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No leap second for December 31, 2021

Timekeepers added the final leap second on December 31, 2016, simply earlier than midnight UTC.

The International Earth Rotation and Reference Systems Service introduced in July 2021 that there’d be no leap second added to the world’s official timekeeping in December 2021. The final leap second was December 31, 2016. Since they at all times add leap seconds on the final day of June or December, the following doable date for a leap second is June 30, 2022.

Timekeepers have added leap seconds 27 instances since 1972. There had been leap seconds added on June 30, 2015, and on June 30, 2012. They’re at all times added to the world’s clocks at 23 hours, 59 minutes and 59 seconds Coordinated Universal Time (UTC), on both June 30 or December 31.

The further second is critical for our official timekeeping primarily to maintain our more and more digital world in sync.

EarthSky lunar calendars are back in stock! We’re guaranteed to sell out. Get one while you can!

Digital clock with red numerals 23:59:60.
Image through NASA.

Why do we’d like a leap second?

Isn’t the size of our day set by the rotation of the Earth? Like the ancients who insisted that every one movement within the heavens have to be good, uniform and unvarying, many people as we speak assume that the Earth’s rotation – its spin on its axis – is completely regular. We discovered, appropriately, that the sun, moon, stars and planets parade throughout our sky as a result of the Earth turns. So it’s straightforward to know why we assume that the Earth’s rotation is exact and unwavering. Yet Earth’s rotation doesn’t keep completely regular.

Instead, in comparison with fashionable timekeeping strategies corresponding to atomic clocks, the Earth is a notoriously poor timepiece. Overall, as many know, Earth’s spin is slowing down. The size of our day is rising. The Wikipedia entry on this topic explains it effectively:

Earth’s rotation is slowing barely with time; thus, a day was shorter previously. This is as a result of tidal effects the moon has on Earth’s rotation. Atomic clocks present {that a} fashionable day is longer by about 1.7 milliseconds than a century in the past, slowly rising the speed at which UTC is adjusted by leap seconds. Analysis of historic astronomical information exhibits a slowing pattern; the size of a day elevated about 2.3 milliseconds per century since the 8th century BCE.

Earth’s rotation is slowing, however it might additionally pace up

The slowing is an general pattern. The speedup is extra of a day-to-day phenomenon. TimeandDate has a very interesting article on a slight speed-up in Earth’s rotation in 2020.

The truth is Earth’s rotation is topic to results which might be laborious to foretell. Other short-term and unpredictable modifications outcome from a wide range of occasions. These vary from slight modifications within the distribution of mass within the Earth’s molten outer core, to motion of huge lots of ice close to the poles, to density and angular momentum variations within the Earth’s environment.

For instance, you would possibly recall the devastating Fukushima earthquake that struck in northeastern Japan in 2011. The quake was magnitude 9.0, the most important ever recorded in Japan. It was the world’s fourth-biggest earthquake since 1900. It unleashed a robust tsunami that precipitated the dying of some 16,000 individuals and precipitated a nuclear meltdown at Fukushima Daiichi Nuclear Power Plant. The quake additionally precipitated displacements of parts of the Earth’s crust that sped up Earth’s rotation, shortening the day by 1.6 millionth of a second.

A sawtooth line graph showing a slight speedup in Earth's rotation in 2020.
Variation of daylength all through 2020. The size of day is the distinction in milliseconds between the Earth’s rotation and 86,400 seconds. Image through the article Earth Is in a Hurry in 2020 at TimeandDate.com.

The tides have an effect on Earth’s rotation

If you could have ever been to the seaside, you may be aware of the principle cause our planet is slowing down. That cause is ocean tides. As our planet rotates, it plows previous the good watery bulges (raised largely by the gravitational interplay of the Earth and moon), which serves to gradual it down very like a brake on a rotating wheel. This impact is small, really very small. According to calculations based mostly on the timing of historic astronomical occasions (eclipses), the Earth’s rotation has slowed down by about 0.0015 to 0.002 seconds per day per century.

In different phrases, the times have gotten longer by about 0.002 seconds per day. But the fee at which this enhance happens additionally grows slowly over time. Currently that rate is also about 0.002 seconds, but per 100 years.

So the Earth is slowing down, very slowly. What occurs, although, is that the each day 0.002-second distinction between the unique definition of a day (86,400 seconds) builds up.

After one day it’s 0.002 seconds. After two days it’s 0.004 seconds. By three days it’s 0.006 seconds and so forth. In a couple of 12 months and a half, the distinction mounts to about 1 second. It is that this distinction that has prompted the addition of a leap second.

Atomic clocks (that are correct to a couple of billionth of a second per day) monitor all of this to an especially excessive precision.

The inconstant Earth

Of course, the slowdown isn’t fixed both. According to the U.S. Naval Observatory, from 1973 to 2008, the variation in Earth’s spin has ranged from plus 4 milliseconds to minus 1 millisecond. Over time, that would necessitate a adverse leap second, signifying a rise within the Earth’s rotation pace.

But, because the idea of leap seconds started in 1973, this has by no means occurred.

And, by the best way, if this high quality element on timekeeping appears esoteric or unimportant to you, notice that it’s to not the telecommunications business.

North and part of South America at night with glowing lines arcing between glittering cities.
Modern telecommunications depends on exact timing, and the addition of a leap second forces many programs to be turned off for a second yearly or two. That’s why there’s typically discussions about abolishing leap seconds.

Not everybody thinks a leap second is a good suggestion

The International Telecommunications Union (ITU), a United Nations physique that governs some international points associated to time, has been considering leap seconds for a while. They thought of abolishing the observe, however in November 2015 – with delegates from greater than 150 nations assembly in Geneva – the ITU announced it had decided not to dump the leap second, at the least not proper now. The ITU said:

The choice … requires additional research relating to present and potential future reference time-scales, together with their influence and purposes. A report might be thought of by the World Radiocommunication Conference in 2023.

So they’re nonetheless excited about it.

Consider the ITU’s scenario. Telecommunications depends on exact timing, and the addition of a leap second forces firms to show off many programs for a second yearly or two. To get all such programs in a world business cycled on and off in sync could be a main headache. Consider additionally that the worldwide positioning system (GPS) doesn’t use the leap second system, which causes additional confusion. Many within the business really feel that the periodic addition of a leap second to maintain the 2 measurements in step is cumbersome and wasteful.

Although dropping the concept of a leap second could be a comfort for telecommunication and different industries, within the lengthy (very lengthy) run, it could trigger clocks to get out of synch with the sun, ultimately inflicting 12 p.m. (midday) to happen in the midst of the evening, for instance. At the present fee of change in Earth’s rotation fee, it could take about 5,000 years to amass only a one-hour distinction between the Earth’s precise rotation fee and the atomic clock.

Measuring the spin distinction

How, you might ask, will we even measure such small modifications within the Earth’s rotation? Historically, astronomers (corresponding to these at Britain’s famed Royal Greenwich Observatory close to London) have used a telescope to look at a star move by means of their eyepiece, crossing an imaginary line referred to as the meridian. Then they time how lengthy it takes for the Earth to carry that star again round to cross the meridian once more. This is very correct for on a regular basis functions. For science, it’s not adequate. The wavelengths and the murkiness of the environment restrict its accuracy.

A way more correct technique is to make use of two or extra radio telescopes separated by 1000’s of miles, in a method referred to as Very Long Baseline Interferometry. By rigorously combining the info from every of the telescopes, astronomers successfully have a telescope 1000’s of miles in dimension, which supplies a lot larger decision (detecting high quality element) and measurement of place. This permits them to find out the planet’s rotation fee to an accuracy of lower than a thousandth of a second. They don’t observe stars, nevertheless, however very distant objects referred to as quasars. The NASA video beneath will inform you extra …

Bottom line: World timekeepers have decreed there might be no leap second on December 31, 2021. Overall the spin of Earth is slowing down, however in 2020 it sped up. They will think about a proposal to dump the leap second in 2023. Stay tuned!

Read more from PhysicsWorld: A brief history of timekeeping

“The Age of Miracles: A Novel” tells the story of an imaginary future in which the Earth’s rotation slows suddenly and dramatically

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