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In modern usage,civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally nationalstandard time in atime zone at afixed offset fromCoordinated Universal Time (UTC), possibly adjusted bydaylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972.[1] Older systems usetelescope observations.
In traditional astronomical usage, civil time wasmean solar time reckoned frommidnight. Before 1925, the astronomical time 00:00:00 meant noon, twelve hours after the civil time 00:00:00 which meant midnight.HM Nautical Almanac Office in theUnited Kingdom usedGreenwich Mean Time (GMT) for both conventions, leading to ambiguity[clarification needed], whereas the Nautical Almanac Office at theUnited States Naval Observatory used GMT for the pre-1925 convention and Greenwich Civil Time (GCT) for the post-1924 convention until 1952. In 1928, theInternational Astronomical Union introduced the termUniversal Time for GMT beginning at midnight.[2]
In modern usage, GMT is no longer a formal standard reference time: it is now a name for thetime zoneUTC+00:00. Universal Time is now determined by reference to distant celestial objects: UTC is derived fromInternational Atomic Time (TAI), and is adjusted byleap seconds to compensate for variations in the rotational velocity of the Earth.[3] Civil Times around the world are all defined by reference to UTC. In many jurisdictions, legislation has not been updated and still refers to GMT; this is taken to mean UTC+0.
The division of the day into times of day has existed since the beginning of the calendar.
People inAntiquity divided the day into twelvehours, but these were reckoned from sunrise rather than midnight.Babylonian hours were of equal length, while Romantemporal hours varied depending on the season.
TheHorae, literally "the hours," were the original Greek goddesses who oversaw regulated life. They were the patron goddesses of the various times of day. In Greek tradition, the twelve hours were counted from just before sunrise to just after sunset.
Roman daytimes were calledhora (hours), with the morning hour ashora prima. The night was divided into four sections calledvigilia (night watch), two before midnight and two after.[4] The Romans originally counted the morning hours backwards: "3 a. m." or "3 hours ante meridiem" meant "three hours before noon", in contrast to the modern meaning "three hours after midnight".
This ancient division has survived in theLiturgy of the Hours:Prime,Terce,Sext, andNones are named after the first, third, sixth and ninth hours of the day. TheMatins, the nocturnal prayer is, according to theRule of Saint Benedict, to be prayed at the "eighth hour of the night", which corresponds to about 2 am.
TheSpanishsiesta derives its name from the Latinhora sexta for the sixth hour (noon).
InSemitic language cultures, the day traditionally begins at nightfall. This is still important today for the beginning ofShabbat andIslamic holidays.
A division of days has survived from Persian, following the Babylonian beginning of the day: The rōsgār (times of day) are hāwan (morning), uapihwin (afternoon), usērin (evening), ēbsrūsrim (sunset to midnight), and ushahin (midnight to dawn). The last two are collectively called shab (night).
The modern division of the day into twenty-four hours of equal length (Italian hours) first appeared in the 14th century with the invention of the mechanical wheel clock and its widespread use inturret clocks.
With the onset ofindustrialization, working hours became tied to the clock rather than to daylight.[5]
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