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One full day on Earth lasted just 19 hours for about one billion years

We often find ourselves racing against the clock to squeeze all our tasks into a 24-hour day. However, had we been residents of Earth billions of years ago, the struggle would have been even more pronounced. Surprisingly, our planet’s day length wasn’t always this long.

A study led by Ross Mitchell, a geophysicist at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, dives into the deep past when Earth’s days were much shorter. 

Why were the days shorter? It turns out our Moon is a bit of a culprit. “Over time, the Moon has stolen Earth’s rotational energy to boost it into a higher orbit farther from Earth,” explained Mitchell. 

Study co-author Uwe Kirscher, now a research fellow at Curtin University in Australia, noted: “Most models of Earth’s rotation predict that day length was consistently shorter and shorter going back in time.” However, their findings contradict the idea of a consistently dwindling day length.

How the study was conducted

The research, which is published in the journal Nature Geoscience, relied on geological records. Scientists used to count the number of sedimentary layers per month from tidal mud flats to determine the number of hours in an ancient day. This method, though, has its limitations due to the rarity of such records and the disputes often arising from their interpretation.

A more reliable method, cyclostratigraphy, has come to the rescue. This geological method uses the rhythm of sedimentary layering to detect astronomical “Milankovitch” cycles that mirror how Earth’s rotation and orbit affect its climate. 

“Two Milankovitch cycles, precession and obliquity, are related to the wobble and tilt of Earth’s rotation axis in space. The faster rotation of early Earth can therefore be detected in shorter precession and obliquity cycles in the past,” said Kirscher.

With an abundance of recently published Milankovitch records, Mitchell and Kirscher set out to test a theory about Earth’s paleorotation. “We realized that it was finally time to test a kind of fringe, but completely reasonable, alternative idea about Earth’s paleorotation,” said Mitchell. 

Testing a theory

This theory speculates that the length of Earth’s day might have reached a plateau in the distant past. Besides the lunar tides we’re familiar with, Earth also experiences solar tides as the atmosphere heats up during the day. 

Although weaker than lunar tides, the impact of solar tides was significant when the Earth was rotating faster and the Moon’s gravitational pull was weaker. Unlike the Moon, which slows down Earth’s rotation, the Sun actually speeds it up.

“Because of this, if in the past these two opposite forces were to have become equal to each other, such a tidal resonance would have caused Earth’s day length to stop changing and to have remained constant for some time,” said Kirscher.

Theory confirmed

Interestingly, the data compilation from the study supports this theory. It appears that the lengthening of Earth’s day ceased and stabilized at about 19 hours between two to one billion years ago. Mitchell refers to this period as the “boring billion.”

This fascinating discovery links the evolution of Earth’s rotation to its atmospheric composition. Timothy Lyons of UC Riverside, who did not take part in the study, marveled at this possibility. “It’s fascinating to think that the evolution of the Earth’s rotation could have affected the evolving composition of the atmosphere,” said Lyons. 

The study thereby bolsters the idea that the modern levels of oxygen on Earth emerged due to longer days, allowing photosynthetic bacteria to generate more oxygen each day. 

So, while we may still scramble to complete our daily tasks, we can find comfort knowing that we have a few more hours at our disposal than our ancestors billions of years ago.

More about the how the moon and Earth interact

The Earth and the Moon share a dynamic relationship, influencing each other in ways that have shaped the evolution of both bodies. Here are a few key aspects of this interaction:

Gravitational pull

The Moon’s primary interaction with the Earth is through gravity. The Moon’s gravitational pull on Earth leads to several observable effects. This pull is the reason for the existence of tides on Earth. When the gravitational forces from the Sun and Moon align, we experience spring tides, which are particularly strong tides. When the Sun and Moon’s gravitational forces are perpendicular to each other, we experience neap tides, which are particularly weak.

Tidal locking

The gravitational pull between the Earth and the Moon has led to a state called tidal locking. This is why we always see the same side of the Moon from Earth. In the past, the Moon rotated at a different speed than it orbited Earth. However, Earth’s gravitational pull created a torque that slowed down the Moon’s rotation over millions of years until it became synchronous with its orbit.

Lengthening of Earth’s day

The Moon’s gravity pulls on the Earth’s equatorial bulge, creating a torque that slows down Earth’s rotation. This interaction gradually lengthens the duration of Earth’s day. Currently, the Moon is causing our day to lengthen by about 1.7 milliseconds per century.

The Moon is moving away

The energy from the slowing rotation of Earth gets transferred to the Moon, causing it to slowly move away from the Earth. The current rate is about 3.8 centimeters per year.

Stabilization of Earth’s axial tilt

The Moon’s presence helps stabilize the tilt of Earth’s axis. Without the Moon, Earth’s axial tilt could vary more dramatically, causing severe climate changes. The Moon’s gravity serves as a stabilizing anchor, keeping our axial tilt relatively steady at about 23.5 degrees, thus giving us stable and predictable seasons.


The interaction between the Earth and the Moon also leads to the celestial phenomena of lunar and solar eclipses. During a lunar eclipse, the Earth comes between the Sun and the Moon and casts a shadow on the Moon. During a solar eclipse, the Moon comes between the Earth and the Sun and casts a shadow on the Earth.

In summary, the relationship between the Earth and the Moon is complex and significant. The gravitational dance they perform together shapes life on Earth in profound ways, affecting everything from our day-night cycle to the tides and even the stability of our climate.


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