TIME
Since the dawn of humanity, the sky and the movement of celestial bodies have dictated the base for all time measurement. Day and night, the seasons, a year, the calendar, and of course the hour, all notions of time are regulated by the movement of the stars.
For religious practices, the Babylonians divided the night into three, then six, then twelve equal periods, using as a reference as many equidistant stars in the celestial vault observable every night of the year. The twelve nocturnal hours were doubled for the day to achieve the twenty-four hours of a day. It was the Sumerians who, in 3000 BC, invented the sexagesimal system (base sixty). A calculation base that would later be used by the Babylonians and then spread throughout Mesopotamia. This is how the sexagesimal system became established in time measurement as well as in astronomy and geometry.
Since 1889, the second has been the standard unit of time. It was first defined by dividing the hour into sixty minutes, which were 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 thus become multiples of this standard second.
The International Bureau of Weights and Measures (BIPM) now calculates the atomic second by averaging several hundred atomic clocks distributed across various laboratories and observatories around the world. New generations of optical frequency atomic clocks use different atoms and significantly increase their precision, reaching a drift of 1 second per 13 billion years. Such clocks could soon redefine the standard second of the International System of Units (SI).
In the past, the second was divided into sixty equal periods: the “tierces”. Nowadays, for practical reasons, the second is divided according to the decimal system.
In watchmaking, the second and its divisions are the universally used units. Beyond timekeeping, the precision of the second and its subdivisions is crucial to most scientific fields. Due to the precision achieved in time measurement, the second has become a unit of distance measurement. The standard metre is defined by the distance travelled by light in 1/299,792,458 of a second. Thus, time is used to define distance.