June 29 is the shortest day on record!

Depuis l’avènement de l’horloge atomique, jamais un jour n’avait été aussi court que le 29 juin 2022. © hacohob, Adobe Stock

Our earth rotates once every 24 hours. This is how the Duration that day. However, the reality is a bit more complicated. Because our planet is not very stable. Over time, it becomes less and less rapid. It usually runs in milliseconds. But for some years, the Earth wants, on the contrary, Spins faster and faster.

As a result, June 29, 2022 is the shortest day on record Atomic clocks. The day in question was 1.59 milliseconds short, so it went completely unnoticed. 24 hours Classic. That’s still less than the record for the shortest day on record… July 19, 2020. That was 1.47 milliseconds short of 24 hours. This record was also broken again on July 26, 2022. One day was less than 1.50 milliseconds.

A question about the oscillation of the Earth’s axis of rotation?

But what happens to Earth? Many factors can cause it Speed Our planet revolves around it. Tidal Powers, The Climate And this Melting polar ice Where Movements internal orAtmosphere of our earth. Even the motion of our satellites. The mechanism is difficult to understand. To date no one has any certainty as to the origin of the current acceleration.

Some think it is related to the Sandler wave. This oscillationEarth’s axis of rotation causing erratic movement Geographical poles on the surface of the globe. About three to four meters. But between 2017 and 2020, that wobble seems to have disappeared.

Whatever the origin of the shortening of days, if this phenomenon persists, it is necessary to introduce a negative leap second. To keep our clocks in step the sun. with the risk of “Wait a second” It causes some problems to our computer systems. But according to experts, we’re not there yet. In fact there is a 70% chance that we will have reached the minimum during the day…

In 2020, Earth Accelerated Its Rotation!

In 2020, the Earth rotated faster than usual for the first time since measurements began 50 years ago. And scientists predict that the situation will be the same in 2021. Can a leap second be used to change time? The question is asked.

The article Nathalie Meyer Published on 01/15/2021

We all wanted to finish this as quickly as possible. The year is 2020. Even Earth seems to have done its best to achieve this. She started spinning a bit faster than usual. Until then, the record for the shortest day was July 5, 2005. Our planet orbits itself in 1.0516 milliseconds, rather than the average of 86,400 seconds in a day. This record was broken in 2020… 28 times! And July 19 set a new record with the shortest day of 1.4602 milliseconds.

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Nothing to worry about though. It can be due to many circumstances Earth’s rotational speed. The motions of its heart, its seas, its atmosphere. Further. Also, on 27 occasions already since 1972, it has been necessary to resort to a leap second to reset astronomical time and the time provided by atomic clocks. In 2016, one second was added on December 31 at 23 hours, 59 minutes and 59 seconds.

Remove a leap second?

But at a time when the international community is questioning the merits of the policy, for the first time, scientists wonder if removing a leap second is necessary. Because they are waiting for 2021 Earth is spinning at least as fast.

According to their calculations, in 2021, the average day should be 0.07 milliseconds shorter than the average of 86,400 seconds. July 9 will mark a new record for a day shorter by 1.88 milliseconds. Throughout the year, the Atomic clocks Some can accumulate up to 24 milliseconds of delay. But in principle, the use of a leap second occurs only when the difference in day length exceeds 400 milliseconds. So in 2016, we surpassed 490 milliseconds.

One more second in 2016: When the Earth slows down

Everyone knows that time is money. To maintain consistency between the time of atomic clocks and the time determined from the Earth’s rotation, which is not constant, an extra second called leap time will be added on December 31, 2016.

The article Laurent Sacco Published on 30/12/2016

Since 1972, the Central Office of the International Geospatial and Reference Systems Service, located at the Paris Observatory, has periodically added a second to Coordinated Universal Time (UTC) rotation of the Earth, which is used to define Universal Time (UT) from the Earth’s orientation. Stars, is not constant over time. So if we want to agree with UTC time, it is necessary to make changes, which is more stable because it is Atomic clocks.

However, keep in mind that changes Earth’s rotation Not done on a regular basis. Therefore, the second of universal time to be added does not occur due to a decrease in the constant speed of our planet’s rotation. As a result, this addition may take place over several consecutive years or, conversely, be postponed indefinitely. One day we may be led to spend a second.

The time of leap second is calculated by…

Still, since the system was introduced, 26 seconds had to be added to UTC. This addition is usually scheduled for midnight on January 1 or July 1. A 27e It is planned to change from 2016 to 2017.

Adding a The second leap A rule that disappears due to the globalization of exchanges Website And the use of satellites, in particular GPS, do not adjust to a time that is not based solely on atomic clocks. Even if UT time disappears in favor of UTC time, it will be accompanied by other issues that need to be addressed. Meanwhile, the leap second practice is being maintained until at least 2023.

Another second in 2005

Paris Watch article published on 12/30/2005

On January 1, 2006, at 1 a.m., the clocks were set back a tiny second. Very exceptionally, the minute between midnight 59 minutes and 1 hour lasts one second longer than normal, i.e. 61 seconds instead of 60. Any clock that counts the usual 60 seconds to this minute will show “1 hour” one second earlier. , and for those requiring at least a statutory period, should be fixed.

On the international “UTC” time scale, this extra second, or so-called “leap” second, occurs just before midnight on December 31, 2005. So scientists think it belongs to 2005. But in France, due to the time difference compared to UTC during daylight hoursWinter (+1 hour), it will actually arrive on January 1… 2006 at 1 o’clock.

The game is played at the Paris Observatory. Indeed, the Department of Time-Space Reference Systems – SYRTE, through its activities in the fields of Earth’s rotation and chronometry, plays an important role in this phenomenon.

The rotation of the Earth, which determines the course of days and nights, decreases over long periods, mainly due to the effects of lunar solar attraction. Additionally, our planet is disturbed by its inner elements (Gor, Code) and external (atmosphere, oceans).

Today, however, time is measured insensitive to Earth’s mood, thanks to 250 atomic clocks from around the world, including 25 in France. Together, it makes possible the calculation of Unified Universal Time – UTC (1).

Because UTC is so regular, there will soon be a shift between it and the time of day and night determined by the Earth’s rotation.

While this change is annoying for some applications, an international agreement signed in 1972 requires that the difference between the two be no more than one second. (2). This is what leap seconds are: when the difference between UTC and the time associated with the Earth’s rotation approaches one second, inserting a leap second into UTC makes it possible to adjust these two quantities between them.

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The Paris Observatory provides scientific services commissioned by national and international organizations. It is in this capacity that a component of the International Geospatial and Reference Systems Service – IERS, located at SYRTE, is responsible for predicting and reporting these leap seconds. This decision is enforced by international and national authorities diffusion Time.

Long-term slowdown in Earth’s rotation speed since 1830 (in red).
The pink curve represents the influence of Earth’s liquid core.

For France, it is LNE-SYRTE (3) The person in charge of this task. This other component of SYRTE makes up the Paris Observatory’s Coordinated Universal Time – UTC(OP). This high-precision reference is used by France Telecom’s talking clock, located at the Paris Observatory, which broadcasts French legal time; You can ask for it by dialing 36 99. Official time based on UTC(OP) is also broadcast transparently by encoding France intercarrier wave so that it can be used by laboratories, industrialists, communities, etc. at any time. , everywhere in metropolitan France (4).
International discussions over the years have led to changes in this system. UTC is then separated from the Earth’s rotation, and we don’t need to add leap seconds.


(1) The International Bureau of Weights and Measures – BIPM, an international organization located in Sèvres, is responsible for calculating UTC. This is the so-called “paper” timescale, which is known to be as late as 6 weeks. Any country that requires efficient timekeeping must create an approximate version of UTC in real time. For France, this reference is the Coordinated Universal Time of the Paris Observatory – UTC(OP).

(2) Until 1960, the unit of second was defined as 86400th of a mean solar day of 1900. Today it is defined as 9,192,631,770 periods of radiation.ground level of the atom Cesium 133.

(3) National Metrology and Testing Laboratory – A framework agreement between the LNE, the Paris Laboratory and the CNRS creates, within SYRTE, LNE-SYRTE, the laboratory responsible for producing and making available time metrology and national references. Frequencies.

(4) The French Chamber of Watchmaking and Microtechnology – a partnership between the CFHM and the LNE – manages this mechanism for broadcasting legal time.

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