hour
Physical unit  

Unit name  hour 
Unit symbol  
Physical quantity (s)  Time , span of time 
Formula symbol  
dimension  
system  Approved for use with the SI 
In SI units  
Named after  Old high German stunta 
Derived from  Day 
See also: year , month , day , minute 
The hour (from Old High German stunta 'standing', 'stay', 'fixed point in time', 'short period', 'hour') denotes the twentyfourth part of a day . In addition to a division into 24 equal parts, there are also other hour terms.
The Latin word is hora , hence the symbol h
or ^{h}
.
For the exact differentiation of an hour (with 60 minutes) e.g. B. of a lesson (often 45 minutes) is also referred to as a hour .
The unit of measurement is hour
The hour is a unit of time . The unit symbol is h (from Latin hora ). The hour does not belong to the International System of Units (SI), but is approved for use with the SI. This makes it a legal unit of measurement . Due to the non decimal subdivision, a conversion into seconds is first necessary for scientific calculations.
Since today's atomic clocks can measure time very precisely and the rotation speed of the earth varies, the hour was redefined as a second, which connects atomic time with universal astronomical time .
Counting and dividing hours
The 24hour counting of a whole day is first attested in ancient Egypt and was later used in ancient Greece around the third century BC, where the 24hour system was derived from the angle . From there it spread over the whole (old) world until the turn of the times .
Various methods of counting hours, i.e. numbering , have been used throughout history :
 In the ancient Egyptian hour counting  as in the counting based on the prayers of the hours in the Middle Ages  the sunrise is the beginning of the first hour of the day ; the sunset as the end of the twelfth hour of the day as well as the dusk as the first hour of the night and the dawn as the twelfth hour of the night
 In the Babylonian hour counting (Greek hours) , sunrise is taken as the beginning of the first hour.
 With the Italian hour counting (also Bohemian or large clock) the first hour begins at sunset . The hours were counted from 1 to 24.
 With the modern 12hour counting (small clock, civil hours) the hour counting starts at midnight and noon ; the hours are counted up from 1 to 12. The two halfdays of 12 hours are called ante meridiem (AM, “before noon”) and post meridiem (PM, “after noon”).
 With the modern 24hour counting (astronomical hours) , the hour counting starts at midnight, the hours are counted up from 1 to 24.
 On the Julian date of the astronomers, the hour counting starts at noon.
The beginning of minute one is called the full hour , for example 08:00:00; it is also called "stroke eight". The term comes from the fact that (in the example above) “the eighth hour is full”. The widespread hour divisions of half an hour , quarter of an hour and the times (chronologically) "quarter eight" (07:15), "half eight" (07:30), "quarter to eight" or "quarter to eight" (07: 45) and "quarter past eight" (08:15).
Other definitions of the length of the hour
The term 'hour'  in addition to today's physicalchronometric term  is also used for the historical chronological and astronomical time systems :
 Equal hours : An hour length is subject to its own definitions and is not tied to a 24hour division.

Temporal hours (Roman hours) : The day arc of the bright day ( sunrise to sunset) was divided into twelve parts and transferred accordingly to the night. Due to the different lengths of day and night in the course of the year, the length of the hours (between 30 and 90 minutes) also change continuously.
 In the horae canonicae , the first hour of the day ( prime ) began between 3 a.m. and 9:30 a.m. and the first hour of the night between 3 p.m. and 9:30 p.m. The English term afternoon refers to the 9th hour ( ninth ), which has been striking at 12 noon since around 1300.
 Seasonal hours : Similar to the temporal hours, the day and night are divided into periods of fluctuating duration, but deviate from a continuous 24hour division.
 Equinox hour : An equinox hour is the twentyfourth part of the sunny day and is independent of the day and night arcs that change with the seasons.
See also
 Clock
 Industrial minute
 Hour angle
 Hour (historically used measure of distance)
 Eighth hour
literature
 Gerhard Dohrnvan Rossum : The story of the hour. Clocks and modern time systems. Hanser, Munich 1992, ISBN 9783446160460 ; Reprint Anaconda, Cologne 2007, ISBN 9783866471399 .
 Hermann Grotefend : Calendar of the German Middle Ages and the modern age. 2 volumes in 3 sections, Hahnsche Buchhandlung, Hanover 1891–1898; Neudruck Aalen 1970, Volume 1, pp. 183189.
Web links
 The International System of Units (SI) . German translation of the BIPM brochure "Le Système international d'unités / The International System of Units (8e édition, 2006)". In: PTBMitteilungen . tape 117 , no. 2 , 2007 ( Online [PDF; 1.4 MB ]).
Individual evidence
 ↑ Time hour that. Duden , 2018, accessed February 12, 2018 .
 ↑ ^{a } ^{b } Le Système international d'unités . 9e édition, 2019 (the socalled "SI brochure", French and English).
 ↑ on the basis of EU Directive 80/181 / EEC in the states of the EU or the Federal Law on Metrology in Switzerland
 ^ Jacques Le Goff: Time, work and culture in the Middle Ages. Translated from the French (1977) by Arthur Goldhammer, Chicago and London 1980, p. 44f.