Generally speaking, the Earth completes one complete turn on its axis every 24 hours. This unique rotation marks a day and drives the cycle of sunrise and sunset that has shaped life patterns for billions of years. But the curtains fell early on June 29, with midnight arriving 1.59 milliseconds earlier than expected. Recent years have seen a flurry of record falls, with shorter days occurring more and more frequently. In 2020, Earth had 28 of the shortest days in the last 50 years, with the shortest, on July 19, shaving 1.47 milliseconds off the 86,400 seconds that make up 24 hours. The June 29 record came close to being broken again last month, when July 26 clocked in at 1.5 milliseconds. So is the world speeding up? Over the long term – the geological timelines that compress the rise and fall of the dinosaurs in the blink of an eye – the Earth is actually spinning more slowly than in the past. Wind the clock back 1.4 billion years and a day would pass in less than 19 hours. On average, then, Earth’s days are getting longer rather than shorter, by about one 74,000th of a second each year. The moon is primarily responsible for the phenomenon: its gravitational pull slightly distorts the planet, causing tidal friction that steadily slows Earth’s rotation. To keep clocks in line with the planet’s rotation, the International Telecommunication Union, a United Nations body, has tried to add occasional leap seconds in June or December – most recently in 2016 – effectively stopping the clocks for a second, so so the Earth can catch up. The first leap second was added in 1972. The next chance is in December 2022, although with the Earth spinning so slowly it is unlikely to be needed. Subscribe to First Edition, our free daily newsletter – every morning at 7am. BST While Earth is slowing down in the long term, the situation is messier on shorter timescales. Inside the Earth is a molten core. Its surface is a mass of shifting continents, swelling oceans and disappearing glaciers. The entire planet is wrapped in a thick blanket of gases and wobbles as it spins on its axis. All of these affect the Earth’s rotation, speeding it up or slowing it down, although the changes are essentially imperceptible. According to NASA, stronger winds during El Niño years can slow the planet’s rotation, lengthening the day by a fraction of a millisecond. Earthquakes, on the other hand, can have the opposite effect. The 2004 earthquake that triggered a tsunami in the Indian Ocean shifted enough rock to shorten the length of the day by nearly three microseconds. Anything that moves mass toward the center of the Earth will speed up the planet’s rotation, just as a roller skater speeds up when pulled on their hands. Geological activity pushing the mass outward from the center will have the opposite effect and slow the rotation. How all these different processes combine to affect the length of a day is a question scientists are still grappling with. But if the trend toward shorter days continues for too long, it could lead to calls for the first “negative leap second.” Instead of adding a second to the clocks, urban time will skip a second to keep up with the faster spinning planet. This in turn could have its own consequences, notably reigniting the debate over whether, after more than 5,000 years, determining time by the motion of the planet is an idea whose time has come.
