Days were 19 hours long in Earth’s remote past.

June 12 () –

Over a billion years of Earth’s history, our days were just 19 hours long, a new study based on cyclostratigraphy finds.

This is a geological method that uses rhythmic sedimentary layers to detect “Milankovitch” astronomical cycles that reflect how changes in Earth’s orbit and rotation affect climate.

The length of the day was shorter because the moon was closer. “Over time, the moon has stolen Earth’s rotational energy to propel it into a higher orbit further from Earth,” said Ross Mitchell, a geophysicist at the Institute of Geology and Geophysics of the Chinese Academy of Sciences and author of a new study published in Nature Geoscience.

“Most models of the Earth’s rotation predict that the length of the day became shorter and shorter going back in time.said Uwe Kirscher, a co-author of the study and now a researcher at Curtin University in Australia.

But a slow and steady change in day length going back in time is not what Mitchell and Kirscher found.

How do researchers measure the length of the ancient day? In decades past, geologists used special sedimentary rock records that preserved very fine-scale layers in tidal marshes. Count the number of sedimentary layers per month caused by tidal fluctuations and you will know the number of hours in an ancient day.

But such tidal records are rare, and those that do survive are often disputed. The new research is based on cyclostratigraphy.

“Two Milankovitch cycles, precession and obliquity, are related to the wobble and tilt of the Earth’s axis of rotation in space. Thus, the faster rotation of the early Earth can be detected in shorter cycles of precession and obliquity in the past“, explained study co-author Uwe Kirscher, now a researcher at Curtin University.

Mitchell and Kirscher took advantage of the recent proliferation of Milankovitch records, with more than half of the antiquity data generated in the last seven years.

“We realized that it was finally time to try some sort of fringe, but entirely reasonable alternative idea about Earth’s paleorotation,” Mitchell said.

One unproven theory is that the length of the day could have stuck at a constant value in Earth’s distant past. In addition to the tides in the ocean related to the pull of the moon, Earth also has solar tides related to the warming of the atmosphere during the day.

Solar atmospheric tides are not as strong as lunar ocean tides, but this would not always have been the case. When Earth was spinning faster in the past, the moon’s pull would have been much weaker. Unlike the pull of the moon, the tide of the sun pushes on the Earth. So while the moon slows down the Earth’s rotation, the sun speeds it up.

“Because of this, if in the past these two opposing forces had been equal to each other, such a tidal resonance would have caused the length of the Earth’s day to stop changing and remain constant for some time,” Kirscher said.

And that’s exactly what the new data compilation showed.

The length of Earth’s day appears to have stopped its long-term increase and leveled off at about 19 hours around two to a billion years ago: “the billion years,” Mitchell noted, “commonly known as the ‘boring’ billion”, where the evolution of the Earth did not undergo great changes.

Interestingly, the time of stagnation is between the two largest increases in oxygen. Timothy Lyons of the University of California, Riverside, who was not involved in the study, said: “It is fascinating to think that the evolution of the Earth’s rotation could have affected the evolution of the composition of the atmosphere.”

Thus, the new study supports the idea that Earth’s rise to modern oxygen levels had to wait longer days for photosynthetic bacteria to generate more oxygen each day.