Time measurement

The definition of time has been based, since antiquity, on the observation of the stars. For centuries, the second was thus determined by the 86,400th part of the average solar day. In other words, the time between two zeniths of the sun is 86,400 seconds.

Time-measurement systems are based on the rotation of the earth around the sun (year), the moon around the earth (month) and the earth on itself (day). In the course of history, these natural rhythms have been adapted and subdivided in various ways. The construction of the abstraction known as objective time began well before the 17th century, in particular with the manufacture of instruments for measuring regular processes: gnomons, sundials, clepsydras, hourglasses, and then, from the 14th century onwards, with the considerable development of the art of watchmaking.

The shadow of a stake planted in the ground, the gnomon, is used to measure the movement of the sun. It is the ancestor of the “modern” sundial, invented by the Arabs in the 13th century. The water clock and the hourglass make it possible to measure time at all times. The first weight clocks appear at the end of the 13^th Century and the watches in the 15^th Century in England, France and Germany.

In the 18th century, the measurement of longitude was a major scientific, technological and economic issue, particularly for the English and Dutch, great conquerors of the seas. It was John Harrison, an English watchmaker, who solved the problem in 1736 with his H1 marine chronometer.

In Switzerland, at the 17th century, thanks to Huguenot refugees fleeing persecution in France, Swiss watchmaking developed from Geneva to Schaffhausen. In 19^th Century, we are moving from salon watchmaking to observatory watchmaking to develop precision watchmaking (Baldi, 2019). Observatories have had the task of providing scientific and technical expertise since the 17th century like Greenwich in England. They become guarantors of measurement standards.

Two kinds of instruments are used to measure time. The first are used to set a time of day, they are used to determine the time of day. The seconds measure the time interval that has elapsed between two moments. When they are set at a given time, they keep it, which is why they are called timekeepers. The modern astronomer determines the time with his special telescopes: meridian telescope, passage instrument, sextant, prism astrolabe, photographic zenithal telescope etc. He keeps the time thanks to the many timepieces at his disposal: watches, chronometers, astronomical clocks, quartz clocks, atomic clocks, etc.

During the 19th century, industrialisation, urbanisation and the acceleration of transport and communications gave new meaning to the time factor for a large part of the population. An increasing number of interdependent events require precise synchronization, made possible by mechanical systems. Although watches have remained luxury items since their invention in the 16th century, they were within the reach of many people from the second half of the 19th century and became indispensable to everyday life.

In 1840, the Scottish watchmaker Alexander Bain made the first electric clock. Energy is supplied by a battery which powers an electromagnet that makes a pendulum swing. He also imagined that a central clock could send electrical signals to synchronize many other clocks. This idea would be implemented many years later.

In Switzerland, the installation of the telegraph network as early as 1852 made the coexistence of different local times problematic, as the acceleration of communications required a unified system. In 1853, the Federal Council adopted the Bern mean time for all postal and telegraphic traffic. From 1860 onwards, this time was set daily by the Neuchâtel Observatory, which communicated it to the telegraph directorate in Bern. The railways also aligned themselves with Berne time, which became the national standard in the second half of the 19th century.

“… If we want time standards that are absolutely permanent, we must look for them not in the movement of our planet, but in the period of vibration of these imperishable, unalterable and perfectly identical molecules. “
About the internal Oscillations of the atoms as a reference clock.
James Clerk Maxwell, 1870

Towards the end of the nineteenth century, the acceleration and internationalisation of transport and communications necessitated the unification and synchronisation of different national times. Sir Sandford Fleming (railway engineer) had the idea of coordinating time between different cities and between states. In 1875, the Metre Convention was signed in Paris by seventeen States and on this occasion the International Bureau of Weights and Measures was founded, whose mission was to ensure the worldwide uniformity of measurements and their traceability.

In 1884, the International Meridian Conference in Washington set the zero meridian at Greenwich. World time divides the globe into 24 time zones of 15 degrees each, starting from the Greenwich reference frame.

Around 1900, electricity began to spread with the establishment of power grids. The first domestic electric clocks were made in 1918 by Henry Ellis Warren, an American electrician.

The first quartz clock was designed in 1927 by Bell Telephone Laboratories in the USA, it had dimensions comparable to those of a refrigerator. The technology was later miniaturized by Seiko, who marketed the first quartz wristwatch in 1969.

In 1949, the first prototype atomic clock was developed by the National Institute of Standards and Technology in the USA, using the ammonia molecule. Then in 1955, to allow the calibration of the reference clocks of the time, the English Louis Essen and Jack Parry developed the first atomic clock integrating the fundamental frequency of the caesium 133 atom.

It is a real revolution in the scientific world, which can count on a precision never reached before. The new definition of the second, a unit of the International System, was defined in 1967 at the 13th General Conference on Weights and Measures in Paris, and the observation of the passage of stars was definitively abandoned. It is based on 9’192’631’770 periods of oscillation of the frequency of caesium 133. Today, the accuracy of current models, ytterbium atomic clocks, is such that they lose 1 second every 13.8 billion years, the age of the Universe!

The new way of counting the time it induces is completely detached from the march of the stars. An independence that will make it possible to demonstrate that the Earth’s rotation slows down on its axis. This singularity can be explained by the dissipation of energy during tides and by the variation of the Earth-moon distance. Although it is imperceptible – the days are now 2 milliseconds longer than a century ago – it still requires occasional adjustment, so that the coordinated universal time delivered by atomic clocks remains as close as possible to mean solar time.

International Atomic Time (TAI) and Coordinated Universal Time (UTC) (former GMT) are obtained from a combination of data from more than 250 atomic clocks kept by more than 70 timing centres, like METAS in Bern. Every day, the International Bureau of Weights and Measures in Paris collects their data in order to calculate the average: this is Coordinated Universal Time (UTC). This is very precious and much in demand: Google responds to some 7 billion automatic computer synchronization requests per day, and some universities in the United States receive up to 20 billion. This number is set to increase further with the expected explosion of connected objects (20 billion in 2020). Not to mention the GPS and Galileo systems, high-frequency banking transactions or the operation of energy networks (smart grids), all of which require a high degree of time accuracy.

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References
Baldi, R. (2019). Les origines de l’Observatoire cantonal de Neuchâtel. Conference at Club 44 in La Chaux-de-Fonds. Quote from her presentation.
Messerli, J. (2015). Mesure du temps. Dictionnaire historique de la Suisse (DHS). Accessed November 24, 2019: https://hls-dhs-dss.ch/fr/articles/012813/2015-01-25/
Le Ru, V. (2017). La construction scientifique du temps. Fabula – La recherche en littérature. Colloque en ligne “L’art, machine à voyager dans le temps”. Accessed April 10 2020: https://www.fabula.org/colloques/document4697.php
Guyot, E. (1956). La détermination de l’heure autrefois et aujourd’hui. Ciel et Terre, Vol. 72, p. 457. Accessed March 6 2019: http://adsabs.harvard.edu/full/1956C%26T….72..457G
Web Links
Watchmaking blog: https://www.atlantime.com/2015/01/temps-utc-et-montres-gmt.html
Journal de la Haute Horlogerie
Various information from Wikipedia