Earth's spin has slowed. But we still may need a negative leap second.

The Earth's orbit has settled down after it accelerated during 2020. Timekeepers warn that we may still need a "negative leap second” in the next ten years.
Each Earth day averages 86,400 seconds. The Earth's rotation doesn't work perfectly. It fluctuates slightly depending on the speed of the core, oceans, and atmosphere. Universal Coordinated Time, or UTC, is the international standard for timekeeping. It's based on the Atomic Clock, which measures time using the movement electrons in atoms that are cooled to absolute zero. Atomic clocks can be precise and invariable.

When the Earth's orbit and the atomic clocks are not in sync, something must give. UTC is adjusted when astronomical time, which is based on Earth’s rotation, differs from UTC by more that 0.4 seconds. This adjustment is called a "leapsecond". Sometimes, leap seconds are added. This was the case last New Year's Eve 2016. A second was added at 23:59 minutes and 59 secs of Dec. 31. According to the National Institute of Standards and Technology, scientists have added a leap second approximately every 18 months since 1972.

Related: Climate change is making Earth darker

A negative leap second is when seconds are subtracted. Although there has not been an international negative leap second, 2020 raises the possibility of one. Earth's orbit accelerated that year, surpassing the 2005 record of 28 seconds for the shortest day. July 19 was the shortest day of 2020. The planet completed its orbit 1.4602 milliseconds quicker than the average of 86.3400 seconds.

According to Time and Date, Earth’s spin has slowed down. Although the first half of 2021 was still fast, the average day length was 0.39 milliseconds shorter than in 2020. However, the average length of a day increased to 0.05 milliseconds from July 1 through Sept. 30 than it was in 2020.

According to Time and Date this means that Earth's rotation is not accelerating. It is still spinning at an average speed. Based on current rotation rates, it is possible that a negative leap second will be needed in approximately 10 years. The International Earth Rotation and Reference Systems Service in Paris will make the final decision.

The planet could have other ideas. It is possible that Earth's rotation will slow down again. This could lead to the addition of another in the future, instead of subtraction. It's impossible to predict it. Scientists don't know what causes the long-term changes in Earth’s rotation.

Nick Stamatakos (one of the U.S.) stated that they have tried to model things internally for the next two years. Time and Date spoke with representatives from Naval Observatory to the IERS Directing Board. "But it is difficult to predict more than six months ahead or one year in advance."

Original publication on Live Science