by If time feels tighter than ever lately, blame the revolution. On June 29 this year, Earth set an unusual record: the shortest day since the 1960s, when scientists began measuring the planet’s rotation with high-precision atomic clocks.
Roughly speaking, the Earth completes one complete turn on its axis every 24 hours. That single spin 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, and midnight came 1.59 milliseconds earlier than expected.
In recent years, a number of records have fallen, with shorter days increasingly being cut up. In 2020, Earth saw 28 of its shortest days in the past 50 years, with the shortest of them, 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 came in 1.5 milliseconds.
So is the world speeding up? In the longer term—the geologic timescales that compress the rise and fall of the dinosaurs into an instant—the Earth actually rotates more slowly than before. Turn the clock back 1.4 billion years and a day will last less than 19 hours. On average, Earth’s days are getting longer rather than shorter, by about 74,000ths of a second each year. The Moon is mostly to blame for the effect: its gravitational pull slightly distorts the planet, producing tidal friction that steadily slows Earth’s rotation.
To keep the clocks in line with the planet’s spin, the International Telecommunication Union, a United Nations agency, has added occasional leap seconds in June or December — most recently in 2016 — effectively stopping the clocks for a second to allow Earth to catch up. The first leap second was added in 1972. The next possibility is in December 2022, although with the Earth spinning so fast lately, it is unlikely to be necessary.
While the Earth is slowing down in the longer term, the situation is messier on shorter time scales. Inside the earth is a molten core; the surface is a mass of shifting continents, swelling oceans and disappearing glaciers. The entire planet is wrapped in a thick blanket of gases and it 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 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 moved 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, much like a spinning skater speeds up when they pull their arms. Geological activity that pushes mass outwards from the center will have the opposite effect and slow down the spin.
How all these different processes come together to affect the length of a day is a question scientists are still struggling with. However, if the trend of shorter days continues for long, it could lead to 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 rotating planet. This may again have its own consequences, not least reviving the debate about whether, after more than 5,000 years, defining time by the movement of the planet is an idea that has had its time.
