In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national

standard time Standard time is the synchronization of clocks within a geographical region to a single time standard, rather than a local mean time standard. Generally, standard time agrees with the local mean time at some meridian that passes through the r ...

in a

time zone A time zone is an area which observes a uniform standard time for legal, Commerce, commercial and social purposes. Time zones tend to follow the boundaries between Country, countries and their Administrative division, subdivisions instead of ...

at a fixed offset from

Coordinated Universal Time Coordinated Universal Time (UTC) is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communicat ...

(UTC), possibly adjusted by

daylight saving time Daylight saving time (DST), also referred to as daylight savings time, daylight time (Daylight saving time in the United States, United States and Daylight saving time in Canada, Canada), or summer time (British Summer Time, United Kingdom, ...

during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use

telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...

observations. In traditional astronomical usage, civil time was

mean solar time Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based ...

reckoned from

midnight Midnight is the transition time from one day to the next – the moment when the date changes, on the local official clock time for any particular jurisdiction. By clock time, midnight is the opposite of noon, differing from it by 12 hours. ...

. 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 the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

used

Greenwich Mean Time Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory, Greenwich, Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being ...

(GMT) for both conventions, leading to ambiguity, whereas the Nautical Almanac Office at the

United States Naval Observatory The United States Naval Observatory (USNO) is a scientific and military facility that produces geopositioning, navigation and timekeeping data for the United States Navy and the United States Department of Defense. Established in 1830 as the ...

used GMT for the pre-1925 convention and Greenwich Civil Time (GCT) for the post-1924 convention until 1952. In 1928, the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

introduced the term

Universal Time Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle wi ...

for GMT beginning at midnight. In modern usage, GMT is no longer a formal standard reference time: it is now a name for the

UTC+00:00. Universal Time is now determined by reference to distant celestial objects: UTC is derived from

International Atomic Time International Atomic Time (abbreviated TAI, from its French name ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 atomi ...

(TAI), and is adjusted by

leap second A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise solar tim ...

s to compensate for variations in the rotational velocity of the Earth. 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.

History

The division of the day into times of day has existed since the beginning of the calendar.

Twelve hours: Babylonian and Roman division of the day

People in Antiquity divided the day into twelve

hour An hour (symbol: h; also abbreviated hr) is a unit of time historically reckoned as of a day and defined contemporarily as exactly 3,600 seconds ( SI). There are 60 minutes in an hour, and 24 hours in a day. The hour was initially establis ...

s, but these were reckoned from sunrise rather than midnight. Babylonian hours were of equal length, while Roman temporal hours varied depending on the season. The

Horae In Greek mythology, the Horae (), Horai () or Hours (, ) were the goddesses of the seasons and the natural portions of time. Etymology The term ''hora'' comes from the Proto-Indo-European ("year"). Function The Horae were originally the ...

, 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 called ''hora'' (hours), with the morning hour as ''hora prima''. The night was divided into four sections called ''vigilia'' (night watch), two before midnight and two after.The Roman Calendar
- www.die-roemer-online.de, after: Johannes Irmscher, Renate Johne (eds.): '' Lexicon of Antiquity''. Wilhelm Heyne Verlag, Munich 1962. 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 the

Liturgy of the Hours The Liturgy of the Hours (), Divine Office (), or ''Opus Dei'' ("Work of God") are a set of Catholic prayers comprising the canonical hours, often also referred to as the breviary, of the Latin Church. The Liturgy of the Hours forms the official ...

Prime A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways ...

Terce Terce is a canonical hour of the Divine Office. It consists mainly of psalms and is held around 9 a.m. Its name comes from Latin and refers to the third hour of the day after dawn. Along with Prime, Sext, None, and Compline, Terce belongs ...

, Sext, and Nones are named after the first, third, sixth and ninth hours of the day. The Matins, the nocturnal prayer is, according to the

Rule of Saint Benedict The ''Rule of Saint Benedict'' () is a book of precepts written in Latin by St. Benedict of Nursia (c. AD 480–550) for monks living communally under the authority of an abbot. The spirit of Saint Benedict's Rule is summed up in the motto of th ...

, to be prayed at the "eighth hour of the night", which corresponds to about 2 am. The Spanish siesta derives its name from the Latin ''hora sexta'' for the sixth hour (noon).

Middle East

Semitic language The Semitic languages are a branch of the Afroasiatic language family. They include Arabic, Amharic, Tigrinya, Aramaic, Hebrew, Maltese, Modern South Arabian languages and numerous other ancient and modern languages. They are spoken by mo ...

cultures, the day traditionally begins at nightfall. This is still important today for the beginning of

Shabbat Shabbat (, , or ; , , ) or the Sabbath (), also called Shabbos (, ) by Ashkenazi Hebrew, Ashkenazim, is Judaism's day of rest on the seventh day of the seven-day week, week—i.e., Friday prayer, Friday–Saturday. On this day, religious Jews ...

and Islamic 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).

Middle Ages and early modern times

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 in

turret clock A turret clock or tower clock is a clock designed to be mounted high in the wall of a building, usually in a clock tower, in public buildings such as Church (building), churches, university buildings, and town halls. As a public amenity to enab ...

s. With the onset of

industrialization Industrialisation (British English, UK) American and British English spelling differences, or industrialization (American English, US) is the period of social and economic change that transforms a human group from an agrarian society into an i ...

, working hours became tied to the clock rather than to daylight.

References

External links