The Earth’s rotation speed has increased, making 2021 shorter than average by about 65 milliseconds.
Many may not realize it, but the year 2021 was shorter than usual. Science communicator Graham Jones from TimeAndDate told Newsweek on December 31 that the past year was shorter than average by about 65 milliseconds, making it the shortest year on record.
The Earth’s rotation speed affects the length of a year. (Photo: NASA)
The reason is the Earth’s rotation speed. Even a slight change in the rotation speed of our planet can cause a day to be slightly longer or shorter than the average of 86,400 seconds.
“If you look at the length of a day throughout the year, you can see peaks and troughs. Some important factors affecting this are the Moon’s orbit and its distance from the Earth,” Graham said. This is a relatively short-term effect, but other factors, such as ocean movements and the internal dynamics of our planet, are believed to have longer-lasting impacts, although scientists do not fully understand how. Therefore, it is very difficult to make predictions about the Earth’s rotation speed more than six months in advance.
Overall, scientists believe that the Earth has been slowing down for a long time, causing years to gradually lengthen, according to Graham. They infer this from factors such as geological characteristics or observations of solar eclipses.
“What was interesting in 2020 was that we noticed the Earth had a sudden increase in speed. The rate of acceleration has decreased, but the Earth continues to spin faster, making 2021 happen more quickly. This year may also be slightly shorter, but it is very hard to say for sure,” Graham explained.
Official data on the length of a day is published by the International Earth Rotation and Reference Systems Service (IERS). This organization also makes decisions about leap seconds if the Earth’s rotation speed deviates too much from the schedule.
Since the 1960s, scientists have measured the length of a day using atomic clocks—extremely accurate timepieces that do not gain or lose more than 0.0000001 seconds per year compared to an ideal clock. Thanks to atomic clocks, scientists can accurately determine when the length of a day deviates from the average.
“When atomic time was internationally approved in 1967, atomic clocks were over 100 times more stable than solar time. Over the past 30 years, atomic clocks have been improved more than a million times,” said Kurt Gibble, an atomic clock expert and physics professor at Pennsylvania State University.
“Today’s atomic clocks can help accurately test our knowledge of fundamental physics, ensure secure financial transactions, and provide time requirements—accurate to a billionth of a second—for mapping systems using the Global Positioning System (GPS) to guide vehicles to almost any address on Earth,” Gibble added.
