Time of day
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Time of day is a measure of the progression of time within a day.
Parts of a day[edit]
The simplest way to measure time within the day is by reference to the position of the sun in the sky. The most important division of the day is between daytime, when the sun is up, and night, when it is not. Noon, the time when the sun is at its highest point in the sky, naturally divides the daytime into morning and afternoon, and midnight similarly divides the night into two parts. The transition between night and daytime is marked by two events: dawn and sunrise, with twilight in between them. Similarly, the transition from daytime to night is marked by sunset and dusk, with a period of twilight in between.
Antiquity[edit]
With the invention of timekeeping devices such as sundials, people began to divide the day up into smaller intervals.
Twelve hours: Babylonian and Roman division of the day[edit]
People in Antiquity divided the day into twelve hours, starting at sunrise. 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.[1] 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[edit]
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[edit]
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.
The need to standardize the definition of time led to the concept of civil time—a system of timekeeping defined by statute or designated by civilian authorities.
Local mean time[edit]
For most of history, time of day was reckoned with reference to the position of the sun in the local sky (apparent solar time). The length of a solar day varies slightly throughout the year, which causes the hours to vary in length over the course of the year. While this works with a sundial, it is inconvenient when keeping time with clocks. In the early 19th century, local mean time was adopted. This system defined an hour as one twenty-fourth of a mean solar day, so that the length of an hour was the same year-round. Each city or town would set its clocks based on its own meridian, so if one travelled 100 km east or west the local time of day would differ by about four minutes.
With local mean time, the time of day at which solar noon occurs varies over the course of the year, becoming as much as 16 minutes off from noon based on the clocks. With the onset of industrialization, working hours became tied to the clock rather than to daylight.[2]
Standard time[edit]
With the introduction of railroads and steamships later in the 19th century, the discrepancy in time as one travelled east or west became a problem. To resolve this standard time was introduced. In this system, all clocks within a geographical region would be synchronized, rather than each city or town setting its clocks based on local (mean) noon. This system was first used in Britain, and was known as railway time.
In the 20th century, this system was expanded globally, and the geographical regions became time zones, referenced to the mean solar time at the Royal Observatory in Greenwich, London, known as Greenwich Mean Time or, later, Universal Time.
Coordinated Universal Time[edit]
In the 1960s, Universal Time based on the mean position of the sun was replaced by systems based on celestial measurements, as these were more accurate. This led to the development of Coordinated Universal Time (UTC). Modern time zones are each defined by a fixed offset from Coordinated Universal Time, possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972.[3] Older systems use telescope observations.
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.[4] 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.
References[edit]
- ↑ The Roman Calendar - www.die-roemer-online.de, after: Johannes Irmscher, Renate Johne (eds.): Lexicon of Antiquity. Wilhelm Heyne Verlag, Munich 1962.
- ↑ 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 (in Deutsch). Institute for Advanced Studies Vienna. Unknown parameter
|trans-work=ignored (help) - ↑ "Coordinated Universal Time (UTC) (CCTF/09-32)" (PDF). Bureau International des Poids et Mesures. p. 3. Retrieved 5 October 2022.
- ↑ "Standard-frequency and time-signal emissions" (PDF). Coordinated universal time (UTC). Rec. ITU-R TF.460-6.
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Related: Belt of Venus · Blue hour · Golden hour · Terminator · Daylight · Moonlight Noon · Afternoon · Midnight · Midnight sun · Polar night · Zodiacal light | |||
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