Once every 24 hours, The Earth makes one rotation around its axis, marking one day on our planet. This reliable rotation period is what allowed humans to develop systems for telling time and what signals they send to humans, animals, and plants when it’s time to rest.
But the Earth’s rotation has not always been so consistent. In fact, a long, long time ago, Earth’s day was much shorter Sarah Milhollandassistant professor of physics at MIT.
“Earth has experienced days that were both shorter and longer than now at various points in history,” Milholland told Live Science in an email. “Most importantly, it is 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 even have been 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, moon-forming effect with a Mars-sized protoplanet accelerating Earth’s angular momentum while shedding enough of the planet’s surface to form the moon. The moon’s tidal effects eventually acted on Earth to slow it back down, Batygin said.
Earth’s day has even been longer than 24 hours, Milholland said, though only by a few milliseconds as a result of subtle changes in the planet’s molten core, oceans or atmosphere.
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Earth’s rotation is actually evidence of its planetary history, Milholland said. How fast a planet spins is determined by how it formed when dust, rock, and gas that orbits the sun in a protoplanetary disk clump together in space. Exactly how hard these pieces collide helps determine the angular momentum of the planetary body, or how fast it is spinning. 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 have looked very different, she said. In this case, the Earth may have become tidally locked to the sun, similar to the way that The Moon is tidally locked to the EarthMilholland said.
If Earth were closer to the sun, it would experience stronger tidal interactions than the sun, which 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 very close to their host stars,” Milholland said. “These planets have a permanent day and night side because only one side of the planet faces the star at all times.”
Earth Day is still changing
Although the length of a day on Earth may seem constant to us, Batygin and Milholland said it’s actually still changing. In particular, it lengthens, albeit very slowly – about 1.7 milliseconds every century.
“The change in Earth’s rotation rate occurs gradually 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 down of the Earth’s rotation. The main culprit is our planet’s relationship with the moon, Batygin said. Tidal forces are created by the moon—the same ones that pull ocean shores friction effect like the moon slowly distanced himself from Earth in time. In fact, this process will be extremely slow—it may take 200 million years the day on Earth to reach 25 hours.
Meanwhile, other planetary and astronomical events can affect the length of Earth’s day, Milholland said. For example, more mundane events such as earthquakes can also have “other smaller 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, Milholland 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 land mass due to melting glaciers, sea level changes and tectonic activity,” Milholland said. “With the melting of the polar ice caps, rotation slows down.”
