It’s challenging to do everything we need to do in a single day. But if we had been alive earlier in Earth’s history, it would have been considerably more challenging.
Considerably while we now take the 24-hour day for granted, there was a time when it was considerably shorter.
Day length was shorter because the moon was closer. “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 in Nature Geoscience.
“Most models of Earth’s rotation predict that day length was consistently shorter and shorter going back in time,” said Uwe Kirscher, co-author of the study and a research fellow now at Curtin University in Australia.
But Mitchell and Kirscher did not discover a gradual increase in day duration over time.
How do researchers measure ancient day length? Geologists previously relied on data from unique sedimentary rocks that preserved extremely fine-scale layering in tidal mud flats. The number of hours in an ancient day can be calculated by counting the number of sedimentary layers produced by tidal changes each month.
But such tidal records are rare, and those preserved are often disputed. Luckily, there’s another means of estimating day length.
Most models of Earth’s rotation predict that day length was consistently shorter and shorter going back in time.
Uwe Kirscher
Cyclostratigraphy is a geologic method that uses rhythmic sedimentary layering to detect astronomical “Milankovitch” cycles that reflect how changes in Earth’s orbit and rotation affect climate.
“Two Milankovitch cycles, precession and obliquity, are related to the wobble and tilt of Earth’s rotation axis in space. The faster rotation of early Earth can therefore be detected in shorter precession and obliquity cycles in the past,” explained Kirscher.
Mitchell and Kirscher took advantage of a recent proliferation of Milankovitch records, with over half of the data for ancient times generated in the past seven years.
“We realized that it was finally time to test a kind of fringe, but completely reasonable, alternative idea about Earth’s paleorotation,” said Mitchell.
The possibility that day length on Earth may have stagnated at a fixed value in the past is one untested notion. Earth also experiences solar tides, which are caused by the atmosphere heating up during the day, in addition to ocean tides caused by the moon’s gravitational attraction.
The strength of solar atmospheric tides is not as great as that of lunar oceanic tides, but this was not always the case. The tug of the moon would have been much weaker in the past when Earth was rotating more quickly.
Unlike the pull of the moon, the sun’s tide instead pushes Earth. So while the moon slows Earth’s rotation down, the sun speeds it up.
“Because of this, if in the past these two opposite forces were to have become 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.
And that’s exactly what the new data compilation showed.
Earth’s day length appears to have stopped its long-term increase and flatlined at about 19 hours roughly between two to one billion years ago “the billion years,” Mitchell noted, “commonly referred to as the ‘boring’ billion.”
The timing of the stalling intriguingly lies between the two largest rises in oxygen. Timothy Lyons of the University California, Riverside, who was not involved in the study, said, “It’s fascinating to think that the evolution of the Earth’s rotation could have affected the evolving composition of the atmosphere.”
The new research thus supports the hypothesis that longer days were required for photosynthetic bacteria to produce more oxygen each day before Earth’s climb to contemporary oxygen levels.
