Science & Technology, Canada (Commonwealth Union) – A group of astrophysicists from the University of Toronto has unveiled a discovery regarding the prolonged pause in the lengthening of Earth’s day caused by the gravitational influence of the moon. They have found that for more than a billion years, a specific atmospheric tide, driven by the sun, counteracted the moon’s effect. This equilibrium maintained a steady rotational rate for Earth, resulting in a consistent day length of 19.5 hours between approximately two billion years ago and 600 million years ago.
This substantial pause in the slowing of Earth’s rotation, as indicated by geological evidence and analyzed through atmospheric research tools, is of great significance. If not for this billion-year period, our current 24-hour day would have expanded to over 60 hours.
The scientists attribute this tidal stalemate between the sun and moon to an incidental yet crucial connection between the temperature of the atmosphere and Earth’s rotational rate.
The team of authors behind the paper comprises Professor Norman Murray, a theoretical astrophysicist from the Canadian Institute for Theoretical Astrophysics (CITA) within the Faculty of Arts & Science, Hanbo Wu, a graduate student affiliated with CITA and the Department of Physics, Kristen Menou, an associate professor in the David A. Dunlap Department of Astronomy and Astrophysics, as well as the Department of Physical and Environmental Sciences at U of T Scarborough, Jeremy Leconte, a CNRS researcher at the Laboratoire d’astrophysique de Bordeaux and former CITA postdoctoral fellow, and Christopher Lee, an assistant professor in the Department of Physics.
The study explores the history of Earth’s rotation and its interaction with the moon’s gravitational pull. When the moon first formed approximately 4.5 billion years ago, the Earth had a day length of less than 10 hours. However, over time, the moon’s gravitational force has gradually slowed down the planet’s rotation, causing the day to lengthen. Presently, Earth’s day lengthens by approximately 1.7 milliseconds every century.
The moon’s impact on Earth’s rotation occurs through the creation of tidal bulges in the planet’s oceans. These bulges generate high and low tides on opposite sides of the Earth. The gravitational pull of the moon on these tidal bulges, combined with the friction between the tides and the ocean floor, acts as a braking mechanism, slowing down the Earth’s rotation.
The findings appeared in the journal Science Advances.
“Sunlight also produces an atmospheric tide with the same type of bulges,” said Murray. “The sun’s gravity pulls on these atmospheric bulges, producing a torque on the Earth. But instead of slowing down Earth’s rotation like the moon, it speeds it up.”
Throughout the majority of Earth’s geological history, the dominant influence on the planet’s rotation has been the lunar tides, surpassing the impact of solar tides by approximately a factor of ten. This dominance has resulted in the gradual slowing of Earth’s rotational speed and the subsequent lengthening of days.
However, around two billion years ago, a noteworthy change occurred. The atmospheric bulges became more significant due to two factors: the warmer state of the atmosphere and its natural resonance, which refers to the frequency at which waves propagate through it. This resonance coincided with the length of the day at that time.
Similar to a bell that produces a particular sound determined by its temperature, the atmosphere exhibits resonance determined by various factors, including temperature. This means that waves, such as those generated by significant volcanic eruptions like the 1883 Krakatoa eruption in Indonesia, travel through the atmosphere at a velocity influenced by its temperature. Consequently, the resonant behavior of the atmosphere can be compared to that of a bell producing a consistent note when its temperature remains constant.
