On June 29, the Earth completed one full spin (rotation)— a day — in 1.59 milliseconds less than its routine 24 hours. It was the shortest day recorded since the 1960s, when scientists first began to use the precise atomic clocks to measure the Earth’s rotational speed.
Key highlights
- Earth completes one full turn on its axis every 24 hours. That single spin marks out a day and drives the cycle of sunrise and sunset that has shaped patterns of life for billions of years.
But the curtains fell early on 29 June, with midnight arriving 1.59 milliseconds sooner than expected.
- The past few years have seen a flurry of records fall, with shorter days being notched up ever more frequently.
- In 2020, the Earth turned out 28 of the shortest days in the past 50 years, with the shortest of those, on 19 July, shaving 1.47 milliseconds off the 86,400 seconds that make up 24 hours.
- The 29 June record came close to being broken again last month, when 26 July came in 1.5 milliseconds short.
Factors affecting planet’s rotation
- While the Earth is slowing down over the longer 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 vanishing glaciers.
- The entire planet is wrapped in a thick blanket of gases and it wobbles as it spins on its axis. All of these influence 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 down the planet’s spin, extending the day by a fraction of a millisecond. Earthquakes, on the other hand, can have the opposite effect.
- The 2004 earthquake that unleashed a tsunami in the Indian Ocean shifted enough rock to shorten the length of the day by nearly three microseconds.
- Anything that moves mass towards the centre of the Earth will speed up the planet’s rotation, much as a spinning ice skater speeds up when they pull in their arms. Geological activity that pushes mass outwards from the centre will have the opposite effect and slow down the spin.
