Once every 24 hours, Soil completes one rotation on its axis and marks one day on our planet. This reliable period of rotation allowed humans to develop systems that tell the time and signal to people, animals and plants when it is time to rest.
But Earth’s rotation hasn’t always been this consistent. In fact, a long time ago, Earth’s day was much shorter Sarah Millhollandan assistant professor of physics at MIT.
“Earth has experienced days that were both shorter and longer than now at various points in history,” Millholland told LiveScience in an email. “Most importantly, it was influenced by tidal interactions with the moon. About a billion years ago, day length 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 LiveScience in an email. This rapid rotation was the result of a huge, moon-forming impact with a Mars-sized protoplanet accelerating Earth’s angular momentum while breaking off enough of the planet’s surface to form the moon. The moon’s tidal effects ultimately acted on Earth to slow it down again, Batygin said.
Earth’s day has actually lasted longer than 24 hours, Millholland said, although only by a few milliseconds due to subtle changes in the planet’s molten core, oceans or atmosphere.
Related: Which way does the earth turn? What about the other planets?
Earth’s rotation is actually evidence of the planetary origin story, Millholland said. How fast a planet rotates is determined by how it formed when dust, rocks and gas orbiting the sun in a protoplanetary disk converged in space. Exactly how forcefully these pieces collide helps determine a planetary body’s angular momentum, or how fast it spins. You can compare it to spinning a bottle: the more force you exert, the faster the bottle spins.
If Earth had formed closer to the sun, our planet’s rotation would look very different, she said. In this case, the Earth might have become stuck to the Sun, similar to how the the moon is tidally connected to the earthsaid Millholland.
If Earth were closer to the Sun, it would experience stronger tidal interactions from the Sun, making its rotation period the same as its orbital period, meaning a day could last a year, she explained. “It is believed that many exoplanets are locked because they are very close to their host stars,” says Millholland. “These planets have permanent day and night sides because only one side of the planet always faces the star.”
Earth Day is still changing
Although the length of the day on Earth seems consistent to us, Batygin and Millholland said it is actually still changing. In particular, it is getting longer, albeit very slowly – about 1.7 milliseconds per century.
“The change in Earth’s rotation speed happens gradually enough that evolutionary processes can adapt to the changes over time,” Batygin said. “The relative change in turnover rate would not be noticeable in everyday life.”
Several factors contribute to the Earth’s slowing rotation. The main culprit is our planet’s relationship with the moon, Batygin said. Tidal forces arise from the moon – the same kind that pull on the ocean shores a friction effect like the moon slowly takes distance of the Earth over time. That process will indeed be extremely slow – it may take time 200 million years that the Earth’s day would last 25 hours.
In the meantime, other planetary and astronomical events could affect the length of the day on Earth, Millholland said. For example, more mundane events such as earthquakes could also have “other, smaller effects on its rotation.”
Unexpected 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 magnitude 8.9 earthquake that struck Japan on March 11, 2011 accelerated the Earth’s rotation. shortened the length of the 24-hour day by 1.8 microseconds.
Man-made disasters are another possible factor.
“Climate change could also play a role due to the redistribution of Earth’s mass due to glacier melting, sea level changes and tectonic activity,” Millholland said. ‘As the polar ice caps melt, the… the rotation slows down.”