you push just as the swing comes to a stop at one end of its path, the push will increase the swing’s momentum and cause it to travel farther and higher. The tide and atmospheric resonance both have that effect.
Murray and his colleagues used global atmospheric circulation models (GCMs) to forecast the temperature of the atmosphere at this time in addition to geological evidence to arrive at their conclusions. The same models that climatologists use to research global warming are known as GCMs. The fact that they performed so well during the team’s research, in Murray’s opinion, is a valuable lesson.
According to Murray, “I’ve spoken to folks who are skeptical about climate change and don’t trust in the global circulation models that are telling us we’re in a climatic crisis. “And I inform them that the global circulation models we utilized in our research were accurate. They perform.
The outcome provides further context for the climate catastrophe despite its remoteness in geological history. Murray points out that this tidal imbalance may be affected by our current warming atmosphere because the atmospheric resonance varies with temperature.
“As Earth’s temperature rises due to global warming, the resonant frequency also rises, displacing our atmosphere further from resonance. Because of this, the torque of the sun is reduced, which causes the day to lengthen sooner than it otherwise would.
Reference: “Why the day is 24 hours long: The history of Earth’s atmospheric thermal tide, composition, and mean temperature” by Hanbo Wu, Norman Murray, Kristen Menou, Christopher Lee and Jeremy Leconte, 5 July 2023, Science Advances.
DOI: 10.1126/sciadv.add2499
