June 29 is the shortest day on record!

Our Earth rotates once every 24 hours. Thus the length of the day is defined. However, the truth is a little more complicated. Because our planet is not very constant. Over time, it tends to slow down and become less prone. It usually plays in milliseconds. But within a few years, the Earth seemed to want, on the contrary, spin faster and faster.

Because of this, June 29, 2022 is the shortest day ever recorded in atomic clocks. This went completely unnoticed because the day in question lasted only 1.59 milliseconds less than 24 hours classics. This is even shorter than the record for the shortest day on record… on July 19, 2020. Previously it was a question of 1.47 milliseconds in less than 24 hours. This record was broken again on July 26, 2022. With a day shorter by 1.50 milliseconds.

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

But what happened on Earth? A number of factors can change the speed of our planet’s rotation. the water forcethe climate and the melting of polar ice where the movements inside oratmosphere on our Earth. Even the movement of our satellites. The mechanism is difficult to understand. And no one has any certainty of the date about the beginning of the current acceleration.

Some think it is related to the Chandler oscillation. This oscillation ofthe axis of rotation of the earth results in irregular movement of the 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.

Whatever the source of the shortening of the days, if the phenomenon continues, it is necessary to consider the introduction of a negative jump in the second. To keep our clocks in step with Sun. At the risk of “skip a second” Caused some problems with our computer systems. But according to experts, we’re not there yet. There is actually a 70% chance that we have reached at least the duration of the day…

The Earth is speeding up its rotation in 2020!

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

Article on Nathalie Mayer published on 01/15/2021

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

However, there is nothing to worry about. Many conditions can cause the speed of the earth’s rotation. The movements of its heart, its oceans, its atmosphere. And more. In addition, 27 times since 1972, it has been necessary to resort to a leap second to readjust astronomical time and the time given by atomic clocks. In 2016, one second was added on December 31 at 23 hours, 59 minutes and 59 seconds.

Take a leap second?

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

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

Another second in 2016: when the Earth slows down

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

Article on 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 occasionally added one second to Coordinated Universal Time (UTC). The rotation of the Earth, which is used to define universal time (UT) from the orientation of the Earth relative to star, not constant over time. So it is necessary to make changes if we want to agree on UTC time, which is more stable because it is based on atomic clocks.

It should be kept in mind, however, that the changes in rotation of the earth not done regularly. Thus, the second universal time added is not the result of a decrease in the constant speed of rotation of our planet. As a result, this increase may occur in several consecutive years or, on the contrary, be delayed for an indefinite period. It may even one day lead us to subtract a second.

In 2012, a leap second should be added. © euronews

The leap second time is counting down…

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

Adding a jump second is destined to disappear because the globalization of exchanges of Internet and the use of satellites, especially GPS, do not fit well with a time that is not based solely on atomic clocks. The loss of a UT time in favor of a UTC time however is accompanied by other problems that need to be solved. Meanwhile, the practice of leap second is continued until 2023.

Another second in 2005

Paris Observatory article published on 12/30/2005

On January 1, 2006, at 1 a.m., the vigils must be delayed by a few seconds. Very strangely, the minute between midnight 59 minutes and 1 hour lasts one second longer than normal, ie 61 seconds instead of 60. Any clock counting the usual 60 seconds for this minute will show “1 hour” with one second in advance , and should be corrected, at least for those who require 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 before midnight. So scientists probably think it belongs to 2005. But in France, because of the time difference compared to UTC in the hours of the day.winter (+1 h), it will actually arrive at 1 o’clock on January 1… 2006.

This second one was played at the Paris Observatory. In fact, the Time-Space Reference Systems – department of SYRTE, through its activities in the field of measuring the rotation of the Earth and metrology of time, has an important role in this phenomenon.

The rotation of the Earth itself, which determines the passage of days and nights, slows down in the long term, mainly due to the effects of lunisolar attraction. Furthermore, our planet is disturbed by its internal components (core, clothing) and outside (atmosphere, oceans).

However, time is now measured in a way that is not sensitive to Earth conditions, thanks to 250 atomic clocks belonging to many countries in the world, including 25 in France. Together, they make it possible to calculate Coordinated Universal Time – UTC (1).

And UTC is so regular that there is a noticeable shift between it and the time of day and night determined by the rotation of the Earth itself.

This lag can be annoying for some appsan international agreement signed in 1972 stipulated that the difference between the two should not exceed one second (2). This is what leap seconds are for: when the difference between UTC and the time associated with the rotation of the Earth is close to one second, inserting a leap second in UTC makes it possible to adjust these two scales.

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

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

For France, it is LNE-SYRTE (3) who is in charge of this task. This other part of SYRTE produces Coordinated Universal Time at the Paris Observatory – UTC(OP). This high precision reference is used by the France Telecom talking clock, located at the Paris Observatory, to broadcast the legal time in France; it can be heard by dialing 36 99. The legal time based on UTC(OP) is also broadcast transparently by encoding the France Inter carrier wave, so that it can be used at any time in laboratories, industrialists, communities, and others. , anywhere in metropolitan France (4).
International discussions that have been going on for several years may lead to a change in this system. UTC can be separated from the Earth’s rotation and there is no need to add leap seconds.


(1) The International Bureau of Weights and Measures – BIPM, an international body located in Sèvres, is responsible for calculating the UTC. This is a so-called “paper” time scale, which is known to have a delay of up to 6 weeks. Any country that needs efficient time metrology should create its own real-time standard version of UTC. For France, this reference is Coordinated Universal Time at the Paris Observatory – UTC(OP).

(2) Until 1960, the second unit was defined as the 86400th part of the average solar day in the year 1900. Today it is defined as 9,192,631,770 periods of radiation corresponding to the transition between two levels of hyperfines in the ground state of atom of cesium 133.

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

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

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