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Make-Up Hater Wears Fenty For A Day
Make-Up Hater Wears Fenty For A Day
Published: 2017/09/22
Channel: Boldly
Kevin Durant Press Conference | Warriors Media Day 2018 NBA Season | Sep 22, 2017
Kevin Durant Press Conference | Warriors Media Day 2018 NBA Season | Sep 22, 2017
Published: 2017/09/22
Channel: Yoni Hoops
Era uma vez (Kell Smith) DAY cover
Era uma vez (Kell Smith) DAY cover
Published: 2017/09/12
Channel: DAY
Meu lugar (DAY) Autoral
Meu lugar (DAY) Autoral
Published: 2017/07/04
Channel: DAY
CNN New Day W/ CHRIS COUMO 9/22/17 BREAKING NEWS: GROWING OPPOSITION TO NEW OBAMACARE REPEAL BILL
CNN New Day W/ CHRIS COUMO 9/22/17 BREAKING NEWS: GROWING OPPOSITION TO NEW OBAMACARE REPEAL BILL
Published: 2017/09/22
Channel: Law Abiding Citizen
100 Push Ups a Day Challenge | Does it really work?
100 Push Ups a Day Challenge | Does it really work?
Published: 2017/09/22
Channel: Calisthenicmovement
Day - Az égig (Official Music Video)
Day - Az égig (Official Music Video)
Published: 2013/03/16
Channel: Antal & Day
Corpo sensual (Pabllo Vittar, Mateus Carrilho) DAY, Dreicon
Corpo sensual (Pabllo Vittar, Mateus Carrilho) DAY, Dreicon
Published: 2017/09/14
Channel: DAY
40 UNEXPECTED LIFE HACKS TO IMPROVE YOUR DAY
40 UNEXPECTED LIFE HACKS TO IMPROVE YOUR DAY
Published: 2017/08/04
Channel: 5-Minute Crafts
Handcuffing My Boyfriend To Cameron Dallas For A Day
Handcuffing My Boyfriend To Cameron Dallas For A Day
Published: 2017/09/19
Channel: Rclbeauty101
Avicii - The Days (Lyric Video)
Avicii - The Days (Lyric Video)
Published: 2014/10/03
Channel: AviciiOfficialVEVO
First Day Of Fall
First Day Of Fall
Published: 2017/09/22
Channel: Stacy Brown
Deixe-me ir (1Kilo) DAY cover
Deixe-me ir (1Kilo) DAY cover
Published: 2017/05/25
Channel: DAY
5 DIY Valentine
5 DIY Valentine's Day Gifts and Room Decor Ideas
Published: 2016/01/30
Channel: IdunnGoddess
ECHT oder doch FAKE? - so erkennst du gefälschte Produkte!
ECHT oder doch FAKE? - so erkennst du gefälschte Produkte!
Published: 2017/09/22
Channel: Made My Day
SURPRISING OUR KIDS WITH A DAY AT THE THEME PARK!
SURPRISING OUR KIDS WITH A DAY AT THE THEME PARK!
Published: 2017/09/22
Channel: KKandbabyJ
DIY Hot Glue Pencil Topper - DAY 7 of 7-Day Marathon Of Glue Gun DIYs
DIY Hot Glue Pencil Topper - DAY 7 of 7-Day Marathon Of Glue Gun DIYs
Published: 2017/07/15
Channel: IdunnGoddess
My 1st Day at Chemotherapy (UCSF)
My 1st Day at Chemotherapy (UCSF)
Published: 2017/09/19
Channel: Wil Dasovich
Solução (Um44k) DAY cover
Solução (Um44k) DAY cover
Published: 2017/08/28
Channel: DAY
WGL EU Extended Season 2017 - Week 3 - Day 1
WGL EU Extended Season 2017 - Week 3 - Day 1
Published: 2017/09/22
Channel: WGL EU
De hora em hora (ADZ) DAY cover
De hora em hora (ADZ) DAY cover
Published: 2017/08/07
Channel: DAY
DAY 8: Sabrina Limon Trial
DAY 8: Sabrina Limon Trial
Published: 2017/09/21
Channel: 23 ABC News | KERO
HOW TO LOOT RED ZONE IN UNDER 15 MINUTES | Last Day On Earth: Survival
HOW TO LOOT RED ZONE IN UNDER 15 MINUTES | Last Day On Earth: Survival
Published: 2017/09/21
Channel: Tripex
Last Day Of School Carpool Ride!
Last Day Of School Carpool Ride!
Published: 2016/06/08
Channel: Rclbeauty101
SHOCKING! NASA Confirm PLANET X NIBIRU will come American the right day 23th September 2017
SHOCKING! NASA Confirm PLANET X NIBIRU will come American the right day 23th September 2017
Published: 2017/09/21
Channel: NAT Worldwide
Melhor sorriso (ADZ) DAY cover
Melhor sorriso (ADZ) DAY cover
Published: 2017/07/21
Channel: DAY
Pico loco (ADZ) DAY cover
Pico loco (ADZ) DAY cover
Published: 2017/06/24
Channel: DAY
Morrissey - Spent the Day in Bed (Official Lyric Video)
Morrissey - Spent the Day in Bed (Official Lyric Video)
Published: 2017/09/19
Channel: MorrisseyOnVEVO
Duro igual concreto (1Kilo) DAY cover
Duro igual concreto (1Kilo) DAY cover
Published: 2017/07/28
Channel: DAY
Day - Szárnyalj (közr. Tkyd, Hesz Ádám) (Official Music Video)
Day - Szárnyalj (közr. Tkyd, Hesz Ádám) (Official Music Video)
Published: 2013/07/28
Channel: Antal & Day
Antal & Day - Még egy kör /Kexx Fesztivál Himnusz 2017/
Antal & Day - Még egy kör /Kexx Fesztivál Himnusz 2017/
Published: 2017/08/15
Channel: Antal & Day
Rally for Rivers - Day 18 - Ahmedabad
Rally for Rivers - Day 18 - Ahmedabad
Published: 2017/09/20
Channel: Sadhguru
We Did 100 Push-Ups Every Day For 30 Days
We Did 100 Push-Ups Every Day For 30 Days
Published: 2017/08/19
Channel: BuzzFeedBlue
Armin van Buuren feat. Josh Cumbee - Sunny Days (Official Music Video)
Armin van Buuren feat. Josh Cumbee - Sunny Days (Official Music Video)
Published: 2017/06/17
Channel: Armin van Buuren
DAY IN MY LIFE AT COLLEGE: GAME DAY AT UA
DAY IN MY LIFE AT COLLEGE: GAME DAY AT UA
Published: 2017/09/19
Channel: Katie Horan
Day - Egy az ezerből
Day - Egy az ezerből
Published: 2013/06/28
Channel: Antal & Day
30 Days Timelapse at Sea | 4K | Through Thunderstorms, Torrential Rain & Busy Traffic
30 Days Timelapse at Sea | 4K | Through Thunderstorms, Torrential Rain & Busy Traffic
Published: 2017/09/10
Channel: JeffHK
WATCH: United Nations General Assembly — Day 2 — Wednesday, Sept. 20, 2017
WATCH: United Nations General Assembly — Day 2 — Wednesday, Sept. 20, 2017
Published: 2017/09/21
Channel: PBS NewsHour
[MV] BeWhy(비와이) - Day Day (Feat. JayPark) (영어/한글 자막) (ENG/KOR SUB)
[MV] BeWhy(비와이) - Day Day (Feat. JayPark) (영어/한글 자막) (ENG/KOR SUB)
Published: 2016/08/14
Channel: D TL
A DAY IN MY LIFE
A DAY IN MY LIFE
Published: 2017/09/17
Channel: Laineybot
Tkyd & Day - Az út (Jason Wankel Remix Video)
Tkyd & Day - Az út (Jason Wankel Remix Video)
Published: 2017/03/09
Channel: Antal & Day
We Followed A 1950s Beauty Routine For A Day
We Followed A 1950s Beauty Routine For A Day
Published: 2017/09/16
Channel: Boldly
Morena (1Kilo) DAY cover
Morena (1Kilo) DAY cover
Published: 2017/06/11
Channel: DAY
FIRST DAY OF SUMMER
FIRST DAY OF SUMMER
Published: 2017/06/21
Channel: CaseyNeistat
[쇼미더머니5 LIVE] 비와이 - Day Day (feat.박재범) @ Semi-Final 160708 EP.09
[쇼미더머니5 LIVE] 비와이 - Day Day (feat.박재범) @ Semi-Final 160708 EP.09
Published: 2016/07/09
Channel: Mnet Official
LIVE | Daihatsu Yonex Japan Open 2017 | Day 3
LIVE | Daihatsu Yonex Japan Open 2017 | Day 3
Published: 2017/09/21
Channel: BadVidLiv
Avicii - For A Better Day
Avicii - For A Better Day
Published: 2015/09/03
Channel: AviciiOfficialVEVO
The Beatles - A Day In The Life
The Beatles - A Day In The Life
Published: 2015/10/28
Channel: TheBeatlesVEVO
HEALTHY WHAT I EAT IN A DAY // Vegan & Easy!
HEALTHY WHAT I EAT IN A DAY // Vegan & Easy!
Published: 2017/09/15
Channel: Olivia Vargus
BTS (방탄소년단)
BTS (방탄소년단) '봄날 (Spring Day)' Official MV
Published: 2017/02/12
Channel: ibighit
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WIKIPEDIA ARTICLE

From Wikipedia, the free encyclopedia
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A day is a unit of time. In common usage, it is either an interval equal to 24 hours[1] or daytime, the consecutive period of time during which the Sun is above the horizon. The period of time during which the Earth completes one rotation with respect to the Sun is called a solar day.[2][3] Several definitions of this universal human concept are used according to context, need and convenience. In 1960, the second was redefined in terms of the orbital motion of the Earth, and was designated the SI base unit of time. The unit of measurement "day", redefined in 1960 as 86 400 SI seconds and symbolized d, is not an SI unit, but is accepted for use with SI.[1] A civil day is usually 86 400 seconds, 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. The word day may also refer to a day of the week or to a calendar date, as in answer to the question, "On which day?" The life patterns of humans and many other species are related to Earth's solar day and the day-night cycle (see circadian rhythms).

In recent decades the average length of a solar day on Earth has been about 86 400.002 seconds[4] (24.000 000 6 hours) and there are about 365.242 2 solar days in one mean tropical year. Because celestial orbits are not perfectly circular, and thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. A day, understood as the span of time it takes for the Earth to make one entire rotation[5] with respect to the celestial background or a distant star (assumed to be fixed), is called a stellar day. This period of rotation is about 4 minutes less than 24 hours (23 hours 56 minutes and 4.1 seconds) and there are about 366.242 2 stellar days in one mean tropical year (one stellar day more than the number of solar days). Mainly due to tidal effects, the Earth's rotational period is not constant, resulting in further minor variations for both solar days and stellar "days". Other planets and moons have stellar and solar days of different lengths from Earth's.

Introduction[edit]

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

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, defined as the time it takes for the Sun to return to its culmination point (its highest point in the sky). Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. On average over the year this day is equivalent to 24 hours (86 400 seconds).

A day, in the sense of daytime that is distinguished from night-time, is commonly defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles. The length of daytime averages slightly more than half of the 24-hour day. Two effects make daytime on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. The difference in time depends on the angle at which the Sun rises and sets (itself a function of latitude), but can amount to around seven minutes.

Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example, being 24 hours from sunset, oldstyle).[6] The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position (longitude as well as latitude), 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).

The Earth's day has increased in length over time. This phenomenon is 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). (See tidal acceleration for details.) 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. The length of day for the Earth before the moon was created is still unknown.[citation needed]

Etymology[edit]

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,[7] and the 210th most common in UK English.[7]

International System of Units (SI)[edit]

A day, symbol d, is defined as 86 400 seconds. The Second is the base unit of time in SI units.

A day according to Coordinated Universal Time (UTC) can include a negative or positive leap second, and can therefore have a length of either 86 399 or 86 401 seconds.

In 1967–68, during the 13th CGPM (Resolution 1),[8] 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.[9]

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

Decimal and metric time[edit]

In the 19th century, an idea circulated to make a decimal fraction (110 000 or 1100 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, ke.

Colloquial[edit]

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

  • 24 hours (exactly)
  • The period of light when the Sun is above the local horizon (that is, the time period from sunrise to sunset)
  • 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[10]
  • 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 "twilight".

Civil day[edit]

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.

Leap seconds[edit]

In order to keep the civil day aligned with the apparent movement of the Sun, positive or negative leap seconds may be inserted from time to time. 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. Leap seconds occur only at the end of a UTC-calculated month, and have only ever been inserted at the end of June 30 or December 31.[citation needed]

Boundaries[edit]

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. 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, and the Eve of Saint Agnes are remnants of the older pattern when holidays began during the prior evening. Common convention 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). Prior to 1926, Turkey had two time systems: Turkish (counting the hours from sunset) and French (counting the hours from midnight).

In ancient Egypt, the day was reckoned from sunrise to sunrise. Muslims fast from sunrise to sunset each day during the month of Ramadan. The "Damascus Document", copies of which were also found among the Dead Sea scrolls, states regarding the observance of the Sabbath that "No one is to do any work on Friday from the moment that the Sun's disk stands distant from the horizon by the length of its own diameter," presumably indicating that the monastic community responsible for producing this work counted the day as ending shortly before the Sun had begun to set.[citation needed]

In many cultures, nights are named after the previous day. For example,"Friday night" usually means the entire night between Friday and Saturday. This difference from the civil day often leads to confusion. Events starting at midnight are often announced as occurring the day before. TV-guides tend to list nightly programs at the previous day, although programming a VCR requires the strict logic of starting the new day at 00:00 (to further confuse the issue, VCRs set to the 12-hour clock notation will label this "12:00 AM"). Expressions like "today", "yesterday" and "tomorrow" become ambiguous during the night. Because Jews and Muslims begin their days at nightfall, "Saturday" night, for example, is what most people would call Friday night.

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 "expiry" date stamped on the pass.

24 hours vs daytime[edit]

To distinguish between a full day and daytime, the word nychthemeron (from Greek for a night and a day) may be used in English for the former, or more colloquially the term 24 hours. In other languages, the latter is also often used. Other languages also have a separate word for a full day.

Midnight Sun[edit]

In places which experience the midnight sun (polar day), daytime may extend beyond one 24 hour period and could even extend to months

See also[edit]

References[edit]

  1. ^ a b BIPM (2014) [2006]. "Non-SI units accepted for use with the SI, and units based on fundamental constants". SI Brochure (8th ed.). 
  2. ^ Weisstein, Eric W. (2007). "Solar Day". Retrieved 2011-05-31. 
  3. ^ Weisstein, Eric W. (2007). "Day". Retrieved 2011-05-31. 
  4. ^ The average over the last 50 years is about 86 400.002. The yearly average over that period has ranged between about 86 400 and 86 400.003, while the length of individual days has varied between about 86 399.999 and 86 400.004 seconds. See this graph: Deviation of day length from SI day.svg (data from "EARTH ORIENTATION PARAMETERS". International Earth Rotation and Reference Systems Service. Archived from the original on April 26, 2015. ).
  5. ^ Certain authors caution against identifying "day" with rotation period. For example: Courtney Seligman. "Rotation Period and Day Length". 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. 
  6. ^ L. Holford-Stevens, The History of Time (Oxford 2005) p. 6
  7. ^ a b "English Words". Oxford Dictionaries Online (ODO). Oxford University Press. Retrieved 2015-10-17. 
  8. ^ "SI Unit of Time (Second)". Resolution 1 of the 13th CGPM (1967/68). Bureau International des Poids et Mesures (BIPM). Retrieved 2015-10-17. 
  9. ^ "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. 
  10. ^ "Definition of NYCHTHEMERON". www.merriam-webster.com. Retrieved 2017-02-01. 

External links[edit]

Media related to Day at Wikimedia Commons

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