If you’re the kind of person who gets a lot done, you’re grateful for every one of the 86,400 seconds that make up a day. On July 9, however, as well as on July 22, and August 5, you won’t get your full complement of seconds. On these days the Earth will be measurably—and, so far, unaccountably— accelerating its rotation, shaving from 1.3 to 1.5 milliseconds off of the usual 24 hours the typical day gets.
[time-brightcove not-tgx=”true”]A millisecond, or one thousandth of a second, is admittedly not much—an eyeblink takes about 100 milliseconds. But with atomic clocks tracking the Earth’s rotational speed so closely that once every year and a half or so, the International Earth Rotation and Reference Systems Service (yes, there is such a thing) has to add a leap second to the year to account for any increase in the planet’s speed, every millisecond counts. So what’s behind the sudden speed-up and what effect could it have?
The shortest day in history
Around the world, there are 450 atomic clocks in operation, tracking the time with an accuracy that sees them gaining or losing only about 1 second every 100 million years. The clocks exist for more than just tidy temporal bookkeeping. Everything from weather satellites to GPS satellites to telecommunications to nuclear missiles and other weapons of war rely on precise timing to operate properly. So when both astronomers and the operators of the clocks notice that the atomic ticks are falling out of step with the rotation of the Earth, people take notice.
The shortest day ever recorded since atomic clocks went into operation in the 1950s occurred last year, on July 5, according to Date and Time, when the Earth shaved off 1.66 seconds. Three years earlier, on June 30, 2022, things moved similarly quickly, when the day came in 1.59 seconds under the 24-hour wire. But with three short days predicted over the next few weeks by astronomers and clock watchers, this summer is something even more rare.
What makes Earth spin faster?
There are a number of reasons the Earth may be changing its speed this way. But it’s not clear yet just which explanation is responsible this time.
The likeliest cause is the position of the moon. Lunar distance is an always-changing thing, with the moon tracing an elliptical orbit around the Earth. At its closest approach—or perigee—the moon is only 224,000 miles distant. At its furthest—or apogee—that gap widens to 251,655 miles. On the three speedy days this summer, the moon will be at or near apogee—which is a puzzle, since lunar gravity is such that the Earth tends to slow down, not speed up, when the moon is farther away.
The moon’s orbit is not only elliptical, however, but cockeyed too, angled anywhere from 18° to 28° relative to the Earth’s equator. The sharper that angle is the faster the Earth orbits, with lunar gravity in this case speeding things up, countering the braking effect that lunar apogee usually applies. On the three days in question this summer, the moon will be close to its 28° peak.
The moon is not the only thing that can shorten earthly days. Earthquakes can too, redistributing subterranean mass and causing the planet to accelerate or slow down in the same way a figure skater can change the speed of her spin when she tucks in or extends her arms. In 2005, an earthquake in Indonesia shifted the Earth’s polar mass about one inch toward the east, decreasing the length of a day by 2.68 microseconds—or millionths of a second. No significant earthquake has occurred recently, however, ruling temblors out as the cause of the current speed up.
Does climate change impact Earth’s rotation?
Climate change—again and seemingly always—may play a role too. Last year, two NASA-funded studies found that since 2000, melting glaciers have caused the axis of the planet—or the centerpoint of its spin—to shift by about 30 ft., changing the speed of rotation. The catch is, in this case the change causes the planet to decelerate, not speed up—by about 1.33 milliseconds per century. If warming continues at its present pace, it is projected to increase the length of our days by 2.62 milliseconds by the end of the century.
Other factors, including rising ocean levels and the warming—and thus swelling—of the atmosphere can make a difference too, slightly increasing planetary circumference. Even the springtime blossoming of trees can play a role. “In northern summer, the trees get leaves,” geophysicist Richard Holme of the University of Liverpool told Live Science. “This means that mass is moved from the ground to above the ground—further away from the Earth’s spin axis.” In all of these cases too, however, the effect is to slow the Earth, not speed it up.
So that brings scientists back to lunar gravity as the cause of the current acceleration—and that’s a reason not to fret the slightly shorter days. The Earth and the moon have been doing their dance for the better part of 4.5 billion years, and it’s always been a stable one. Here’s betting they’ve got a few billion more years yet in their run.
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