If time feels tighter than ever lately, blame it on the revolution. On June 29 this year, Earth achieved an unusual record: the shortest day since the 1960s, when scientists began measuring the planet’s rotation with high-precision atomic clocks.
Generally speaking, the Earth completes one full revolution on its axis every 24 hours. This single turn marks a day and drives the cycle of sunrise and sunset that has shaped the patterns of life for billions of years. But the curtains fell early on June 29, and midnight arrived 1.59 milliseconds ahead of schedule.
In recent years there has been a drop in records, with shorter days being recorded with increasing frequency. In 2020, Earth had 28 of the shortest days in the last 50 years, and the shortest of those days, on July 19, cut 1.47 milliseconds out of the 86,400 seconds that make up 24 hours. The June 29 record was close to being broken again last month, when it fell by 1.5 milliseconds on July 26.
So is the world speeding up? Over the long term, geological time scales that compress the rise and fall of the dinosaurs into the blink of an eye, the Earth spins more slowly than before. Turn back the clock 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 74,000 of a second each year. The moon is primarily responsible for the effect: the gravitational pull slightly distorts the planet, producing tidal friction that constantly slows the Earth’s rotation.
To keep clocks in line with the planet’s spin, the International Telecommunication Union, a United Nations body, has pushed to add occasional leap seconds in June or December, most recently in 2016, stopping clocks for a second so that the Earth can get caught. up The first leap second was added in 1972. The next chance is in December 2022, although with the Earth spinning so fast lately, it’s unlikely to be necessary.
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While the Earth is slowing down in the long term, the situation is messier on shorter time scales. Inside the Earth there is a molten core; its surface is a mass of shifting continents, rising 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 this influences the Earth’s rotation, speeding it up or slowing it down, although the changes are essentially imperceptible.
According to NASA, the strongest winds in El Niño years can slow the planet’s spin, 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 dislodged 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 spinning ice skater speeds up when they pull their arms. Geological activity pushing the mass outward from the center will have the opposite effect and slow the spin.
How all these different processes come together to affect the length of a day is a question scientists are still grappling with. But if the trend of shorter days continues for a long time, it could prompt calls for the first “negative leap second.” Instead of adding a second to the clocks, civil time would skip a second to keep up with the faster spinning planet. This in turn could have its own consequences, including igniting debate over whether, after more than 5,000 years, defining time by the movement of the planet is an idea that has had its time.