Earth’s reliable 24-hour rotation, marking a day on our planet, is a fundamental aspect of life, allowing us to develop time-keeping systems and providing a signal for rest for humans, animals, and plants. However, this rotation has not always been consistent. In the distant past, Earth’s day was significantly shorter.
According to an assistant professor of physics at MIT, Earth has experienced days both shorter and longer than the current 24 hours throughout its history. The primary influence on these fluctuations is the tidal interaction with the moon. Approximately a billion years ago, the length of a day was only 19 hours.
Early in Earth’s formation, the rotation may have been even faster, potentially less than 10 hours. This rapid spinning was a result of a massive impact with a Mars-sized protoplanet, which accelerated Earth’s angular momentum while also contributing to the formation of the moon. Over time, the moon’s tidal forces gradually slowed Earth’s rotation.
Although Earth’s day has also been longer than 24 hours, this variation was only by a few milliseconds, caused by subtle changes in the planet’s molten core, oceans, and atmosphere.
The rate of a planet’s rotation is determined by its formation process, specifically how dust, rocks, and gas in the protoplanetary disk collide and come together. The force of these collisions influences the planet’s angular momentum, determining its spin rate.
If Earth had formed closer to the sun, its rotation would be drastically different. In this scenario, Earth might have become tidally locked to the sun, similar to the moon, where one side always faces the sun. This would lead to a day lasting an entire year.
Many exoplanets, especially those orbiting close to their stars, are believed to be tidally locked, resulting in a permanent dayside and nightside as only one side faces the star.
While Earth’s day appears constant to us, it is actually still changing, albeit very slowly, and is gradually lengthening.
The primary factor contributing to this lengthening is the moon’s gravitational pull. The tidal forces from the moon, similar to those affecting ocean shorelines, are causing a gradual transfer of angular momentum from Earth to the moon, resulting in a slowing of Earth’s rotation. This process will take an incredibly long time, potentially billions of years, for Earth’s day to reach 25 hours.
Besides the moon’s influence, other planetary and astronomical events can also impact the length of Earth’s day. Earthquakes, for example, can have smaller effects on Earth’s rotation.
Unexpected collisions with asteroids, by applying torque to Earth’s rotation, can also change the day’s length. The magnitude 8.9 earthquake that struck Japan in 2011 accelerated Earth’s spin, shortening the day by a fraction of a microsecond.
Human activities, including glacial melting, sea level changes, and tectonic activity, can also play a role in the redistribution of Earth’s mass, impacting its rotation. As polar ice caps melt, the water shifts towards the equator, contributing to the slowing of Earth’s spin.
