We Know How Fast Earth Spins ... Don’t We?

·3 min read
Photo credit: Lawrence Lawry - Getty Images
Photo credit: Lawrence Lawry - Getty Images
  • The Earth-moon system is responsible for our planet’s gradually slowing spin, leading to longer days.

  • Factors like the “Chandler Wobble” may be temporarily speeding up our spin rate.

  • Scientists use multiple ways to ascertain Earth’s spinning speed, including evidence from shells.

Earth spins on its axis at about 1,000 miles per hour, or 1,525 feet per second at the equator. This speed maintains our familiar day-night pattern as a 24-hour cycle.

But this time measurement we’ve been taking for granted throughout human history is temporary. In the long run, Earth days will lengthen. For hundreds of millions of years, the moon’s gravity has been exerting a pull on our planet, causing high tides and slowing down our spin. In fact, the difference between proto-Earth’s days and our modern days is stark; when the Earth-moon system formed about 4.5 billion years ago, Earth spun much faster, making a day last only four hours.

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This slowing effect will continue, and our days will keep stretching out longer. If humans are still hanging around our home planet in 250 million years, we’ll finally get what we have always longed for: an extra hour in the day.

Scientists know that Earth spun faster from a variety of evidence, including the growth patterns of mollusk shells. About 70 million years ago, a type of mollusk that has the same shell-growth patterns as mollusks today experienced a 23.5-hour day. By closely studying the growth rings of the mollusk shells using high-definition imaging tools, researchers could pinpoint individual days in the animal’s life. They observed that the rings, which grow faster during the day and slower at night, showed that the animal had about 30 minutes less time each day.

Despite all signs pointing to longer days in the future, our slowly but steadily decreasing spin rate can get thrown out of whack from time to time.

Weirdly, scientists determined that we experienced our shortest day ever on June 29, 2022. Not by much, though—Earth sped up just enough to shave off 1.59 milliseconds. This time change is an anomaly that keeps occurring, so much so that it could introduce the need for a negative leap second. (Normally, we’ve been adding a leap second every so often to keep intensely time-sensitive calculations, like those for GPS instruments and astronomy, running smoothly.)

Climate change could have affected the length of our days, especially through melting glacier ice that has caused seas to rise. The resulting massive amounts of water create bulges that didn’t exist before, and so subtly alter the planet’s spin rate. In fact, the displacement of seas has shifted our planet’s axis eastward, even beyond climate model predictions. When the weight distribution of any ball shifts, so does its rotational axis, the invisible central pole about which the ball spins, creating a “Chandler Wobble.”

Photo credit: Boy_Anupong - Getty Images
Photo credit: Boy_Anupong - Getty Images

Discovered in the 1800s, the Chandler Wobble points to a natural jiggle in Earth’s movement. Researchers haven’t been able to point to a singular cause, but it could be due to the fact that Earth isn’t a perfect sphere, and it’s simply overflowing with water. Our planet is a shifting mass of molten rock that continually migrates beneath the crust; even the deep mantle and its overlying upper mantle are fidgety, causing earthquakes. Even oceanic tidal forces could affect planetary movements. The spinning motion itself causes Earth to bulge slightly along its equator.

However it wobbles or alters its speed, Earth will keep spinning for billions of years longer. Will we reap the benefits of a lengthening day? Only time will tell.

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