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The division of the day into specific units is Egyptian and Chaldean. First divided the day into 24 parts, 12 and 12 for the night for the day. The second divided their day into sixty parts as in Vedic calendars of India from the seventh century BC. BC, the Babylonians borrowed from Egypt the division of the day into twelve parts. Many people have subsequently defined their notion of time in cutting into twelve parts both periods of varying lengths depending on the season.

The invention of the minute and the second would also be of Babylonian origin, although it is very unlikely that they have been able to be in time with a precision better than a few tens of minutes.

The ancient Egyptians used online timer a cutting of the night in 12 hours. For the night, they took into account the stars to determine the beginning of the offices. Some summer nights, only twelve stars rose to horizon3. Subsequently, they also divided the day into 12 hours, depending on the season, these hours were longer or shorter. Presumably to 12 subdivisions allowed to divide the day in third easily, into quarters or sixths. In addition, twelve was already used to divide the year into months or lunations, by observing the lunar cycles during the year (see history of time measurement).

It is actually very likely that the division was based on the observation of the Sun's position in the sky with the naked eye or with an instrument. The Sun's position also served to focus on land and sea

On the other hand, the second appeared among the Babylonians, roughly corresponded to the period of resting heart rate, easy to count, and also appreciated the following for the definition of rhythm in music. The time had about 3600 seconds it was easy to divide into multiples of 60, which is also a multiple of 12. The symbolism of the number 12, and the ease of dividing it into 2, 3 or 4, and the ease of dividing the minute and hour 5 in this system can only use integers would have led to the creation of sexagesimal system to divide the time in minutes and seconds equal and whole.

The use in Roman times, taken from the Greek civilization was to divide the period from sunrise to sunset in exactly 12 hours, and divide the same night in exactly 12 hours. The length of time thus varied with the season. Every online stopwatch hour corresponded to a military watchtower. The first hour was that which followed the morning, the sixth hour match at noon, the ninth hour in the middle of the afternoon and the twelfth (and last) time that before dusk. This usage is maintained in the notation of the canonical hours.

France adopted the time of the Greenwich meridian (and thus the universal time of 24 hours instead of 2 times 12 hours, decided in October 1884 to the International Conference Washington4), March 9, 1911, but is not the subject of an explicit law (only the "Paris time" is mentioned which were subtracted 9 minutes and 20 seconds to match GMT) 5. The adoption of daylight saving time, advocated by André Honnorat MP since 1916, was passed by Parliament March 19, 1917 by 291 votes against 1775. The principle of the DST is abandoned by the Provisional Government of the French Republic, August 14, 1945, at the same time as time is GMT +1 adoptée5 and is restored in 1976, after the first shock pétrolier5 1973.

Some used traditionally continues to formulate hours twice twelve hours, for example: 4 hours (afternoon) for 16 hours.

Tanzania and Kenya, Swahili is used to start the day at dawn. Day length varies little with the season because of the proximity to the equator, the beginning of the day was made at 6 am in Western notation.