The slow, steady lengthening of a day on Earth, caused by the moon’s gravitational pull, has been stalled for more than a billion years, work by University of Toronto scientists shows.
The reason? An atmospheric tide caused by the Sun offset the effect of the Moon from about 2 billion years ago to 600 million years ago. The phenomenon has thus stabilized the Earth’s rotation speed so that a day lasts only 19.5 hours.
Without this billion-year pause in our planet’s rotation slowing, our current 24-hour day would be longer than 60 hours, astrophysicist Norman Murray and his colleagues explain in their work published in the journal Science Advances (new window).
Earth Day
A day on Earth lasted less than 10 hours when the moon formed about 4.5 billion years ago. But since the arrival of a natural satellite in Earth’s sky, the Moon’s gravitational pull has slowed its rotation, resulting in longer and longer days.
Even today, the day lengthens by about 1.7 milliseconds per century.
The Moon slows the planet’s rotation by attracting Earth’s oceans and creating tidal bulges on opposite sides of the globe, a phenomenon that takes the form of high and low tides.
The moon’s gravitational pull on these bulges and the friction between the tides and the seabed act as a brake on the planet’s rotational speed. However, sunlight also creates such bulges, but instead of slowing down the Earth’s rotation, it speeds it up, says Professor Murray.
For most of Earth’s evolution, this atmospheric resonance was not synchronized with the planet’s rotational speed. Today, each of the two atmospheric tides takes 22.8 hours to circle the globe. Because this resonance and the Earth’s 24-hour rotation period are out of sync, the atmospheric tide is relatively weak.
Under the influence of the sun
In their work, the Ontario team combined geological data and global atmospheric circulation models to predict the temperature of the atmosphere over the billion-year period when the atmosphere was warmer and more resonant for about 10 hours. In addition, the rotation of the earth – slowed down by the moon – reached 20 hours at the beginning of this epoch.
As atmospheric resonance and day length approached equal factors (10 and 20), atmospheric tide intensified, bulges increased, and the solar tidal effect became large enough to counteract the lunar tide.
The tidal impasse between the Sun and the Moon results from the fortuitous but vitally important relationship between the temperature of the atmosphere and the Earth’s rotational speed, the researchers write.
For much of Earth’s geological evolution, the moon’s tides were ten times greater than the sun’s tides, causing Earth’s rotation speed to slow and increasing day length, they note.
However, about two billion years ago, the atmospheric bulges were larger because the atmosphere was warmer and because its natural resonance — the frequency at which waves propagate through it — matched the length of the day.
Climatic changes
The models used in this work are the same ones climatologists use to study global warming. Murray says the fact that they worked so well in his research is a timely lesson.
In his opinion, the result, while distant in geological history, adds an additional perspective to the current climate crisis, as atmospheric resonance changes with temperature. Thus, the warming of the atmosphere could have an impact on the tidal imbalance.