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In 1960, the [[second]] was redefined in terms of the orbital motion of the Earth in the year 1900, and was designated the [[SI base unit]] of [[time]]. The [[Units of measurement|unit of measurement]] "day", was redefined as 86,400 SI seconds and symbolized ''d''. In 1967, the second and so the day were redefined by [[atomic electron transition]].<ref name=Second>{{cite web |url=https://www.bipm.org/en/publications/si-brochure/second.html |title=Unit of time (second) |edition=8th |date=2014 |orig-year=2006 |website=SI Brochure |author=BIPM |authorlink=International Bureau of Weights and Measures}}</ref> A '''civil day''' is usually 24 hours, plus or minus a possible [[leap second]] in [[Coordinated Universal Time]] (UTC), and occasionally plus or minus an hour in those locations that change from or to [[daylight saving time]].
In 1960, the [[second]] was redefined in terms of the orbital motion of the Earth in the year 1900, and was designated the [[SI base unit]] of [[time]]. The [[Units of measurement|unit of measurement]] "day", was redefined as 86,400 SI seconds and symbolized ''d''. In 1967, the second and so the day were redefined by [[atomic electron transition]].<ref name=Second>{{cite web |url=https://www.bipm.org/en/publications/si-brochure/second.html |title=Unit of time (second) |edition=8th |date=2014 |orig-year=2006 |website=SI Brochure |author=BIPM |authorlink=International Bureau of Weights and Measures}}</ref> A '''civil day''' is usually 24 hours, plus or minus a possible [[leap second]] in [[Coordinated Universal Time]] (UTC), and occasionally plus or minus an hour in those locations that change from or to [[daylight saving time]].

== Introduction ==
[[File:Dagr by Arbo.jpg|thumb|[[Dagr]], the [[List of Norse gods and goddesses|Norse god]] of the day, rides his horse in this 19th-century painting by [[Peter Nicolai Arbo]].]]


== Colloquial ==
== Colloquial ==

Revision as of 03:33, 13 October 2020

For other uses, see Day (disambiguation).

A day is approximately the period of time during which the Earth completes one rotation on its axis.[1] A solar day is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times.[2]

In 1960, the second was redefined in terms of the orbital motion of the Earth in the year 1900, and was designated the SI base unit of time. The unit of measurement "day", was redefined as 86,400 SI seconds and symbolized d. In 1967, the second and so the day were redefined by atomic electron transition.[3] A civil day is usually 24 hours, plus or minus a possible leap second in Coordinated Universal Time (UTC), and occasionally plus or minus an hour in those locations that change from or to daylight saving time.

Colloquial

The word refers to various similarly defined ideas, such as:

Full day
  • 24 hours (exactly) (a nychthemeron)
  • A day counting approximation, for example "See you in three days." or "the following day"
  • The full day covering both the dark and light periods, beginning from the start of the dark period or from a point near the middle of the dark period
  • A full dark and light period, sometimes called a nychthemeron in English, from the Greek for night-day;[4] or more colloquially the term 24 hours. In other languages, 24 hours is also often used. Other languages also have a separate word for a full day.
  • Part of a date: the day of the year (doy) in ordinal dates, day of the month (dom) in calendar dates or day of the week (dow) in week dates.
  • Time regularly spend at paid work on a single work day, cf. man-day and workweek.
Daytime
  • The period of light when the Sun is above the local horizon (that is, the time period from sunrise to sunset)
  • The time period from 06:00–18:00 (6:00 am – 6:00 pm) or 21:00 (9:00 pm) or another fixed clock period overlapping or offset from other time periods such as "morning", "evening", or "night".
  • The time period from first-light "dawn" to last-light "dusk".
Other
  • A specific period of the day, which may vary by context, such as "the school day" or "the work day".

Boundaries

Sun and Moon, Hartmann Schedel's Nuremberg Chronicle, 1493

For most diurnal animals, the day naturally begins at dawn and ends at sunset. Humans, with their cultural norms and scientific knowledge, have employed several different conceptions of the day's boundaries. Common convention among the ancient Romans,[5] ancient Chinese[6] and in modern times is for the civil day to begin at midnight, i.e. 00:00, and last a full 24 hours until 24:00 (i.e. 00:00 of the next day). In ancient Egypt, the day was reckoned from sunrise to sunrise. The Jewish day begins at either sunset or nightfall (when three second-magnitude stars appear).

Medieval Europe also followed this tradition, known as Florentine reckoning: in this system, a reference like "two hours into the day" meant two hours after sunset and thus times during the evening need to be shifted back one calendar day in modern reckoning.[citation needed] Days such as Christmas Eve, Halloween, and the Eve of Saint Agnes are remnants of the older pattern when holidays began during the prior evening. Prior to 1926, Turkey had two time systems: Turkish (counting the hours from sunset) and French (counting the hours from midnight).

Validity of tickets, passes, etc., for a day or a number of days may end at midnight, or closing time, when that is earlier. However, if a service (e.g., public transport) operates from for example, 6:00 to 1:00 the next day (which may be noted as 25:00), the last hour may well count as being part of the previous day. For services depending on the day ("closed on Sundays", "does not run on Fridays", and so on) there is a risk of ambiguity. For example, a day ticket on the Nederlandse Spoorwegen (Dutch Railways) is valid for 28 hours, from 0:00 to 28:00 (that is, 4:00 the next day); the validity of a pass on Transport for London (TfL) services is until the end of the "transport day" – that is to say, until 4:30 am on the day after the "expires" date stamped on the pass.

See also

References

  1. ^ Weisstein, Eric W. (2007). "Day". Retrieved 2011-05-31.
  2. ^ Weisstein, Eric W. (2007). "Solar Day". Retrieved 2011-05-31.
  3. ^ BIPM (2014) [2006]. "Unit of time (second)". SI Brochure (8th ed.).
  4. ^ "Definition of NYCHTHEMERON". www.merriam-webster.com. Retrieved 2017-02-01.
  5. ^ See Plutarch, Quaestiones Romanae, 84.
  6. ^ s:zh:清史稿/卷48: 起子正，盡夜子初。
  • Media related to Day at Wikimedia Commons
  • The dictionary definition of day at Wiktionary
  • Quotations related to Day at Wikiquote
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