Thank the moon for Earth's lengthening day

June 4, 2018, University of Wisconsin-Madison
Credit: CC0 Public Domain

For anyone who has ever wished there were more hours in the day, geoscientists have some good news: Days on Earth are getting longer.

A new study that reconstructs the deep history of our planet's relationship to the moon shows that 1.4 billion years ago, a day on Earth lasted just over 18 hours. This is at least in part because the moon was closer and changed the way the 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," explains Stephen Meyers, professor of geoscience at the University of Wisconsin-Madison and co-author of the study published this week [June 4, 2018] in the Proceedings of the National Academy of Sciences.

It describes a tool, a statistical method, that links 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 .

"One of our ambitions was to use astrochronology to tell time in the most distant past, to develop very ancient geological time scales," Meyers says. "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 helps determine variations in the Earth's rotation around and wobble on its axis, and in the orbit the Earth traces around the sun.

These variations are 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 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, 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.

The solar system has 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 is currently moving away from the Earth at a rate of 3.82 centimeters 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 neighbors were doing billions of years ago in order to understand the effect they had on Earth and its Milankovitch cycles. This was the problem he brought with him to a talk he gave at Columbia University's Lamont-Doherty Earth Observatory while on sabbatical in 2016.

In the audience that day was Alberto Malinverno, Lamont Research Professor at Columbia. "I was sitting there when I said to myself, 'I think I know how to do it! Let's get together!'" says Malinverno, the other study co-author. "It was exciting because, in a way, you dream of this all the time; I was a solution looking for a problem."

The two teamed up to combine a that Meyers developed in 2015 to deal with uncertainty across time—called TimeOpt—with astronomical theory, geologic data and a sophisticated statistical approach called Bayesian inversion that allows the researchers to get a better handle on the uncertainty of a study system.

They then tested the approach, which they call TimeOptMCMC, 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 .

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

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

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, 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.

"The geologic record is an astronomical observatory for the early solar system," says Meyers. "We are looking at its pulsing rhythm, preserved in the rock and the history of life."

Explore further: From rocks in Colorado, evidence of a 'chaotic solar system'

More information: Stephen R. Meyers el al., "Proterozoic Milankovitch cycles and the history of the solar system," PNAS (2018). www.pnas.org/cgi/doi/10.1073/pnas.1717689115

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13 comments

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granville583762
1.8 / 5 (10) Jun 04, 2018
Orbital and axis spin are two independent rotations
It is the mass of the moon and the mass of the earth rotating about their barycentre, nothing to do with the earth rotating on its centre of axis, they are two independent rotations, one is orbital and one is spin. The earth and moon are weightless in the vacuum of space, also the moon is in orbit about the sun separately from earth.
PoppaJ
1.7 / 5 (6) Jun 05, 2018
This is some of the oldest news. This was being taught in high school in the 80's. Stephen Meyers, professor of geoscience at the University of Wisconsin-Madison is a lazy bumb and needs to stop republishing old research.
carbon_unit
3 / 5 (6) Jun 06, 2018
Orbital and axis spin are two independent rotations
It is the mass of the moon and the mass of the earth rotating about their barycentre, nothing to do with the earth rotating on its centre of axis, they are two independent rotations, one is orbital and one is spin. The earth and moon are weightless in the vacuum of space, also the moon is in orbit about the sun separately from earth.
But spin and orbit aren't independent. Tidal forces tap Earth's rotational energy to accelerate the Moon away while slowing the Earth's rotation. And the Moon is most certainly in Earth orbit, not an independent solar orbit. Where do you come up with these notions?

Pity the Moon is moving away. I wish it had stayed closer so we'd have more solar eclipses

ZoeBell
1 / 5 (5) Jun 06, 2018
There is way more variability behind length of day: the dark matter in conjunctions of planets (Jupiter in particular) or even galactic equator. The systematic effect of Moon tides is quite negligible in this context.

Many physicists today are spending billions of tax payers money in search for various quantum gravity effects - but the ignorance of length of day variability convicts them from occupation driven hypocrisy, because the Earth is very robust and also cheap flywheel for testing various gravity and relativity extension theories.
granville583762
3 / 5 (4) Jun 06, 2018
Orbital and axis spin
granville583762> It is the mass of the moon and the mass of the earth rotating about their barycentre, nothing to do with the earth rotating on its centre of axis, they are two independent rotations, one is orbital and one is spin. The earth and moon are weightless in the vacuum of space; also the moon is in orbit about the sun separately from earth.

carbon_unit> But spin and orbit aren't independent. Tidal forces tap Earth's rotational energy to accelerate the Moon away while slowing the Earth's rotation. And the Moon is most certainly in Earth orbit, not an independent solar orbit. Where do you come up with these notions?

Some planets barrel roll - Orbital and axis spin are two independent rotations, the same pole remains in the direction of the sun
granville583762
3 / 5 (4) Jun 06, 2018
That is; the gyroscopic force keeps the poles fixed in space as the planet orbits the sun, planets and moons do not require to all spin and orbit about the same angular axis. They can be at right angles to each other
barakn
5 / 5 (4) Jun 06, 2018
There is way more variability behind length of day: the dark matter in conjunctions of planets (Jupiter in particular) or even galactic equator. The systematic effect of Moon tides is quite negligible in this context. -ZoeBell/Zephir

What a load of shit. The variability in rate of spin (other than that attributed to the Moon) is because the moment of inertia of the Earth changes as mass moves toward or away from the axis of rotation - mass like denser bodies of water or air as currents move them around, glaciers melting, or even the crust itself as earthquakes thrust up huge chunks of land. Moment of Inertia. Look it up. It's fundamental physics, which is why you are apparently oblivious to it.
granville583762
5 / 5 (2) Jun 07, 2018
Perpendicular Rotation and Solar Orbit
The moon can orbit the earth at right angles to the earth's axis of rotation, such that any rotation of the earth on its axis cannot add or subtract to the moment of inertia of the moon-earth rotating about their respective barycentre
The moon clearing its orbit as it is leaving its self to orbit the sun alone
carbon_unit> And the Moon is most certainly in Earth orbit, not an independent solar orbit. Where do you come up with these notions?

The moon is most certainly in solar orbit dear chap, just like the earth!
granville583762
5 / 5 (2) Jun 07, 2018
Circular continuous loops is solar orbit

Saying the moon is not in solar orbit carbon_unit, as it is in orbit round the earth which is in orbit at 64,000mph round the sun - the moon is also travelling at 64,000mph round the sun.
That is he definition of solar orbit, two particles travelling the same speed round their star at the same velocity completing circular continuous loops is solar orbit.

And you got 5stars for saying the moon is not in solar orbit!
Anonym262722
1 / 5 (2) Jun 07, 2018
The physics of lengthening day, decelerating expansion speed C4 of 4-radius R4 of the 3-D space and slowing speed of light C in the 3-D space together with slowing of all atomic processes (such as decay rate) are detailed in the e-books and many annual papers by 'Suntola Dynamic Universe' replacement of BB and some 5-10 postulates of the traditional relativity and quantum theories. The continual Earth-Moon range sensing of Apollo program during the past 47 years by DU shows the energy balanced global expansion rate to be 2.8 cm/yr with a local tidal effect of 1 cm in the observed 3.8 cm/yr value. A mistake in GR based theory should predict an artificial bump of some 10 cm not supported by the observations. See other coral based DU proofs of lengthening day during the past 0.4-0.8 B years at Suntola's PFS web site.
granville583762
5 / 5 (2) Jun 07, 2018
Gravitational solar, planetary and moonly orbits are each separate independent orbital entities.

The moon can orbit the earth at right angles to the earth's axis of rotation, there is not any rotation reduction of the earth on its axis, cannot add or subtract to the moment of inertia of the moon-earth rotating about their respective barycentre as that is a completely separate independent rotation

granville583762> Orbital and axis spin are two independent rotations
It is the mass of the moon and the mass of the earth rotating about their barycentre, nothing to do with the earth rotating on its centre of axis, they are two independent rotations, one is orbital and one is spin. The earth and moon are weightless in the vacuum of space, also the moon is in orbit about the sun separately from earth.

Just as the moon in solar orbit, it is a completely separate orbital rotation to its orbit round the earth.
granville583762
5 / 5 (2) Jun 07, 2018
There are two separate spin axis
The earth is rotating about its moon earth barycentre is effectively wobbling once a month on a separate axis, while at the same time as spinning on a separate earthly daily axis.
granville583762
3.7 / 5 (3) Jun 07, 2018
What will change is the lunar month
The earths daily spin will remain, what will change is the earth moon period about their barycentre will acquire a longer period as the moon moves further away from earth.
The lunar month will grow longer than an earthly month.

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