Day
A day is the
A collection of sequential days is organized into calendars as dates, almost always into weeks, months and years. A solar calendar organizes dates based on the Sun's annual cycle, giving consistent start dates for the four seasons from year to year. A lunar calendar organizes dates based on the Moon's lunar phase.
In common usage, a day starts at
In specific applications, the definition of a day is slightly modified, such as in the
Etymology
The term comes from the
Definitions
Apparent and mean solar day
Several definitions of this universal human concept are used according to context, need, and convenience. Besides the day of 24 hours (86,400 seconds), the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the solar day, the time it takes for the Sun to return to its culmination point (its highest point in the sky). Due to an orbit's eccentricity, the Sun resides in one of the orbit's foci instead of the middle. Consequently, due to Kepler's second law, the planet travels at different speeds at various positions in its orbit, and thus a solar day is not the same length of time throughout the orbital year. Because the Earth moves along an eccentric orbit around the Sun while the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. In recent decades, the average length of a solar day on Earth has been about 86,400.002 seconds[3] (24.000 000 6 hours). There are currently about 365.2421875 solar days in one mean tropical year.
Ancient custom has a new day starting at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example, being 24 hours from sunset, old style).[4] The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position (longitude and latitude, as well as altitude), and the time of year (as indicated by ancient hemispherical sundials).
A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of such a day is nearly constant (24 hours ± 30 seconds). This is the time as indicated by modern sundials.
A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth moves along its orbit around the Sun (due to both its velocity and its axial tilt).
In terms of Earth's rotation, the average day length is about 360.9856°. A day lasts for more than 360° of rotation because of the Earth's revolution around the Sun. With a full year being slightly more than 360 days, the Earth's daily orbit around the Sun is slightly less than 1°, so the day is slightly less than 361° of rotation.
Elsewhere in the Solar System or other parts of the universe, a day is a full rotation of other large astronomical objects with respect to its star.[5]
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
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 called a calendar day.
A day is commonly divided into 24 hours, with each hour being made up of 60 minutes, and each minute composed of 60 seconds.
Sidereal day
A
Besides a stellar day on Earth, other bodies in the Solar System have day times, the durations of these being:[10][11]
Name | Daylength (hours) |
---|---|
Mercury | 4222.6 |
Venus | 2802 |
Earth's Moon | 708.7 |
Mars | 24.7 |
Ceres | 9[12]–9.1[13] |
Jupiter | 9.9 |
Saturn | 10.7 |
Uranus | 17.2 |
Neptune | 16.1 |
Pluto | 153.3 |
In the International System of Units
In the
In decimal and metric time
Various
Other definitions
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;[19] 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 week dates.
- Time regularly spend at paid work on a single 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", "afternoon", or "evening".
- 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".
Variations in length
Mainly due to
Since the rotation rate of the Earth is slowing, the length of a SI second fell out of sync with a second derived from the rotational period.[20] This arose the need for leap seconds, which insert extra seconds into Coordinated Universal Time (UTC).[20] 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. Other than the two-millisecond variation from tidal deceleration, other factors minutely affect the day's length, which creates an irregularity in the placement of leap seconds.[22] 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.
Geological day lengths
Discovered by paleontologist John W. Wells, the day lengths of geological periods have been estimated by measuring sedimentation rings in coral fossils,[23][21] due to some biological systems being affected by the tide.[21] The length of a day at the Earth's formation is estimated at 6 hours.[21] Arbab I. Arbab plotted day lengths over time and found a curved line.[21] Arbab attributed this to the change of water volume present affecting Earth's rotation.[21]
Date | Geological period | Number of days per year[23] | Duration of the day |
Present | Current | 365 | 24 hours |
−100 million years | Cretaceous | 380 | 23 hours and 20 minutes |
−200 million years | Triassic | 390 | 22 hours and 40 minutes |
−300 million years | Carboniferous | 400 | 22 hours |
−400 million years | Devonian | 410 | 21 hours and 20 minutes |
−500 million years | Cambrian | 425 | 20 hours and 40 minutes |
Boundaries
This section needs additional citations for verification. (May 2023) |
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.
In the Hebrew Bible, Genesis 1:5 defines a day in terms of "evening" and "morning" before recounting the creation of the Sun to illuminate it: "And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day." 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. Days such as Christmas Eve, Halloween (“All Hallows’ Eve”), and the Eve of Saint Agnes are remnants of the older pattern when holidays began during the prior evening.
The common convention among the
In ancient Egypt the day was reckoned from sunrise to sunrise.
Prior to 1926, Turkey had two time systems: Turkish, counting the hours from sunset, and French, counting the hours from midnight.
Parts
Humans have divided the day in rough periods, which can have cultural implications, and other effects on humans' biological processes. The parts of the day do not have set times; they can vary by lifestyle or hours of daylight in a given place.[26]
Daytime
Daytime is further divided into morning, afternoon, and evening. Morning occurs between sunrise and noon.[29] Afternoon occurs between noon and sunset,[30] or between noon and the start of evening. This period of time sees human's highest body temperature,[31] an increase of traffic collisions,[32] and a decrease of productivity.[33] Evening begins around 5 or 6 pm, or when the sun sets, and ends when one goes to bed.[34][35][36]
Twilight
Night
Night is the period in which the sky is dark;[39] the period between dusk and dawn when no light from the sun is visible.[40] Light pollution during night can impact human and animal life, for example by disrupting sleep.[41][42]
See also
- Determination of the day of the week
- Holiday
- ISO 8601
- Season, for a discussion of daylight and darkness at various latitudes
- Synodic day
- World Meteorological day
References
- ISBN 978-0-19-929668-2.
- ^ a b "English Words". Oxford Dictionaries Online (ODO). Oxford University Press. Archived from the original on 2016-01-25. Retrieved 2015-10-17.
- ^ "Earth Orientation Parameters". International Earth Rotation and Reference Systems Service. Archived from the original on April 26, 2015.
- ^ L. Holford-Stevens, The History of Time (Oxford 2005) p. 6
- ^ "day". Encyclopædia Britannica. Archived from the original on 2022-07-10. Retrieved 2022-08-17.
- ^ Certain authors caution against identifying "day" with rotation period. For example: Seligman, Courtney. "Rotation Period and Day Length". Archived from the original on 2018-09-29. Retrieved 2011-06-03.
A Cautionary Note: Because the rotation period of the Earth is almost the same as the length of its day, we sometimes get a bit sloppy in discussing the rotation of the sky, and say that the stars rotate around us once each day. In a similar way, it is not unusual for careless people to mix up the rotation period of a planet with the length of its day, or vice versa.
- ^ a b "sidereal day". Encyclopædia Britannica. Archived from the original on 2022-09-26. Retrieved 2022-08-17.
- ISBN 0-387-98746-0. Archivedfrom the original on 2011-12-09. Retrieved 2022-08-17.
- ^ Komhyr, Walter Dmyro (June 1980). "Operations Handbook – Ozone Observations with a Dobson Spectrophotometer". gml.noaa.gov. p. 122. Archived from the original on 2021-06-12. Retrieved 2022-08-17.
- ^ "Planetary Fact Sheet – Metric". nssdc.gsfc.nasa.gov NASA. Archived from the original on 19 July 2012. Retrieved May 29, 2021.
- ^ Griggs, Mary Beth (18 January 2019). "Shaky rings help scientists measure Saturn's days – Speedy planet". The Verge. Archived from the original on 19 January 2019. Retrieved 18 January 2019.
- ^ "planets/dwarf-planets/ceres/in-depth". nasa.gov. Archived from the original on June 2, 2021. Retrieved May 30, 2021.
- ^ Tate, Karl (21 November 2012). "Dwarf Planets of Our Solar System (Infographic)". www.space.com. Archived from the original on 18 May 2021. Retrieved May 30, 2021.
- ^ BIPM (2014) [2006]. "Non-SI units accepted for use with the SI, and units based on fundamental constants". SI Brochure (8th ed.). Archived from the original on 2014-11-11. Retrieved 2015-01-27.
- ^ "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.
- ^ "Unit of Time (Second)". SI Brochure: The International System of Units (SI) (8 ed.). Bureau International des Poids et Mesures (BIPM). 2014 [2006]. Archived from the original on 2018-06-13. Retrieved 2015-10-17.
- ^ Veitch, Harriet (2008-04-02). "Why don't we have metric time?". The Sydney Morning Herald. Archived from the original on 2022-08-21. Retrieved 2022-08-21.
- ^ from the original on 2022-08-21. Retrieved 2022-08-21.
- ^ "Definition of NYCHTHEMERON". www.merriam-webster.com. Archived from the original on 2017-02-02. Retrieved 2017-02-01.
- ^ from the original on 2022-05-31. Retrieved 2022-08-20.
- ^ a b c d e f Arbab, Arbab I. (January 2009). "The Length of the Day: A Cosmological Perspective" (PDF). Department of Physics, Faculty of Science, University of Khartoum. 1. Archived (PDF) from the original on 2022-08-20. Retrieved 2022-08-20.
- IERS. 2013. Archivedfrom the original on August 29, 2016. Retrieved August 6, 2016.
- ^ Bibcode:1969LAstr..83..411K. Archivedfrom the original on 21 December 2019. Retrieved 5 June 2021.
- ^ See Plutarch, Quaestiones Romanae, 84. Archived 2021-02-09 at the Wayback Machine
- ^ s:zh:清史稿/卷48: 起子正,盡夜子初。
- ^ "Parts of the Day: Early morning, late morning, etc". Britannica Dictionary. Archived from the original on 2022-08-22. Retrieved 2022-08-22.
- ^ 32′⁄2 + 34′ = 50′
- ^ 50°/60 ÷ 360° × 2(for sunrise and set) × 24 hours ≈ 7 min
- ^ "Definition of MORNING". www.merriam-webster.com. Archived from the original on 2022-08-22. Retrieved 2022-08-22.
- ^ "Definition of AFTERNOON". www.merriam-webster.com. Archived from the original on 2022-08-22. Retrieved 2022-08-22.
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- ^ Ray, James T. (1960). Human Performance as a Function of the Work–Rest Cycle. National Academy of Sciences. p. 11.
- ^ "evening, n." www.oed.com. Oxford English Dictionary. Retrieved 18 September 2023.
The close of day, esp. the time from about 6 p.m., or sunset if earlier, to bedtime; the period between afternoon and night.
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- ^ "Definitions from the US Astronomical Applications Dept". USNO. Archived from the original on 2019-09-27. Retrieved 2011-07-22.
- ^ "Glossary of Marine Navigation" (PDF). Archived from the original (PDF) on 2017-08-29.
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- ^ "Definition of NIGHT". www.merriam-webster.com. Archived from the original on 2022-08-22. Retrieved 2022-08-22.
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- ^ "Light pollution harms wildlife and ecosystems". DarkSky International. Retrieved 2023-10-30.