The year 2020 will be remembered for many things, most of them unpleasant, but it will also be remembered for being one of the fastest on record, due to our planet’s accelerating rate of spin. Should this trend continue, it could result in an unprecedented “negative leap second.”
Our clocks are falling out of sync, but instead of running slow like they usually do, they’re starting to run a bit fast. The reason for this has to do with the surprising number of short days experienced last year, the result of our planet’s spin accelerating ever so slightly. As Time and Date reports, 2020 had the 28 shortest days since 1960.
It takes Earth 24 hours, or 86,400 seconds, to make a complete rotation around its axis, which scientists refer to as a mean solar day. The term “mean” is key, however, as slight variances are experienced each day. This became apparent in the 1960s with the development of atomic timekeeping. Atomic clocks measure Earth’s spin relative to a distant astronomical object, typically a fixed star. Scientists learned that the length of a single day can deviate by a few milliseconds (ms), in which 1 ms is equal to 0.001 seconds.
The variability in our planet’s spin is nothing to worry about, and you certainly don’t need to hold on to your couch for fear of being flung into space. The variability of Earth’s rotational speed is a normal phenomenon, and it’s influenced by factors such as the internal sloshing of our planet’s molten core, oceans, winds, and atmospheric pressure.
To be clear, we’re talking about really tiny numbers. Today, for example, is expected to last for 24 hours, 0 minutes, and 0.0792 ms, whereas yesterday lasted for 24 hours, 0 minutes, and 0.2561 ms, according to Time and Date, a website run by journalists and researchers. That’s a difference of 0.1769 ms, so yeah, minuscule stuff. Some days can be off to an unusual degree, however, such as July 5, 2005, when Earth’s rotation was 1.0516 ms less than the mean solar day.
The year 2020 was quite extraordinary in this regard, beating the 2005 record no less than 28 times. The shortest of these was on July 19, when Earth’s rotation was 1.4602 ms below the mean solar day.
Interestingly, we can expect more of the same in 2021. “[A]n average day in 2021 will be 0.05 ms shorter than 86,400 seconds,” reports Time and Date, which means that, over the course of the entire year, “atomic clocks will have accumulated a lag of about 19 ms.”
Typically, these clocks run fast by a few hundred milliseconds each year, requiring an added leap second to keep clocks in sync.
“A leap second is a second added to Coordinated Universal Time (UTC) in order to keep it synchronised with astronomical time,” according to the U.S. National Institute of Standards and Technology. “UTC is an atomic time scale, based on the performance of atomic clocks that are more stable than the Earth’s rotational rate.”
The last time this happened was in 2016. Leap seconds are typically added at the stroke of midnight on New Year’s Eve, so if you celebrated at the time and didn’t wait for an extra full second, you ushered in 2017 a bit prematurely.
We haven’t had to invoke a leap second since 2016, and given the acceleration of Earth’s spin, we may eventually have to do something we’ve never done before, which is to take away a full second. In other words, a negative leap second.
This action would serve the same purpose as a positive leap second, which is to keep UTC in step with our atomic clocks. That said, the International Earth Rotation and Reference Systems Service, which decides on such matters, currently has no plans to do so.
It could happen, though. And should we need to invoke a negative leap second at some future juncture, you can impress your friends by ushering in the new year exactly one second before everyone else. You’re welcome.