Civil time

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In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. [1] Older systems use telescope observations.

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In traditional astronomical usage, civil time was mean solar time reckoned from midnight. 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 used Greenwich Mean Time (GMT) for both conventions, leading to ambiguity[ clarification needed ], whereas the Nautical Almanac Office at the United States Naval Observatory 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 introduced the term Universal 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 the time zone UTC+00:00. Universal Time is now determined by reference to distant celestial objects: UTC is derived from International Atomic Time (TAI), and is adjusted by leap 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.]

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 hours, 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, 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. [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 the Liturgy of the Hours: Prime, Terce, 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, 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

In Semitic language cultures, the day traditionally begins at nightfall. This is still important today for the beginning of Shabbat 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 clocks.

With the onset of industrialization, working hours became tied to the clock rather than to daylight. [5]

See also

Related Research Articles

International Atomic Time 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 atomic clocks in over 80 national laboratories worldwide. It is a continuous scale of time, without leap seconds, and it is the principal realisation of Terrestrial Time. It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface and which has leap seconds.

A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours. As a day passes at a given location it experiences morning, noon, afternoon, evening, and night. This daily cycle drives circadian rhythms in many organisms, which are vital to many life processes.

The term ephemeris time can in principle refer to time in association with any ephemeris. In practice it has been used more specifically to refer to:

  1. a former standard astronomical time scale adopted in 1952 by the IAU, and superseded during the 1970s. This time scale was proposed in 1948, to overcome the disadvantages of irregularly fluctuating mean solar time. The intent was to define a uniform time based on Newtonian theory. Ephemeris time was a first application of the concept of a dynamical time scale, in which the time and time scale are defined implicitly, inferred from the observed position of an astronomical object via the dynamical theory of its motion.
  2. a modern relativistic coordinate time scale, implemented by the JPL ephemeris time argument Teph, in a series of numerically integrated Development Ephemerides. Among them is the DE405 ephemeris in widespread current use. The time scale represented by Teph is closely related to, but distinct from, the TCB time scale currently adopted as a standard by the IAU.
<span class="mw-page-title-main">Greenwich Mean Time</span> Time zone of Western Europe, same as WET

Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a particular time unless a context is given. The term "GMT" is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.

<span class="mw-page-title-main">Hour</span> Unit of time equal to 60 minutes

An hour is a unit of time historically reckoned as 1⁄24 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.

<span class="mw-page-title-main">Leap second</span> Extra second inserted to keep civil time in sync with the Earths rotation

A leap second is a one second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment. The leap second was introduced in 1972. Since then, 27 leap seconds have been added to UTC, with the most recent occurring on December 31, 2016. All have so far been positive leap seconds, adding a second to a UTC day; while it is possible for a negative leap second to be needed, one has not happened yet.

<span class="mw-page-title-main">Second</span> SI unit of time

The second is the unit of time in the International System of Units (SI), historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.

The Julian day is the continuous count of days since the beginning of the Julian period, and is used primarily by astronomers, and in software for easily calculating elapsed days between two events.

A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.

Universal Time 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 with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle. UT1 is the same everywhere on Earth. UT1 is required to follow the relationship

In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to perturbations and vary with time. These time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, the node of its orbit relative to a reference plane, the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit.

The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. and p.m.. Each period consists of 12 hours numbered: 12, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11. The 12-hour clock has been developed since the second millennium BC and reached its modern form in the 16th century.

<span class="mw-page-title-main">24-hour clock</span> Timekeeping convention

The modern 24-hour clock is the convention of timekeeping in which the day runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours passed since midnight, from 00(:00) to 23(:59), with 24(:00) as an option to indicate the end of the day. This system, as opposed to the 12-hour clock, is the most commonly used time notation in the world today, and is used by the international standard ISO 8601.

<span class="mw-page-title-main">Midnight</span> Transition time from one day to the next

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.

Time in New Zealand is divided by law into two standard time zones. The main islands use New Zealand Standard Time (NZST), 12 hours in advance of Coordinated Universal Time (UTC) / military M (Mike), while the outlying Chatham Islands use Chatham Standard Time (CHAST), 12 hours 45 minutes in advance of UTC / military M^ (Mike-Three).

<span class="mw-page-title-main">Philippine Standard Time</span> Time zone used in the Philippines (UTC+08:00)

Philippine Standard Time, also known as Philippine Time (PHT), is the official name for the time zone used in the Philippines. The country only uses a single time zone, at an offset of UTC+08:00, but has used daylight saving time for brief periods in the 20th century until July 28, 1990.

Nautical time is a maritime time standard established in the 1920s to allow ships on high seas to coordinate their local time with other ships, consistent with a long nautical tradition of accurate celestial navigation. Nautical time divides the globe into 24 nautical time zones with hourly clock offsets, spaced at 15 degrees by longitudinal coordinate, with no political deviation.

A tropical year or solar year is the time that the Sun takes to return to the same position in the sky – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronomical seasons. For example, it is the time from vernal equinox to the next vernal equinox, or from summer solstice to the next summer solstice. It is the type of year used by tropical solar calendars.

<span class="mw-page-title-main">Coordinated Universal Time</span> Primary time standard

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 communication, navigation, scientific research, and commerce.

<span class="mw-page-title-main">Time in Finland</span>

Finland uses Eastern European Time (EET) during the winter as standard time and Eastern European Summer Time (EEST) during the summer as daylight saving time. EET is two hours ahead of coordinated universal time (UTC+02:00) and EEST is three hours ahead of coordinated universal time (UTC+03:00). Finland adopted EET on 30 April 1921, and has observed daylight saving time in its current alignment since 1981 by advancing the clock forward one hour at 03:00 EET on the last Sunday in March and back at 04:00 EET on the last Sunday in October, doing so an hour earlier for the first two years.

References

  1. ↑ "Coordinated Universal Time (UTC) (CCTF/09-32)" (PDF). Bureau International des Poids et Mesures. p. 3. Retrieved 5 October 2022.
  2. ↑ McCarthy, Dennis D.; Seidelmann, P. Kenneth (2009). Time : from Earth rotation to atomic physics. P. Kenneth Seidelmann. Weinheim: Wiley-VCH. p. 17. ISBN   978-3-527-40780-4. OCLC   429605165.
  3. ↑ "Standard-frequency and time-signal emissions" (PDF). Coordinated universal time (UTC). Rec. ITU-R TF.460-6.
  4. ↑ The Roman Calendar - www.die-roemer-online.de, after: Johannes Irmscher, Renate Johne (eds.): Lexicon of Antiquity . Wilhelm Heyne Verlag, Munich 1962.
  5. ↑ Andrej, Isabella (1993). "Der 'blaue Montag' – Eine Form des Widerstandes gegen die industrielle Arbeitszeitdisziplinierung" [The 'blue monday' – a form of resistance to industrial disciplining of working hours]. Seminar Neue Geschichte WS 1993/94: Arbeit und Arbeiterkämpfe in Europa 18. bis 20. Jahrhundert[Labor and Workers' Struggles in Europe, 18th to 20th Century] (in German). Institute for Advanced Studies Vienna.
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