Quick question: how long is a day? If you said 24 hours, we're going to be pedantic and tell you no, you are wrong. So wrong, in fact, that if we left you in charge of satellites there would be utter chaos.
The Earth's rotation has altered significantly over time. Right now, the Earth rotates just over 365 times on its axis in the time it takes to orbit around the Sun. By looking at ancient corals, however, we can figure out that this wasn't always the case, and the Earth used to spin a lot faster than it does now. Hundreds of millions of years ago, in fact, the Earth made a full 420 spins in the time it took to go around the Sun.
While coral grows, it puts down a fine layer of calcium carbonate every day. Since corals grow more in the dry season than the wet, you can then count up the lines of calcium carbonate deposits between the seasons, ending up with a number of days in a year. It's essentially like a more accurate version of a tree.
Through this, scientists have figured out that 444-419 million years ago, the Earth span 420 times a year, while a few million years later it slowed to 410.
There are all sorts of factors that affect the speed of rotation, such as changing sea levels and shifts within the Earth, though the biggest factor is that the Moon is moving away from the Earth (who can blame it) and as the two bodies interact, the result is the Earth slowing down.
Normally, a leap second is introduced every now and then in order to account for the slowing of the Earth. However, in 2020 the reverse has been true, the Earth's rotation has been speeding up again.
The record for the shortest day (since we began measurements with precise atomic clocks in the 1960s) was set in 2005. In 2020, that record was broken 28 times, Timeanddate.com reports. Since records began, the average day has been getting longer, until 2020 when on average during the year, the days have been about 0.5 milliseconds shorter.
The result is that the world's International Earth's Rotation Service may need to add a negative leap second for the first time in history, should the Earth continue to rotate so fast in 2021.
"It is certainly correct that the Earth is spinning faster now than at any time in the last 50 years," senior research scientist with National Physical Laboratory’s time and frequency group Peter Whibberley told the Telegraph, adding that it may be that the change leads to the leap second system being removed altogether.
“It’s quite possible that a negative leap second will be needed if the Earth’s rotation rate increases further, but it’s too early to say if this is likely to happen."
It is predicted that an average day in 2021 will be 0.05 ms shorter than the usual 86,400 seconds.
While a leap or negative leap second may not affect your own timekeeping significantly (you can't use it as an excuse for why you're late for work, for instance), it can have big impacts on satellite communications, which rely on solar time (measured by Earth’s rotation relative to the Sun) aligning with Coordinated Universal Time (UTC). It can also have an impact on all sorts of other computer systems that aren't ready for the change.
In 2012, a big outage famously took place on Reddit when the Linux operating system they were using failed to take into account the leap second. Not everyone is confident that tech will be ready for a negative leap second.