For the first time in history, astronomers have been able to estimate the length of the day and the distance to the Moon as it was 2.5 billion years ago. Back then, our day lasted only 17 hours.
We have to go back to 1969, when NASA’s Apollo missions installed reflective panels on the Moon that have shown that the Moon is currently moving 1.5 inches away from Earth each year.
Why is the Moon receding?
At a slow rate, the Moon moves away from the Earth and the Earth rotates more slowly around its axis. As the Moon revolves around the Earth, the tides rise. These bulges extend in the direction of the Moon and on the opposite side of the Earth. But the Earth is not still; it’s also rotating, and that rotation advances the tidal bulge relative to the Moon’s current position. The extra gravitational pull that this bulge puts on the Moon is propelling it into a more distant orbit.
An international team of scientists affiliated with the University of Utrecht, the University of Geneva, and the University of Quebec in Montreal have succeeded in accurately determining the distance to the Moon 2.46 billion years ago, using the so-called Milankovitch cycles. The climatic precession cycle, for example, arises from the precession movement (wobble) or the change in orientation of the Earth’s spin axis over time. This cycle currently lasts approximately 21,000 years, but this period would have been shorter in the past when the Moon was closer to Earth.
The location chosen for the geological survey was Karijini National Park in Western Australia. There are 2.5 billion-year-old layers there that are among the oldest parts of the Earth’s crust. In this type of formation, layers rich in iron alternate with more clayey layers. According to the researchers, this characteristic pattern is related to periodic changes in the shape of the Earth’s orbit and the orientation of its spin axis.
days on early earth
They found that this characteristic cycle pattern also gradually changed over time. By looking at evidence of layers in Australian rock and confirming with evidence of similar formations in South Africa, the researchers worked backwards to show that the precession signal reflected a distance between the Earth and the Moon 2.46 billion years ago. They found that at that time the day lasted only 17 hours, much less than the current 24.
“Today, this distance is about 384,300 kilometers. On average, of course, because the Moon doesn’t make a perfect circle around the Earth, its orbit is an ellipse. During the time interval we studied, the Earth-Moon distance was much shorter: about 321,800 kilometers,” the authors explain in their study published in the journal Proceedings of the National Academy of Sciences.
In a few hundred million years, the Moon, our only natural satellite, will not only be farther from our sky in our view, but it will even be too small for us to count on the famous and expected total solar eclipses. Although there is still a lot of time left for that, it may serve as a strong argument for trying not to miss these spectacular astronomical events.
Referencia: Margriet L. Lantink et al, 2022. Milankovitch cycles in banded iron formations constrain the Earth–Moon system 2.46 billion years ago. Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2117146119