June 29th was the shortest day ever!

Our Earth rotates once every 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 unfolds in milliseconds. But for some years it seems that the Earth wants the opposite spinning faster and faster.

As a result, June 29, 2022 was the shortest day ever recorded by atomic clocks. It went completely unnoticed because the day in question lasted only 1.59 milliseconds less than 24 hours classics. That’s still shorter than the record for the shortest day that had been recorded… on July 19, 2020. It was then a matter of 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 about oscillation of the earth’s axis of rotation?

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

Some believe it is related to the Chandler oscillation. This swing ofthe earth’s axis of rotation resulting in irregular movement of geographic poles on the surface of the globe. About three to four meters. But between 2017 and 2020, that wobble seems to have just…disappeared.

Regardless of the origin of the shortening of the days, if the phenomenon were to continue, it would be necessary to consider introducing a negative leap second. To keep our clocks in step with Sun. With the risk that “skip a second” causing some problems for our computer systems. But according to experts, we are not there yet. There is actually a 70% chance that we have reached a minimum during 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 normal. And scientists predict it will be the same in 2021. Will we resort to a leap second to adjust time? The question is asked.

Article by Nathalie Mayer published on 15/01/2021

We all wanted to get it over with as quickly as possible. year 2020. And it seems that even Earth has done everything to achieve this. She started purring a little faster than usual. Until then, the record for the shortest day was held on July 5, 2005. Our planet had then turned on itself in 1.0516 milliseconds less than the average 86,400 seconds 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 worry about though. A number of circumstances can cause the earth’s rotation speed. The movements of its heart, its oceans, its atmosphere. And even more. Furthermore, already since 1972, on 27 occasions it has been necessary to resort to a leap second to adjust 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.

Want to remove a leap second?

But at a time when the international community is questioning the validity of the principle, scientists are wondering for the first time whether it will be necessary to withdraw a leap second. Because in 2021 they are waiting The Earth rotates at least as fast.

According to their calculations, the average day in 2021 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. During the whole year atomic clocks could accumulate a delay of about 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 is well known, time is timesilver. In order to maintain agreement between the time of the atomic clocks and that determined from the Earth’s rotation, which is not constant, on 31 December 2016 another so-called leap second is added.

Article by 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 Earth’s rotation, which is used to define universal time (UT) based on the Earth’s orientation with respect to stars, is not constant over time. It is therefore necessary to make adjustments if we want to agree with UTC time, which is much more stable because it is based on atomic clocks.

However, it must be remembered that changes in rotation of the earth not done regularly. Thus, the second of universal time to be added is not a result of a decrease at a constant rate of the rotation of our planet. As a result, this addition may take place over several consecutive years or, on the contrary, be postponed indefinitely. It may even be that one day we are led to subtract a second.

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

The time for the leap second is counted…

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

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

One more second in 2005

Paris Observatory article published on 30/12/2005

On January 1, 2006, at 1 a.m., the clocks must be set back by one 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 counting the usual 60 seconds for this minute will therefore show “1 hour” one second ahead, and will need 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. Researchers therefore tend to believe that it belongs to 2005. However, in France due to the time difference compared to UTC during daytimewinter (+1 h), it will actually arrive at 1 o’clock on January 1st… 2006.

This second takes place at the Paris Observatory. In fact, the Time-Space Reference Systems – SYRTE department, through its activities in the 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 course of days and nights, slows down in the long term, mainly due to the effects of lunisolar attraction. In addition, our planet is disturbed by its internal components (core, Coat) and external (atmosphere, oceans).

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

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

This delay 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 associated with the earth’s rotation approaches one second, the insertion of a leap second in UTC allows these two scales to be readjusted between them.

The Paris Observatory provides scientific services trusted by national and international organizations. It is in this capacity that a component of the International Earth Rotation and Reference Systems Service – IERS, located on 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 Earth’s rotation rate since 1830 (in red).
The pink curve represents the influence of the Earth’s liquid core.

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


(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 that needs effective 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 second was defined as the 86,400th. part of an average solar day in 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, Paris Observatory and CNRS creates, within SYRTE, LNE-SYRTE, a laboratory responsible for producing and making available the national references for time and metrology. frequencies.

(4) A partnership between the French Chamber of Horology and Microtechnology – CFHM and LNE manages this way of sending legal time.

Interested in what you just read?

Leave a Comment