THE STRUGGLE FOR TIME MEASUREMENT AND PRECISION HAS BEEN AS LONG AS THE HISTORY OF HUMAN KIND. When the very first second of the new year 2005, January 1st ticks off in the Southern Pacific islands of Fijis and, from there, around the world 24 time zones, few people will realise that the exact time is being calculated in Paris using the Global Positioning Satellite network to compare atomic clocks from different continents. "We are constantly using GPS satellites to work out the average of approximately 200 atomic clocks located in about 60 countries around the world," says Felicitas Arias, the head of the Time Section at the BIPM in Paris which is in charge of setting the official time for 50 leading countries, including the USA. The precision with which the second is calculated today is in the order of 10-15. That is to say it can lose or gain one second in 30 million years! Since 1967, Cesium atomic clocks have revolutionized time measurement standards. Before them, time was based on astronomical Earth revolution. In 1967, the atomic second was defined as the interval of time taken to complete 9,192,631,770 oscillations of the atom of Cesium-133. The atomic time is so exact that, since 1961, 32 additional seconds have been introduced to bring universal Time into compliance with real Earth's time. Because of the slowing down of the Earth rotation, days get longer by several millionths of a second. The last leap second has been introduced on 1 January 1999, at 0 h and none will be added next year. For the past 15 years, half a dozen of leading laboratories around the world, like the Observatoire de Paris, have been developing new techniques to elaborate a new generation of atomic clocks, using different atoms maintained at ultra-low temperatures to improve time measurement precision. History of the time measurement instruments shows that the same efforts were constant over centuries. In the 16th century, the best sundials had a precision of a minute. From 1967 to 2001, the second accuracy has improved by a factor of 10 000 to reach the actual precision of 10-15. The research and the competition for new technical tools to gain time measurement accuracy are far from over. In 1995, the first fountain clock, built by the BNM-SYRTE of the Observatoire de Paris was first to get the best performances in the world in terms of stability with an accuracy of 10-15. But more promising is the next generation of atomic clocks, the so-called “optical clocks”, with the Strontium atom. The gain in precision would be so great that the science of horology could undergo another revolution that may well require that the second be once again redefined as in 1967. But serious obstacles remain for the “optical clocks”. Because of the effects of Einstein relativity, optical clocks are a nightmare to set up. But why do time makers want clocks accurate to one second every 30 billion years? Because super-accurate clock technology will be used immediately in new communications technologies, in testing fundamental theories in physics and in probing human diseases. They should also improve navigation with GPS to the centimetre accuracy. © text : Frédéric Castel
François Arago, a famous scientist and director of the Observatoire de Paris, initiated in 1843 the construction of the huge cupola on the roof of this 17th century building. The whole cupola, which protects a 38 cm diameter telescope, is a rotating structure.
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World time is calculated in Paris. Felicitas Arias (in the center), the head of the Time Section at the International Bureau of Weight and Measurements in Paris (BIPM), with the team in charge of setting the international uniform time scale for the world
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Pierre Arman's assemblages of clocks in front of the Saint Lazare railway station (1985) in Paris. This public sculpture, which ironically shows different times in front of the train station’s main clock, is named "Everybody’s time" (“Le Temps de Tous”).
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In order to constantly improve the accuracy and sometimes even go beyond the 10-15 range of accuracy, experts at the Observatoire de Paris laboratory constructed in 1998 a dual fountain -- still in use -- that can operate with both Rubidium and Cesium.
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On December 9, 1874, Captain Ernest Mouchez led a major astronomical expedition in the Indian Ocean to study and to take photographs of Venus passing in front of the Sun. Later on, he was appointed admiral and was assigned in 1878 as director of the Obser
ZEF0023155 © Franco Zecchin
The small cupola on the top of the Observatory of Paris with the Eiffel tower in the skyline of Paris. This cupola with its telescope was used in the 18th and 19th century before a larger cupola was erected in 1850. The Observatoire, built in 1672, is the
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Laser are used as tools to compare different atomic clocks. Daniele Rovera, a researcher of the BNM, is the lab where stabilized lasers with a stability below 10-14 at 1 second. These lasers are used as secondary standards and local oscillators in freque
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A few years ago, the BNM-SYRTE lab of the Observatoire de Paris, like other famous labs around the world, initiated work on “optical clocks” by using the Strontium atom. Even if the performances of this new atomic clock are not yet as precise as the curre
ZEF0023019 © Franco Zecchin
At the BNM, one can see an optical fiber carrying some visible light of the 'femtosecond' laser and its spectrum on a piece of paper just in front.
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Optical fibers, protected in flexible metallic pipe, are used to polarize the laser light, and then to send it to the vacuum chamber to cool the atoms with lasers.
ZEF0023018 © Franco Zecchin
The huge cupola on the roof of the Observatoire protects a 38 cm objective telescope with a focal length of 9 m, is an amazing rotating structure which enables -- up to now -- astronomical observations of 360 degrees in the sky of Paris. This masterpiece
ZEF0023156 © Franco Zecchin
This is the optics table where several laser beams are going through a fiber-connected network with diodes, fiber ports and Rubidium cells that control the laser beam. This is a Rubidium table part of the dual atomic fountain clock which is also a Cesium
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D’Ons-en-Bray's astronomical clock. The nicely decorated astronomical pendulum was built by the famous 18th century Paris clockmaker P. Fardoil in 1710 for Louis-Leon Pajot d’Ons-en-Bray, the director of the French postal service at the time. Its main di
ZEF0023162 © Franco Zecchin
On the left side, part of the Rubidium table of a dual atomic fountain clock which is also a Cesium atomic clock. On the right, the vertical metallic box contains a vacuum system and the surrounding magnetic shields, where the cold atoms are produced.
ZEF0023017 © Franco Zecchin
The simplest sundial, built with a vertical stick rising from a flat horizontal surface, has been used for at least 3000 years. As the Sun rises, then passes the highest point in its path and finally sets, the shadow rotates around the stick in a clockwis
ZEF0023153 © Franco Zecchin
This is a mobile quadrant of the early 18th century made out of metal and equipped with four legs and two refractors. The first instrument of this type was built in 1668 by Abbé Jean Picard. One can measure time based on the rotation of the Earth by obser
ZEF0023163 © Franco Zecchin
To use this kind of 18th century mobile telescope, called a transit instrument, its base axle has to be put in East-West direction in order to observe the transit of the stars at the meridian according to the astronomical star time schedule. With this mod
ZEF0023164 © Franco Zecchin
Rodolphe Le Targat, a PhD student at the Observatoire, working on the optical clock using Strontium.
ZEF0023020 © Franco Zecchin
An optical fiber carries some visible light of femtosecond laser.
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Daniele Rovera working on the femtosecond laser. The “femtosecond” lasers are used to compare state-of-the-art optical atomic clocks between them, and with the Cesium clocks.
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The technical improvements made to this fountain, together with new measuring procedures could allow to explore some day the 10-16 range of accuracy for both kinds of atoms. From left to right: Harold Marion, a PhD. student from Paris VI/ P & M. Curie, Lu
ZEF0023023 © Franco Zecchin
Rodolphe Le Targat working on the optical clock using Strontium. In the middle, one can see the vacuum chamber where the Strontium atoms are cooled by laser technique.
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This chronograph, used in the early part of the XXth century, was connected to an electrical meridian instrument, in order to print the exact time on a roll of paper, when a star was observed. The transit instruments are still used around the world for as
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Because of too many calls to the Observatoire asking for the official time and making the phone line constantly busy, its director, Ernest Esclangon, in 1933, decided to build the first speaking clock in the world. On February 13, the new technique combin
ZEF0023154 © Franco Zecchin
In 1952, at the Observatory workshop, André Danjon, built this new electric astrolabe which works like a marine sextant with a mobile prism and determines time with an incredible accuracy of 1/125th second. The innovation of Danjon’s prismatic astrolabe,
ZEF0023028 © Franco Zecchin
This is a mobile atomic clock of the 60’s enclosed in a trunk which was transported by plane to compare time in several observatories before the GPS network which enables clock synchronizing today. This heavy piece of luggage from the Observatoire de Pari
ZEF0023152 © Franco Zecchin
These two clocks are set up on the base of the 38 cm telescope of the Observatoire de Paris and they are crucial to locate and target stars in the sky during astronomical observations. This state of the art telescope in 1854 was an essential tool for sky
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In front of Pierre-Simon Laplace statue, the famous 19th C. mathematician, the container of a Cesium atomic clock whose atomic tube has been removed. Historians credit L. Essen and J. Parry in England with the first successful atomic clock in 1955 using C
ZEF0023165 © Franco Zecchin