Basic elements › Units of measurement

TIME

Since the dawn of humanity, it is the sky and the movement of the celestial bodies that have established themselves as the basis for measuring time. Day and night, the seasons, a year, the calendar and, of course, the time of day: all notions of time are governed by the movement of the celestial bodies.

1. The sexagesimal system

For religious practices, the Babylonians divided the night into three, then six, then twelve equal periods, taking their guide mark from as many equidistant stars in the celestial vault, observable every night of the year. The twelve nighttime hours were doubled for the day in order to obtain the twenty-four hours of a day. It is to the Sumerians that we owe, in 3000 BC, the sexagesimal system (base sixty). A calculation base that would later be used by the Babylonians, then spread throughout Mesopotamia. This is how the sexagesimal system became established in the measurement of time as well as in astronomy or geometry.

2. The second: official unit of measurement

Since 1889, the second has been the standard value of time. It was first defined by dividing the hour into sixty minutes, themselves divided into sixty seconds. Since 1967, the second has been defined as a multiple of the frequency of caesium 133 (9,192,631.770 Hz). The minute and the hour have since themselves become multiples of this standard second.

The International Bureau of Weights and Measures (BIPM) now defines the atomic second by averaging several hundred atomic clocks located in various laboratories and observatories around the world. New generations of atomic clocks with optical frequencies use other atoms and make it possible to further significantly increase their precision, reaching a drift of 1 second in 13 billion years. Such clocks could soon redefine the standard second of the International System of Units (SI).

In the past, the second was itself divided into sixty equal periods: thirds. Nowadays, for practical reasons, the second is divided according to the decimal system.

In watchmaking, the second and its subdivisions are the universally used units. Beyond time measurement, the precision of the second and its subdivisions is essential to most sciences.

3. The second: unit of measurement of distance

Thanks to the precision achieved in time measurement, the second has become a unit of measurement of distance. The metre is defined as the distance travelled by light in a vacuum in 1/299,792,458 of a second. A time is thus measured in order to determine the distance.