For a few days this summer, your days will be just a smidge shorter

Article content

You’re not running late — Earth is just moving faster, at least for a few days this summer.

On July 22, Earth will spin about 1.38 milliseconds faster than its typical 86,400 seconds in a day. If that’s not quick enough, Earth will rotate 1.51 milliseconds faster on August 5.

Those numbers are calculated by the International Earth Rotation and Reference Systems Service, according to timeanddate.com. The IERS tracks Earth’s orientation in space and schedules leap seconds, which are added to help keep our clocks synchronized with astronomical time (when Earth moves a bit more sluggish).

People already experienced a shorter day on July 9 — but maybe didn’t know it because it’s only 1.3 milliseconds faster. The fastest day since the introduction of the atomic clock occurred on July 5, 2024, when the day was truncated by 1.66 milliseconds.

“Since we are talking [about] 1 millisecond, it’s not something you’d notice,” Duncan Agnew, geophysicist at Scripps Institution of Oceanography, University of California, San Diego, said in an email. But while the changes may not register to people, scientists track them to keep our technology accurate, including GPS systems that tells us where exactly we are.

Shorter days happen from time to time. They tend to occur during the summer, when Earth spins faster than other times of the year, Agnew said. But there’s also added boosts on these days from the moon and maybe even mysterious processes in Earth’s core.

During the summer, the northern hemisphere is tilted toward the sun, which minimizes the difference in temperature between the equator and Earth’s poles. This smaller temperature variation slows down the jet stream — a narrow band of strong winds around 30,000 feet above us — and moves it northward. (The slower jet stream also explains why storms are more sluggish during the summer in the northern hemisphere.)

To recall a lesson from physics class, the angular momentum in this Earth-atmosphere system is conserved. When the atmosphere begins moving slower, Earth’s rotation speeds up.

But some days are even shorter than the rest, thanks to the moon. Agnew explained that the moon isn’t perfectly aligned with Earth’s equator, orbiting on an incline. It travels over the equator twice a month and also travels overhead at higher and lower latitudes twice a month. The moon reaches its most extreme north and southern positions about every 18.6 years — called a lunar standstill — which is occurring in 2024 and 2025. On July 22 and Aug. 5, the moon will be close to its peak angle, 28 degrees, to Earth. The steeper angle causes Earth to rotate faster.

“So twice a month, when the Moon is North or South, it spins faster,” said Agnew.

That’s important information to know for GPS operators, for instance. GPS determines accurate positioning by sending signals from satellites to receivers on Earth. To do so, it relies on precise measurements of Earth’s rotational speed. If a GPS system doesn’t account for a faster rotation at a particular time, then it may arrive to a point on the ground earlier than expected and create positioning errors.

Earth’s spin has varied throughout its history. When Earth was first formed and the moon was closer, days were much shorter. Days were 19 hours long for about 1 billion years. As the moon has drifted away from us, our more recent days have been some of the longest in history.

“Out of the trillion days or so of the Earth’s existence, almost all have been shorter: very very roughly, maybe 100,000 have been longer,” said Agnew. “It’s just that the long days have all been recent.”

Some processes like the melting of the ice sheets may have contributed to slowing down Earth’s rotation, too. The meltwater is moving toward the equator, making our planet bulkier and rotate slower.

Even as Earth is moving much slower than historical timelines, Earth’s rotation has mysteriously been speeding up in recent decades. Agnew said the boost could be due to processes deep within our core, which is hard to confirm or predict because of limited observations. Given all the factors, it’s also hard to know if Earth will continue to speed up or tap the brakes in upcoming years.

At the end of the day, all we can do is make the most out of 86,400 seconds, give or take.