Around 1.4 billion years ago, a day on Earth lasted approximately 18.7 hours, according to a study published in the Proceedings of the National Academy of Sciences. And this is at least in part because the Moon was closer and changed the way our planet spun around its axis.
Earth’s movement in space is influenced by the other astronomical bodies that exert force on it, like other planets and the Moon. This helps determine variations in the Earth’s rotation around and wobble on its axis, and in the orbit the planet traces around the Sun.
Collectively known as Milankovitch cycles, these variations determine where sunlight is distributed on the planet, which also means they determine Earth’s climate rhythms.
Geoscientists have observed this climate rhythm in the rock record, spanning hundreds of millions of years.
But going back further, on the scale of billions of years, has proved challenging because typical geologic means do not provide the precision needed to identify the cycles.
It’s also complicated by lack of knowledge of the history of the Moon, and by what is known as solar system chaos, a theory posed by Parisian astronomer Jacques Laskar in 1989.
Last year, University of Wisconsin-Madison’s Professor Stephen Meyers and co-authors cracked the code on the chaotic Solar System in a study of sediments from a 90 million-year-old rock formation that captured Earth’s climate cycles.
Still, the further back in the rock record the researchers have tried to go, the less reliable their conclusions.
For instance, the Moon is currently moving away from the Earth at a rate of 3.82 cm/year.
“Using this rate, scientists extrapolating back through time calculated that beyond about 1.5 billion years ago, the Moon would have been close enough that its gravitational interactions with the Earth would have ripped the Moon apart. Yet, we know the Moon is 4.5 billion years old,” Professor Meyers said.
So, Professor Meyers and his colleague, Professor Alberto Malinverno of Columbia University, sought a way to better account for just what our planetary neighbors were doing billions of years ago in order to understand the effect they had on Earth and its Milankovitch cycles.
The researchers combined sophisticated statistical methods with astronomical theory and geologic data.
They then tested the approach on two stratigraphic rock layers: the 1.4 billion-year-old Xiamaling Formation from Northern China and a 55 million-year-old record from Walvis Ridge, in the southern Atlantic Ocean.
With the approach, they could reliably assess from layers of rock in the geologic record variations in the direction of the axis of rotation of Earth and the shape of its orbit both in more recent time and in deep time, while also addressing uncertainty.
They were also able to determine the length of day and the distance between the Earth and the Moon.
“Application of the approach to 1.4-billion-year-old rock layers indicates a precession constant of 85.79 arcsec/year, an Earth-Moon distance of 340,900 km, and length of day of 18.68 hours, with dominant climatic precession cycles of 14,000 years and eccentricity cycles of 131,000 years,” the scientists said.
“In the future, we want to expand the work into different intervals of geologic time,” Professor Malinverno added.
Stephen R. Meyers Alberto Malinverno. Proterozoic Milankovitch cycles and the history of the Solar System. PNAS, published online June 4, 2018; doi: 10.1073/pnas.1717689115