For over a billion years, a delicate balance between the Sun and the Moon locked Earth’s rotation in a steady rhythm, keeping the length of a day at roughly 19.5 hours. According to a new study by researchers at the University of Toronto and the University of Bordeaux, without this centuries‑long pause the day would have stretched to more than 60 hours by now.
The work, published in Science Advances, combines sophisticated mathematical models with geological evidence—such as sedimentary layers that record ancient tidal cycles—to trace how Earth’s spin evolved over the past 4.5 billion years.
When the Moon first formed about 4.5 billion years ago, it orbited close to Earth, and our planet spun rapidly, making a day last less than 10 hours. Today, the Moon is slowly receding and Earth’s rotation is slowing, adding about 1.7 milliseconds to the length of a day each century.
The Moon’s gravitational pull creates oceanic bulges—tides—on opposite sides of the planet. These bulges drag against the seabed, producing friction that extracts angular momentum from Earth’s rotation and gradually lengthens the day.
In contrast, solar heating generates atmospheric tides that can accelerate Earth’s spin. The atmosphere bulges in response to the Sun’s thermal forcing, and the resulting torque can slightly speed up the rotation. Historically, the Moon’s effect has been roughly ten times stronger, dominating the slow‑down process.
During the period between 2 billion and 600 million years ago, the Sun and Moon’s opposing torques were in near‑exact resonance. The Earth’s atmosphere, warmed by a higher global temperature, supported standing waves that completed a full cycle every 10 hours—exactly twice per Earth rotation. This resonant condition amplified the atmospheric bulges, allowing the Sun to add momentum to Earth’s rotation and counteract the Moon’s braking effect. The result was a plateau in the day’s length at ~19.5 hours that lasted for a billion years.
As the atmosphere cooled and the resonant frequency slipped, the delicate balance was lost. Since then, the day has been lengthening, and today the Sun’s atmospheric tide takes 22.8 hours to complete a full cycle, lagging behind the 24‑hour day.
Importantly, the study confirms the accuracy of the global atmospheric circulation models used by climate scientists. By reproducing past temperatures and tidal behaviors, the researchers demonstrate that these models can reliably predict how future climate change will influence atmospheric tides—and, by extension, the length of a day.
Because rising global temperatures could shift the atmospheric resonance further away from its historic balance, the Sun’s ability to accelerate Earth’s rotation may diminish, potentially accelerating the day‑lengthening trend. This subtle yet profound effect underscores another way in which human activity is reshaping our planet.
