Earth may currently be spinning faster, but a day on our planet lasts 24 hours largely because of the presence of the moon—and it’s gradual drift away from us.
There are other factors—including processes in the Earth’s core and solar tides related to the atmosphere heating up during daytime—but tidal forces from our moon are the most critical in determining the length of a day on Earth.
However, it appears that the length of a day on Earth hasn’t got slightly longer over deep time. There appears to have been a stalling, which may be responsible for a flourishing of plant life—and a surge in oxygen levels.
Dramatic results
“Over time, the moon has stolen Earth’s rotational energy to boost it into a higher orbit farther from Earth,” said Ross Mitchell, geophysicist at the Institute of Geology and Geophysics of the Chinese Academy of Sciences and lead author of a new study published today in Nature Geoscience.
It seems that Earth’s changing orbit and the angle it spins at could have as much an influence—with dramatic results for ancient Earth.
Not slow and steady
The study does not find a slow and steady change in day length going back in time, as most models of Earth’s rotation predict.
Most such research studies tidal mud flats preserved in sedimentary rocks—with the number of layers indicating the the number of tides, which give away the number of hours in an ancient day.
This new research uses the science of cyclostratigraphy, which uses rhythms found in vast layers of sediment to add long-term astronomical cycles to the mix. These so-called Milankovitch astronomical cycles include, according to NASA:
- The shape of Earth’s orbit (its eccentricity)
- The angle Earth’s axis, which is tilted with respect to Earth’s orbital plane (obliquity).
- The direction Earth’s axis of rotation (precession).
Milankovitch cycles
Those last two—obliquity and precession—are the result of Earth wobbling as it spins. It’s something you can see with a spinning top—set it going and soon enough the angle of the axis will change. “The faster rotation of early Earth can therefore be detected in shorter precession and obliquity cycles in the past,” said Kirscher.
Precession takes place over 22,000 years, with the direction of Earth’s axis changing to point to a variety of stars. For example, for the moment the star Polaris is known as the North Star because Earth’s axis points straight to it. However, 12,000 years ago bright star Vega was the North Star—and it will be again in 12,000 years.
Such astronomical phenomena are required to calculate the so-called paleorotation of Earth, but it’s controversial due to a lack of data sources. Half of the data on Milankovitch cycles for ancient times has been generated in the past seven years, say the researchers. “We realized that it was finally time to test a kind of fringe, but completely reasonable, alternative idea about Earth’s paleorotation,” said Mitchell.
19 hour day
Using that new data led to evidence that day length may have have stalled at a constant value in Earth’s distant past—about 19 hours—not because of the moon slowing down the planet’s rotation, but because of the sun speeding it up.
One to two billion years ago Earth rotated more quickly, which would have made the tidal pull of the moon weaker. Solar atmospheric tides would have been stronger. When they reached a certain point they could have canceled each other out. “Because of this, if in the past these two opposite forces were to have been equal to each other, such a tidal resonance would have caused Earth’s day length to stop changing and to have remained constant for some time,” said Kirscher.
Oxygen surge
One to two billion years ago is also between the times of two of the biggest surges in oxygen levels on Earth. That suggests that the length of day stalling at 19 hours for a billion years may have been instrumental in allowing plants enough time to photosynthesize and therefore thrive on Earth—and so help create lots of oxygen for in the planet’s atmosphere. “It’s fascinating to think that the evolution of the Earth’s rotation could have affected the evolving composition of the atmosphere,” said Timothy Lyons of the University California, Riverside, who was not involved in the study.
Wishing you clear skies and wide eyes.
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