It’s hard to do everything we want to do in one day. But it would have been much more difficult if we lived earlier in Earth’s history.
Although we take the 24-hour day for granted, days were shorter in Earth’s deep past.
The length of the day was shorter because the moon was closer. Ross Mitchell, a geophysicist at the Institute of Geology and Geophysics of the Chinese Academy of Sciences and lead author of a new study published in the journal Nature, said: Natural Earth Sciences.
“Most models of Earth’s rotation predict that the length of the day has always been shorter and shorter as you go back in time,” said Uwe Kircher, co-author of the study and a research fellow now at Curtin University in Australia.
But a slow but steady change in the length of a day that moves back in time is not what Mitchell and Kircher found.
How do researchers measure the length of an ancient day? In past decades, geologists have used records of special sedimentary rocks that preserve very fine layers in tidal mud flats. Calculate the number of layers of sediment 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 preserved records are often disputed. Fortunately, there is another way to estimate the length of the day.
Cyclostratigraphy is a geological method that uses rhythmic sedimentary layers to detect astronomical “Milankovitch” cycles that reflect how changes in the Earth’s orbit and rotation affect climate.
“Two Milankovitch cycles, precession and tilt, are related to the oscillation and tilt of the Earth’s rotation axis in space. Thus, faster Earth’s rotation can be detected earlier and shorter deflection cycles in the past,” Kircher explained.
Mitchell and Kircher benefited from the recent proliferation of Milankovitch records, with more than half of the data on antiquities generated in the past seven years.
“We realized it was finally time to test some kind of fringe, but entirely plausible, alternative idea to petrified Earth,” said Mitchell.
One unproven theory is that day length may have stopped at a fixed value in Earth’s distant past. In addition to the ocean tides associated with the pull of the Moon, the Earth also has solar tides associated with the warming of the atmosphere during the day.
Tides in the heliosphere are not as strong as oceanic lunar tides, but this has not always been the case. When the Earth rotated faster in the past, the pull of the Moon was much weaker. Unlike the moon’s pull, the sun’s tide pushes the earth instead. So while the moon slows down the Earth’s rotation, the sun speeds it up.
“Because of this, if these two opposing forces in the past had become equal to each other, such a tidal resonance would have caused the length of Earth’s day to stop changing and remain constant for some time,” Kircher said.
And this is exactly what the new data compilation showed.
The length of day on Earth appears to have halted its long-term increase and has stabilized at roughly 19 hours between 2 billion and 1 billion years—”a billion years,” Mitchell noted, “commonly referred to as the ‘boring’ billion.”
The timing of the stall lies interestingly between the two biggest oxygen spikes. “It’s fascinating to think that the evolution of Earth’s rotation may have affected the evolving composition of the atmosphere,” said Timothy Lyons of the University of California, Riverside, who was not involved in the study.
The new study thus supports the idea that Earth’s rise to modern oxygen levels would have to wait days longer for photosynthetic bacteria to generate more oxygen each day.
the quote: For a billion years of Earth’s history, our days were only 19 hours long, finds a new study (2023, June 12) Retrieved June 12, 2023 from https://phys.org/news/2023-06-billion-years-earth-history -days. html
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