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

June 29 was the shortest day on record!

Our Earth rotates once in 24 hours. This is how the length of the day is defined. However, the reality is a bit more complex. Because our planet is not quite so constant. Over time, it tends to turn less and less quickly. This usually plays out in milliseconds. But for some years, the Earth seems to want, on the contrary, spinning faster and faster.

As a result, June 29, 2022 was the shortest day ever recorded by the atomic clocks. It went completely unnoticed because the day in question lasted only 1.59 milliseconds less than the 24 hours classics. It’s still shorter than the record for the shortest day that was recorded… July 19, 2020. It was then 1.47 milliseconds less than 24 hours. This record was also broken again on July 26, 2022. With a day shorter by 1.50 milliseconds.

A question of oscillation of the axis of rotation of the Earth?

But what is happening to the Earth? Several factors can vary the speed at which our planet rotates. The tidal forcesthe climate and the melting polar ice where the movements internal oratmosphere of our Earth. Even the movement of our satellites. The mechanism is difficult to decipher. And no one has any certainty to date as to the origin of the current acceleration.

Some think it is related to the Chandler oscillation. This oscillation of theearth’s axis of rotation results in irregular movement of the geographical poles on the surface of the globe. About three to four meters. But between 2017 and 2020, that wobble seems to have just…disappeared.

Whatever the origin of the shortening of the days, if the phenomenon were to persist, it would be necessary to consider introducing a negative leap second. To keep our clocks in step with the Sun. With the risk that “skip a second” poses some problems to our computer systems. But according to experts, we are not there yet. There is indeed a 70% chance that we have reached a minimum in the duration of the day…

The Earth has accelerated its rotation in 2020!

In 2020, for the first time since measurements began 50 years ago, the Earth has rotated faster than usual. And scientists predict that it will be the same in 2021. Will we resort to a leap second to readjust the time? The question is asked.

Article of Nathalie Mayer published on 01/15/2021

We all wanted to get this over with as quickly as possible. year 2020. And it seems that even the Earth has done everything possible to achieve this. She started spinning a little faster than usual. Until then, the record for the shortest day was held by July 5, 2005. Our Planet had then turned on itself in 1.0516 milliseconds less than the average 86,400 seconds that a day lasts. In 2020, this record was broken… 28 times! And July 19 set a new record with a shorter day of 1.4602 milliseconds.

Nothing to be alarmed about however. A number of circumstances may cause the earth rotation speed. The movements of its heart, its oceans, its atmosphere. And even more. Moreover, on 27 occasions already since 1972, it has been necessary to resort to a leap second to readjust the astronomical time and the time given by the atomic clocks. In 2016, a 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 validity of the principle, for the first time, scientists are wondering whether it will be necessary to withdraw a leap second. Because in 2021, they are waiting for the Earth spins at least as fast.

According to their calculations, in 2021, the average day should last 0.07 milliseconds less than the average 86,400 seconds. July 9 could mark a new record with a day 1.88 milliseconds shorter. Over the whole year, the atomic clocks could accumulate a delay of some 24 milliseconds. But in principle, the use of a leap second only occurs when the difference in the length of the day exceeds 400 milliseconds. So in 2016, we had exceeded 490 milliseconds.

One more second in 2016: when the Earth slows down

It’s well known, time is timesilver. In order to maintain consistency between the time of the atomic clocks and that determined from the rotation of the Earth, which is not constant, an additional so-called leap second of time will be added on December 31, 2016.

Article of Laurent Sacco published on 30/12/2016

Since 1972, the Central Office of the International Earth Rotation and Reference Systems Service located at the Paris Observatory has sporadically added a second to Coordinated Universal Time (UTC). The rotation of the Earth, which is used to define the universal time (UT) from the orientation of the Earth with respect to the stars, is not constant over time. It is therefore necessary to make adjustments if we want to be in agreement with the UTC time, which is much more stable because it is based on atomic clocks.

It should be kept in mind, however, that changes to the rotation of the earth are not done on a regular basis. Thus, the second of universal time to be added does not result from a decrease at a constant speed of that of the rotation of our planet. As a result, this addition may take place over several consecutive years or, on the contrary, be deferred for an indefinite period. It may even be that one day we will be led to subtract a second.

Already in 2012, a leap second had to be added. © euronews

The time of the leap second is counted…

Still, since the introduction of this system, we had to add 26 seconds to the UTC. These additions are generally scheduled either for January 1 or for July 1 at midnight. A 27e is planned for the transition from 2016 to 2017.

Adding a leap second is destined to disappear because the globalization of exchanges with Internet and the use of satellites, in particular with the GPS, do not adapt well to a time that is not solely based on atomic clocks. The disappearance of a UT time in favor of a UTC time will nevertheless be accompanied by other problems that must be resolved. In the meantime, the practice of the leap second is maintained at least until 2023.

One more second in 2005

Paris Observatory article published on 12/30/2005

On January 1, 2006, at 1 o’clock in the morning, the watches will have to be delayed by a small second. Very exceptionally, the minute between midnight 59 minutes and 1 hour will last one second longer than normal, i.e. 61 seconds instead of 60. Any clock that counts the usual 60 seconds for this minute will therefore display “1 hour” with a second in advance, and will have to be corrected, at least for those who need legal time to the nearest second.

In the international “UTC” time scale, this extra second, or “leap” second as it is called, will occur on December 31, 2005 just before midnight. Scientists therefore tend to consider that it belongs to 2005. But in France, because of the time difference compared to UTC during daylight hourswinter (+1 h), it will indeed arrive at 1 o’clock on January 1… 2006.

This second is being played out at the Paris Observatory. Indeed, the Time-Space Reference Systems – SYRTE department, through its activities in the fields of measurement of the Earth’s rotation and time metrology, plays a key role in this event.

The rotation of the Earth on itself, which determines the passage of days and nights, slows down in the long term, mainly because of the effects of lunisolar attraction. In addition, our planet is disturbed by its internal constituents (core, coat) and external (atmosphere, oceans).

However, time is today measured by means insensitive to the moods of the Earth, thanks to 250 atomic clocks belonging to several countries of the globe, including 25 in France. Together, they make it possible to calculate the Coordinated Universal Time – UTC (1).

And the UTC is so regular that there quickly appears a shift between it and the time of days and nights determined by the rotation of the Earth on itself.

This lag can be annoying for some appsan international agreement signed in 1972 stipulates that the difference between the two must never exceed one second (2). This is what leap seconds are for: when the difference between UTC and the time linked to the rotation of the Earth approaches one second, the insertion of a leap second in UTC makes it possible to readjust these two scales. between them.

The Paris Observatory provides scientific services entrusted by national and international organizations. It is in this capacity that a component of the International Earth Rotation and Reference Systems Service – IERS, located at SYRTE, is responsible for predicting and announcing these leap seconds. This decision is then implemented by the international and national authorities responsible for diffusion time.

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

For France, it is the LNE-SYRTE (3) who is in charge of this task. This other component of SYRTE produces the Coordinated Universal Time of the Paris Observatory – UTC(OP). This high-precision reference is used by France Telecom’s speaking clock, housed at the Paris Observatory, to broadcast French legal time; it can be listened to by dialing 36 99. The legal time based on UTC(OP) is also broadcast transparently by encoding on the France Inter carrier wave, so that it can be used at any time by laboratories, industrialists, communities, etc., everywhere in metropolitan France (4).
International discussions underway for several years could lead to a modification of this system. UTC would then become dissociated from the rotation of the Earth and we would no longer have to add leap seconds.

Notes:

(1) The International Bureau of Weights and Measures – BIPM, an international body located in Sèvres, is responsible for calculating UTC. This is a so-called “paper” time scale, known with a delay of up to 6 weeks. Any country in need of efficient time metrology should make its own rough 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 the 86400th part of an average solar day of the year 1900. Today it is defined as 9,192,631,770 periods of the radiation corresponding to the transition between two levels hyperfines of the ground state of the atom of cesium 133.

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

(4) A partnership between the French Chamber of Watchmaking and Microtechnology – CFHM and the LNE manages this means of broadcasting legal time.

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