Once in 24 hours, earth performs a rotation on its axis, marking a day on our planet. This reliable rotation period is what allowed humans to develop systems for telling time and what signals to humans, animals, and plants when it’s time to rest.
But the rotation of the Earth has not always been so stable. In fact, long ago, Earth’s day was much shorter, he said Sarah Millhollandan assistant professor of physics at MIT.
“Earth has experienced days that were shorter and longer than it is now at various points in history,” Millholland told Live Science in an email. “Most importantly, it was affected by tidal interactions with the moon. About a billion years ago, the length of the day was only about 19 hours.”
Early in the planet’s history, Earth’s rotation may have been even shorter than 10 hours, Konstantin Batygin, a professor of planetary science at Caltech, told Live Science in an email. This rapid rotation was the result of a influence on the formation of the moon with a Mars-sized protoplanet that accelerated Earth’s angular momentum tearing off enough of the planet’s surface to form the moon. The moon’s tidal effects finally worked on Earth to slow it down, Batygin said.
Earth’s day has been even longer than 24 hours, Millholland said, though only by a few milliseconds as a result of subtle changes in the planet’s molten core, oceans or atmosphere.
Connected: In which direction does the Earth rotate? What about the other planets?
The rotation of the Earth is actually evidence of the history of its planetary origins, Millholland said. How fast a planet spins is determined by how it formed when the dust, rocks and gas that surrounded the sun in a protoplanetary disk coalesced in space. Exactly how hard these pieces collide helps determine a planetary body’s angular momentum, or how fast it spins. You can think of it like spinning a bottle: the more force you apply, the faster the bottle will spin.
If Earth had formed closer to the sun, our planet’s rotation would look very different, she said. In this case, the Earth may be tidally locked to the sun, similar to how The Moon is tidally bound to the EarthMillholland said.
If Earth were closer to the sun, it would experience stronger tidal interactions from the sun that would make the rotational period the same as the orbital period, meaning a day could last a year, she explained. “Many exoplanets are thought to be tidally locked because they are so close to their host stars,” Millholland said. “These planets have perpetual day and night because only one side of the planet faces the star at any time.”
Earth Day is still changing
While the length of Earth’s day may seem constant to us, Batygin and Millholland said it’s actually still changing. In particular, it is lengthening, albeit very slowly – about 1.7 milliseconds every century.
“The change in the Earth’s rotation speed is occurring gradually, so much so that evolutionary processes can adapt to the changes over time,” Batygin said. “The relative change in orbital velocity would not be noticeable in everyday life.”
Several factors are contributing to the slowing of the Earth’s rotation. The main culprit is our planet’s relationship with the moon, Batygin said. Tidal forces from the moon—the same kind that pull ocean shores—are creating a friction effect like the moon distance slowly from Earth over time. Indeed, that process will be extremely slow – it could take a long time 200 million years for the Earth’s day to reach 25 hours.
Meanwhile, other planetary and astronomical events can affect the length of Earth’s day, Millholland said. For example, more common events like earthquakes can also have “other, smaller effects on its rotation.”
Sudden collisions with asteroids can also change the length of the day — for example, by applying torque in the direction of Earth’s rotation to make it spin faster, Millholland said. For example, the 8.9 magnitude earthquake that struck Japan on March 11, 2011 accelerated the Earth’s rotation, which shortened the length of the 24-hour day by 1.8 microseconds.
Man-made disasters are another possible factor.
“Climate change it may also play a role due to the redistribution of Earth’s mass due to glacial melting, sea-level changes and tectonic activity,” Millholland said. “With the melting of the polar ice, rotation is slowing down.”
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