Once in 24 hours, Earth it completes one rotation on its axis, which means one day on our planet. This reliable rotation period is what allowed humans to develop systems to tell time and what signals to humans, animals, and plants when it’s time to rest.
But the Earth’s rotation was not always so consistent. In fact, a long, long time ago, Earth’s day was much shorter, he said Sarah Millhollandassistant professor of physics at MIT.
“Earth has experienced days that were both shorter and longer than now at various times 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.”
At the beginning of the planet’s history, the rotation of the Earth could even be 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 huge, impact in the formation of the moon with a Mars-sized protoplanet that accelerated Earth’s angular momentum while breaking off enough of the planet’s surface to form the Moon. The moon’s tidal effects eventually acted on Earth to slow it down, Batygin said.
Earth’s day was even longer than 24 hours, Millholland said, if only by a few milliseconds due to subtle changes in the planet’s molten core, oceans or atmosphere.
Related: Which way does the earth spin? What about the other planets?
Earth’s rotation is actually evidence of its planetary origin story, Millholland said. How fast a planet spins is determined by how it formed when the dust, rocks and gas that orbited the Sun in a protoplanetary disk coalesced in space. How exactly these pieces collide helps determine the planetary body’s angular momentum, or how fast it is spinning. You can think of it like spinning a bottle: the more force you apply, the faster the bottle spins.
If Earth had formed closer to the Sun, our planet’s rotation would have looked very different, she said. In this case, Earth could become tidally locked to the Sun, similar to The Moon is tidally locked to EarthMillholland said.
If the Earth were closer to the Sun, there would be stronger tidal interactions from the Sun, making the rotation period match the orbital period, meaning a day could last a year, she explained. “Many exoplanets are thought to be tidally locked because they are very close to their host stars,” Millholland said. “These planets have a constant day and night side because only one side of the planet always faces the star.”
Earth Day keeps changing
While the length of Earth’s day may seem consistent to us, Batygin and Millholland said it actually keeps changing. In particular, it lengthens, albeit very slowly — about 1.7 milliseconds every century.
“The change in Earth’s rotation rate is gradual enough 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 contribute to the slowing of the Earth’s rotation. The main culprit is our planet’s relationship to the moon, Batygin said. Tidal forces from the moon are created – the same kind that pull on ocean shores frictional effect like the moon slowly moving away from Earth over time. In fact, this process will be extremely slow – it may take time 200 million years to make Earth’s day reach 25 hours.
Meanwhile, other planetary and astronomical events could affect the length of an Earth day, Millholland said. For example, more common events such as earthquakes can also have “other minor effects on its rotation”.
Unexpected collisions with asteroids could also change the length of the day — for example, by applying a 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 a 24-hour day by 1.8 microseconds.
Another possible factor is man-made disasters.
“Climate change it may also play a role due to the redistribution of land mass due to melting glaciers, sea level changes and tectonic activity,” Millholland said. “As the polar ice caps melt, the rotation slows down.”