Think days are short now? They once lasted just 18 hours

An Earth day lasted three quarters as long 1.4 billion years ago - partly because the moon was closer and changed the way Earth spun around its axis. Photo / 123RF

If you've been grumpy about the days getting shorter as we approach June 21's winter solstice, think how much gloomier it would have been if we'd existed 1.4 billion years ago.

At that point in our planet's history, scientists say, a day on Earth lasted just over 18 hours.

The US geoscientists behind a just-published study, and who have also just been collaborating with New Zealand researchers, have further explained how our days are slowly getting longer.

The study, featured this week in the scientific journal Proceedings of the National Academy of Sciences, has reconstructed the deep and intertwined history of Earth's relationship with the moon.

Its authors explained a day lasted three quarters as long 1.4 billion years ago, partly because the moon was closer and changed the way Earth spun around its axis.

"As the moon moves away, the Earth is like a spinning figure skater who slows down as they stretch their arms out," explained Professor Stephen Meyers, of the University of Wisconsin-Madison.

The study described a statistical method that linked astronomical theory with geological observation - called astrochronology - to look back on Earth's geologic past, reconstruct the history of the solar system and understand ancient climate change as captured in the rock record.

"One of our ambitions was to use astrochronology to tell time in the most distant past, to develop very ancient geological time scales," Meyers said.

"We want to be able to study rocks that are billions of years old in a way that is comparable to how we study modern geologic processes."

Earth's movement in space is influenced by the other astronomical bodies that exert force on it, like other planets and the moon.

This helped determine variations in Earth's rotation around and wobble on its axis, and in the orbit Earth traces around the sun.

These variations were collectively known as Milankovitch cycles and they determine where sunlight is distributed on Earth, which also means they determine Earth's climate rhythms.

Scientists like Meyers 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, like radioisotope dating, did 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.

The solar system had many moving parts, including the other planets orbiting the sun.

Small, initial variations in these moving parts can propagate into big changes millions of years later; this is solar system chaos, and trying to account for it can be like trying to trace the butterfly effect in reverse.

Last year, Meyers and colleagues 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 he and others have tried to go, the less reliable their conclusions.

For instance, the moon was currently moving away from Earth at a rate of 3.82cm per year.

Using this present day 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," Meyers explains.

Yet, we know the moon is 4.5 billion years old.

So Meyers sought a way to better account for just what our planetary neighbours were doing billions of years ago in order to understand the effect they had on Earth and its Milankovitch cycles.

He later teamed up with Professor Alberto Malinverno of Columbia University's Lamont-Doherty Earth Observatory to combine a new statistical method with astronomical theory, geologic data and a sophisticated statistical approach called Bayesian inversion.

Earth's movement in space is influenced by the other astronomical bodies that exert force on it, like other planets and the moon. Photo / 123RF

They then tested the combined 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 Earth and the moon.

"In the future, we want to expand the work into different intervals of geologic time," Malinverno said.

The study complemented two other recent studies that rely on the rock record and Milankovitch cycles to better understand Earth's history and behaviour.

A research team at Lamont-Doherty used a rock formation in Arizona to confirm the remarkable regularity of Earth's orbital fluctuations from nearly circular to more elliptical on a 405,000 year cycle.

And another team in New Zealand, in collaboration with Meyers and led by Victoria University's Professor James Crampton, looked at how changes in Earth's orbit and rotation on its axis have affected cycles of evolution and extinction of marine organisms called graptoloids, going back 450 million years.

"This research is very exciting, because the relationship between these orbital changes and extinction has never been shown before in truly ancient ecosystems," Crampton said last month.

"There's a strong debate in science about the impact on extinction and evolution of environmental change versus interactions between species - such as competition for food.

"With this study we can provide evidence of the impact of environmental changes on life on Earth.

"The evolution cycle changes we see occurred relatively soon after the first evolution of complex ecosystems, and during one of the greatest bursts of biodiversity increase in the history of life."

Normally, Crampton said, changes in Earth's orbit would be calculated by astronomers, rather than palaeontologists.

"Astronomers can clearly calculate changes in Earth's orbit about 50 million years into the past, but beyond that point the calculations become impossible due to the effects of what we call chaos theory, which makes the calculations too complex to complete," he said.

"But we can see the effects of changes in Earth's orbit in the fossil record, so we can provide information to astronomers that they previously couldn't find out."

Meyers said the new geological record would ultimately prove an "astronomical observatory" for the early solar system.

"We are looking at its pulsing rhythm, preserved in the rock and the history of life."