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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.

Introduction

Dagr, the Norse god of the day, rides his horse in this 19th-century painting by Peter Nicolai Arbo.

Etymology

The term comes from the Old English dæg, with its cognates such as dagur in Icelandic, Tag in German, and dag in Norwegian, Danish, Swedish and Dutch. All of them from the Indo-European root dyau which explains the similarity with Latin dies though the word is known to come from the Germanic branch. As of October 17, 2015, day is the 205th most common word in US English,[4] and the 210th most common in UK English.[4]

International System of Units (SI)

A day, symbol d, defined as 86,400 seconds, is not an SI unit, but is accepted for use with SI.[5] The second is the base unit of time in SI units.

In 1967–68, during the 13th CGPM (Resolution 1),[6] the International Bureau of Weights and Measures (BIPM) redefined a second as

... the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom.[7]

This makes the SI-based day last exactly 794, 243, 384, 928, 000 of those periods.

Leap seconds

Further information: Leap second and Tidal acceleration

Mainly due to tidal effects, the Earth's rotational period is not constant, resulting in minor variations for both solar days and stellar "days". The Earth's day has increased in length over time due to tides raised by the Moon which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86, 400.002 seconds, and is increasing by about 1.7 milliseconds per century (an average over the last 2, 700 years). The length of a day circa 620 million years ago has been estimated from rhythmites (alternating layers in sandstone) as having been about 21.9 hours.

In order to keep the civil day aligned with the apparent movement of the Sun, a day according to Coordinated Universal Time (UTC) can include a negative or positive leap second. Therefore, although typically 86,400 SI seconds in duration, a civil day can be either 86,401 or 86,399 SI seconds long on such a day.

Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service (IERS), which measures the Earth's rotation and determines whether a leap second is necessary.

Civil day

For civil purposes, a common clock time is typically defined for an entire region based on the local mean solar time at a central meridian. Such time zones began to be adopted about the middle of the 19th century when railroads with regularly occurring schedules came into use, with most major countries having adopted them by 1929. As of 2015, throughout the world, 40 such zones are now in use: the central zone, from which all others are defined as offsets, is known as UTC±00, which uses Coordinated Universal Time (UTC).

The most common convention starts the civil day at midnight: this is near the time of the lower culmination of the Sun on the central meridian of the time zone. Such a day may be referred to as a calendar day.

A day is commonly divided into 24 hours of 60 minutes, with each minute composed of 60 seconds.

Decimal and metric time

Main article: Metric time

In the 19th century, an idea circulated to make a decimal fraction (1⁄10, 000 or 1⁄100, 000) of an astronomical day the base unit of time. This was an afterglow of the short-lived movement toward a decimalisation of timekeeping and the calendar, which had been given up already due to its difficulty in transitioning from traditional, more familiar units. The most successful alternative is the centiday, equal to 14.4 minutes (864 seconds), being not only a shorter multiple of an hour (0.24 vs 2.4) but also closer to the SI multiple kilosecond (1, 000 seconds) and equal to the traditional Chinese unit, kè.

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;[8] 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,[9] ancient Chinese[10] 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. ^ a b "English Words". Oxford Dictionaries Online (ODO). Oxford University Press. Retrieved 2015-10-17.
  5. ^ BIPM (2014) [2006]. "Non-SI units accepted for use with the SI, and units based on fundamental constants". SI Brochure (8th ed.).
  6. ^ "SI Unit of Time (Second)". Resolution 1 of the 13th CGPM (1967/68). Bureau International des Poids et Mesures (BIPM). Archived from the original on 2011-01-10. Retrieved 2015-10-17.
  7. ^ "Unit of Time (Second)". SI Brochure: The International System of Units (SI) (8 ed.). Bureau International des Poids et Mesures (BIPM). 2014 [2006]. Retrieved 2015-10-17.
  8. ^ "Definition of NYCHTHEMERON". www.merriam-webster.com. Retrieved 2017-02-01.
  9. ^ See Plutarch, Quaestiones Romanae, 84.
  10. ^ s:zh:清史稿/卷48: 起子正，盡夜子初。
  • Media related to Day at Wikimedia Commons
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