From the time atomic clocks began to be widely used in the 1960s, Earth's timekeepers have had to add the occasional "leap second" to them in order to keep them in sync with solar time. Solar time is determined by the Earth's position in relation to the Sun, the Moon, and the stars.
A second is defined by atomic clocks as 9,192,631,770 periods of the radiation emitted by a cesium-133 atom as it transitions between the two hyperfine levels of its ground state. Over the past 50 years, it has taken Earth slightly more than 24 hours, or 86,400 seconds, to complete a single rotation about its axis, and because of this, timekeepers have added 27 "leap seconds" to atomic clocks since the 1970s.
Leap seconds are added at 23 hours, 59 minutes, and 59 seconds Coordinated Universal Time (UTC), usually on June 30th or December 31st. Leap seconds were added on June 30th in both 2012 and 2015, and the last time a "leap second" was added was on New Year's Eve 2016, when clocks the world over paused for a second.
Leap seconds are added simultaneously around the world, for example, the leap second added on December 31, 2005, occurred at 23:59:60 UTC, 18:59:60 (6:59:60 p.m.) in U.S. Eastern Standard Time and January 1, 2006, 08:59:60 (a.m.) in Japan Standard Time.
2020 was a very odd year
Due to COVID-19, 2020 was already a very odd year, and then something really odd started happening — instead of slowing, as it had over the course of the last century, Earth's rotation sped up. It now it takes slightly less than 24 hours to complete a full rotation, and timekeepers at the Paris-based International Earth Rotation and Reference Systems Service (IERS) were faced with the decision of whether to add a "negative leap second," which has never been done before.
The changeover to a faster rotation occurred at the midpoint of 2020, in July. July 19, 2020 was the shortest day on record, at 1.4602 milliseconds shorter than a full 24 hours. (A millisecond is 1/1000 of a second). In fact, 28 days of 2020 set records for the shortest day, each surpassing the previous record holder for the shortest day, which occurred back in 2005.
Senior research scientist at the Time and Frequency Group at the UK's National Physical Laboratory recently told The Telegraph newspaper, "It is certainly correct that the Earth is spinning faster now than at any time in the last 50 years. 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."
In July 2020, IERS announced that they wouldn't add a negative leap second on December 31, 2020, however, their next opportunity to add a negative leap second is June 30, 2021.
Planetary scientists are not concerned about the speeding up; they have learned that there are many factors that have an impact on the planetary spin—including the moon's pull, snowfall levels, mountain erosion, and possibly global warming. Computer scientists, on the other hand, have more to worry about, as so much of modern technology is based on what can be described as "true time."
On July 1, 2012, a leap second was added, and it brought down many of the day's most popular websites including, Reddit, FourSquare, Yelp, LinkedIn, Gawker, and Qantas.
The extra second confused the servers of many companies, which expected every minute to last for 60 seconds, and not 61 seconds. A commenter on the Programming subreddit recently wrote, "There has never been a negative leap second, and if there is one, everyone who deals with NTP or kernel timekeeping code expects that it will be an appalling sh**show."
The effects of a shorter day
On average, today's days are .5 second shorter than a full 24 hours, and while you're not likely to notice the effect yourself, it has an enormous impact on satellites, telecommunications equipment, and computers, all of which require that solar time match the time kept by atomic clocks.
Software programmers have difficulty dealing with either positive or negative leap seconds because they make time appear to go either backwards or forwards. For financial markets in the U.S., the addition of a leap second occurs during a trading day and can cause havoc.
After the insertion of a leap second on June 30, 2015, the Intercontinental Exchange, which owns clearing houses and stock exchanges, including the New York Stock Exchange, had to cease operations for 61 minutes. That leap second also caused a 40-minute interruption on Twitter, Instagram, Pinterest, Netflix, Amazon, and Apple's streaming music service.
Google's servers dealt with the June 30, 2015 leap second by "smearing" it out over a 24-hour period. On its servers, Amazon followed a different format, which led to Google and Amazon being timewise out of sync with each other for a day.
A commenter on the Y Combinator website explained the problem of adding a negative leap second as: "Going back one second is what generally happens now when a Linux box does the "inserting leap second" thing. It goes from 23:59:59.999999 to 23:59:59.000000, then runs that whole second again. You get to 23:59:59.999999 again, and then you finally roll over to 00:00:00.000000. From the perspective of the typical time_t rendering of Unix time, there is no way to uniquely represent that 61st second. It just 'disappears'.
What's causing Earth's rotation to speed up?
A 2015 study in Science Advances attributed Earth's faster rotation to global warming, which is causing glaciers to melt, thus increasing sea levels. This redistribution of mass is making the Earth more round, and this in turn is making it spin more quickly.
On April 11, 2011, a devastating magnitude-9.0 earthquake struck northeastern Japan. Besides unleashing a powerful tsunami that killed around 16,000 people and caused a meltdown at the Fukushima Daiichi Nuclear Power Plant, the earthquake also displaced portions of the Earth's crust. This sped up the planet's rotation, causing April 11th to be 1.6 millionths of a second shorter than other days that year.
So, if you thought that 2020 dragged on forever, or else flew by, Earth clearly had its own opinion on the matter, and only time will tell if this speedup will continue.