Unit of time
A unit of time is a physical unit of the time measurement. In the International System of Units, the basic unit of time is the second ( s ), defined as 9,192,631,770 times the period of a microwave that is with a selected transition between two energy levels in the cesium atom in resonance.
There is the second ( s ), minute (1 min = 60 s), hour (1 h = 60 min), day ( 1 D = 24 h), a week ( for 7 days ), month (4 weeks 0-3 d or 28-31 d), year ( 1 a = 12 months or 52 weeks 1-2 d or 365-366 d), the decade (10 years), the century (10 decades), the millennium (10 centuries ). A million years is based on the SI units = Ma " mega- years" ( Megannum ), abbreviated one billion years with Ga ( Gigannum ).
History
24 hours a day
The division of the day into twelve parts probably comes from a version of the sundial the ancient Egyptians. Since the timing of long happened with the help of sunlight, could not be so easily divided the night. Later Egyptian astronomers discovered 36 stars who divided the sky into equal parts. The night was split with 18 of these stars. Each three stars were used for the morning and evening twilight, as it was difficult at this time to recognize the stars. The time of the total eclipse was determined by the remaining twelve stars. Between 1550 and 1070 BC, this system has been simplified by 24 stars were used, with twelve were for the night. The clepsydra was also used for timing.
Later Hipparchus proposed between 147 BC and 127 BC, to divide the day into 24 hours of equal length. Such a classification, however, was common until the 14th century with the advent of mechanical watches.
60 minutes and 60 seconds
The Babylonians made astronomical calculations in the 60 system. This has taken over later Hipparchus. Eratosthenes used the 60 system to divide a circle into 60 parts. He has built an early system for latitude. This system Hipparchus has improved. Claudius Ptolemy has divided the 360 degrees in his book Almagest in two stages in each back 60 units. He called the first unit partes minutae great - the minutes - the second partes minutae secundae - our seconds.