History of the measurement of time

See also: Unit of time

The Histoire of the measurement of time by the Homme goes back to his origins. The measurement of the Temps was always one of its major concerns, in particular to organize the social life, nun and economic companies. The periodic phenomena of the medium where he lived - as the return of the sun with the Saison S or the lunar Cycle - served as first references.

But with time, it took as a starting point physical phenomena, of which it had noticed the periodic character, to conceive and develop dispostifs of measurement of increasingly precise time. The mechanical clocks are an example, which made it possible to the Men to constantly know the hour and in any place.

Observation of the Earth

To locate itself in the Time, the Men thus first of all based themselves on the terrestrial periodic phenomena. The first men counted the Hiver S or the be S to restore the last events. The calendar quite naturally rose from these observations

The calendar

See also: Calendar

The installation of a calendar is difficult. Indeed, the majority of those are defined compared to the Sun or with the the Moon. However, one solar year counts approximately 365,24219 days and a lunar month approximately 29,53 days. One thus includes/understands the difficulty to divide one year into equal shares.

Several calendars followed one another or coexisted through the Histoire. One can quote in particular the calendars Gregorian, orthodoxe Julien , , Hebrew, Moslem, copte, Hindu, Egyptian, hahai' S, zoroastrien, INCA, Chinese or republican. Each great civilization had its own calendar; it was a means of marking its time.

The first traces of true “national” calendars go back with the Egypt and the Mayas and Aztèques, at which the priests was as astronomers and had a calendar much more precise as in Europe.

In -45, Jules César request with the Greek astronomer Sosigène of Alexandria to reform the Roman Calendar not very precise. It thus creates the calendar “Julien” which innovates, creating the years Bissextile S. the one year intermediate duration is thus 365,25 days, which approaches the true astronomic year, with a shift of only three days every 400 years.

In 1582, the Julien calendar showed already a delay of 11 days over the astronomical time. Not enough so that people are affected but sufficient so that the monks have the feeling not to celebrate more Easter at the good date. The Pape Gregoire XIII is addressed to the astronomer Luigi Giglio who proposes that the years centenaries are bissextile only if they are divisible by 400; moreover it refixe the date of the vernal equinox to March 21st. It thus gives rise to the Gregorian Calendrier which we currently use. The one year intermediate duration of the Gregorian calendar is 365,2425 days, which approaches even more precisely the true duration of the astronomic year 365,2422… days. Its installation involved also suppression the 11 days to correct the existing shift between the dates of the seasons and their real occurrence.

The calendar is very important from a point of view religious. Indeed, can one celebrate the festivals every year at the same date? The catholic festivals and the orthodoxe festivals are shifted 13 days currently, the orthodoxe Church not having adopted the Gregorian calendar.

Gnomon with the sundial

The first to be itself concerned about the division of the day in units of time are the Égyptiens, but initially with a religious aim. The first period divided into " heures" was the night, there are approximately forty and one centuries. To locate the flow of time, the sky had been divided into 36 decans associated with divinities, each decan consisted of one or more stars. The night observers supervised the procession of the décans  ; and according to the time of the year the number of decans visible of the twilight at dawn was variable. With the Solstice of summer, when the nights are shortest and when the to raise heliacal of Sirius approach, only 12 decans were observable with certainty, the others were lost in the gleams of raising or the setting one. Towards 2100 before our era it was decided to preserve only the observation of 12 decans during the night: this one was thus divided into 12 parts which were maintained all the year. Thus the first period, other that the year, precisely marked out was the night and not the day. But that related to only the Pharaon, and his relation with the gods. One found in sarcophagi of the tables giving the decans dividing the night into 12 parts.

Six centuries later the texts also indicate a division of the day in 12 hours, undoubtedly by symmetry with the night. With this division is associated the first true Sundial known: it is an L-shaped part. It is directed in the East-West direction, the shade projected by the vertical upright on the other part indicates the hours on both sides midday. The graduations are fixed and do not take account of the influence of the season: one day is divided into 12 hours whatever its duration. The hours indicated thus do not have the same length throughout the year, longer the summer than the winter. In Egypt the variation is rather weak (40  %) not to be too sensitive (contrary to Western Europe where, between the winter and the summer, the duration of the day varies the simple one with the double). This division twice 12 hours is preserved and adopted by the Chaldée NS towards the VII E then spreads themselves in Greece and remains thereafter, until our days. Our 24 hours are Egyptian.

The Babylonian system of numeration was sexagesimal, the division of the hour and of the minutes this system began again. Thus, the hour is divided into 60 minutes and the minute in 60 seconds. Sixty is a number which with the characteristic to have a great number of dividing whole (1, 2,3,4,5,6,10,12,15,20,30 and 60), which facilitates astronomical calculations.

Periodicity of the physical phenomena

But the Man was not satisfied to observe and use nature. He also knew to use his direction of the observation and his intelligence to design measuring instruments of the time which is not based inevitably on natural phenomena which are not under its control. The uses of the daily life (speaking time in a council, a lawsuit, ringings of the bells for the hours of the offices, meetings of councils of cities, the curfew, etc) provided him the motivation of it.

The clepsydre and the sand glass

The Égyptiens used the Clepsydre, large vase bored at its base, graduated inside and which lets escape a thin thread of water. The Greek improved it to make it more precise. They added a dial and a needle to him, transforming it into a genuine measuring instrument.

The Sablier, him, is based on the same principle, except water is replaced by Sable. It is used more to measure intervals of time that to indicate the hour. A usually quoted anecdote is that of Christophe Colomb which, in 1492, at the time of its voyage towards the America, used to take stock a sand glass that it turned over since his departure every half-hour. Its origin is unknown.

These tools becoming not very precise on long periods and the variations of accumulating time, it became urgent that the scientists find a solution.

The mechanical clock

The first Horloge S mechanics appear at the 14th century. At the beginning they sound the bells, do not have a dial and, when they are equipped with it at the 15th century, there will not be that a needle, that of the hours.

These first clocks consist schematically of a driving weight which actuates a spur gearing, which incidentally makes turn the needles. The whole constitutes a clock only if one can control the fall of the weight. It is the appearance of the exhaust which will transform this simple assembly of gears into genuine clock. The exhaust alternatively makes it possible to release then to block the fall of the weight, thanks to an oscillating mechanism .

In these first clocks this mechanism is a foliot , simple stem at the ends of which two masses are hung, which can oscillate horizontally around a vertical axis supporting it in its medium. The masses confer the Inertie necessary to him to stop the fall of the weight. Interdependent of the axis of oscillation, two pallets alternatively come to block the wheel of meeting (which gives its name to this first type of exhaust) that involves the driving weight.

Here for example illustrations of this principle will be found. This very clever mechanism is also very delicate to precisely regulate; frictions and shocks are important, difficult to control. And especially, each element takes part indiscernablement in the two functions motor coach and regulating.

The pendulum and the spiral spring

Among many other phenomena, Galileo studied the Pendule oscillating and foot-note that the period (duration of an outward journey and complete return) of the pendulum seemed to be remarkably constant for a given pendulum. It drew in 1641 a project of Horloge regulated by an oscillating pendulum without building it. It is finally Christiaan Huygens and Solomon Coster which built the first Horloge with pendulum in 1657.

Technological advance is important; conceptual progress is even more. The functions regulating and driving are clearly identified and separated, which will make possible of the precise adjustments. The first clocks delay the fall of a weight thanks to an irregular mechanism oscillating through an exhaust. The pendulum clocks maintain the regular movement oscillating the pendulum by taking through the exhaust right energy necessary to a weight which goes down.

In 1675, Huygens also invents the spiral Ressort, which will play the part of the pendulum in the watches.

Towards a removable measuring instrument…

One saw it with the example of Christophe Colomb, the measurement of time in the marine is essential. In particular to be able to determine the Longitude, operation which implies to preserve on board the hour of the wearing of departure. So much so that the governments British and Spanish offered strong rewards to the scientist who will succeed in building a transportable stop watch having a precision and, especially, a stability sufficient to make a complete point at sea. Because it is impossible to make function a pendulum on a boat because of the Roulis.

Such an measuring instrument is invented by the British clock and watch maker John Harrison in 1737. It creates an enormous stop watch of an astonishing precision. It gains the price in 1764 only with its fifth prototype which, in two months of voyage, shifted only few seconds, performance up to that point ever reached. In reward, it receives, after some tergiversations, of king de Grande-Bretagne a pretty revenue for its old days…

The clock for all

During the 18th century, it is of good tone, when one has the means of them, to have a clock. The refinement of its decoration and its precision indicate the richness of its owner. This precision is not of any utility in the everyday life but the taste of the " the last cri" technological is not a mania appeared with the 21e century. During the 19th century, the industrialization of the clock industry will gradually make it possible all to have a clock or a clock, at the same time as the diffusion of the hour will be spread with the Télégraphe and than the standardization of time will become necessary in particular with the development of the Railroad which obliges to synchronize the clocks of a whole country. Then time also will be introduced into the factories with the measurement of the working time and the productivity.

Modern means

From now on, the mechanical clocks are not any more with the day order. More precise and more compact means were developed. On the matter, the clock with quartz constituted a true progress.

Quartz is a form of Dioxyde of silicon (SiO) which abounds in nature. Like all rigid materials, it resounds with a Fréquence which is clean for him (for example 32  768 Hz for quartz of the current watches). Moreover, its hardness enables him to have frequencies of raised vibrations, which is very favorable for the precision. However when a quartz crystal vibrates, of weak electric charges appear and disappear on its surface. It is the piezoelectric effect. These loads are detected and are used for controlling and stabilizing the operation of an electronic oscillator. The precision obtained is ten times more important than the best of the mechanical mechanisms of clock industry. The first oscillator with quartz to be functioned on this principle appears in 1933 but its size is closer to a refrigerator than of a watch bracelet. Produced of a great miniaturization, the latter appears only in the Années 1970.

But to meet the increasing need for precision of science and state-of-the-art technologies, quartz alone appears still too vague. The following stage will be the Atomic clock. In this one, the stability of an electronic oscillator does not rest any more on the oscillations of a crystal alone but on those of the electromagnetic wave (of comparable nature that the Lumière) emitted by a electron at the time of its transition from an energy level to another inside the Atome. The first atomic clock appeared in 1947; it used the atomic transitions from the molecule of Ammoniac. Then one have recourse to the Rubidium then, especially, with the Cesium. These is the latter body which currently ensures the most exact operation and most stable for an atomic clock. The first clock with cesium appeared in 1955. Since, it did not cease improving. The current performances of the clocks correspond to a one second shift all the 3 million years. The cesium fountains with cold atoms are ten times more powerful. Atomic transitions from other elements like the Ytterbium, at optical frequencies much higher than the frequency used in the clocks with cesium, are being studied in the whole world and will make it possible to still gain a factor from ten to hundred.

The next stage will be the miniaturization of the atomic oscillators which will become components then being able to fit in a watch or a receiver GPS or Galileo. The American organization of the National Institute of Standards and Technology (NIST) currently works (2007) in this direction.

See too

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