A day (symbol d) is a unit of time equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI.12 The SI unit of time is the second.
The word 'day' can also refer to the (roughly) half of the day that is not night, also known as 'daytime'. Both refer to a length of time. Within these meanings, several definitions can be distinguished. 'Day' may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?".
Contents 1 Introduction 2 Etymology 3 International System of Units (SI) 4 Astronomy 5 Colloquial 6 Civil day 7 Leap seconds 8 Boundaries of the day 9 24 hours vs daytime 10 See also 11 Notes and references 12 External links // Introduction 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 the zenith (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 amounts to almost seven minutes at least.
Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example) The exact moment of, and the interval between, two sunrises or two sunsets depends on the geographical position (longitude as well as latitude), and the time of year. This is the time 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. The original length of one day, when the Earth was new about 4.5 billion years ago, was about six hours as determined by computer simulation. It was 21.9 hours 620 million years ago as recorded by rhythmites (alternating layers in sandstone). 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.
EtymologyThe term comes from the Old English dæg, with similar terms common in all other Indo-European languages, such as Tag in German, dies in Latin, dydd in Welsh or dive in Sanskrit or even dag in Norwegian, Danish and Swedish.
International System of Units (SI)A day is defined as 86,400 seconds.
A day on the UTC time scale can include a negative or positive leap second, and can therefore have a length of 86,399 or 86,401 seconds.
The International Bureau of Weights and Measures (BIPM) currently defines 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.3
This makes the SI-based day last exactly 794,243,384,928,000 of those periods.
In the 19th century it had also been suggested to make a decimal fraction (1⁄10,000 or 1⁄100,000) of an astronomic day the base unit of time. This was an afterglow of decimal time and calendar, which had been given up already.
AstronomyA day of exactly 86,400 SI seconds is the astronomical unit of time (the second is not preferred in astronomy).4
For a given planet, there are two types of day defined in astronomy:
sidereal day - a single rotation of a planet with respect to the distant stars mean solar day - average time of a single rotation of a planet with respect to its star.For Earth, the sidereal day is about 3 minutes 56 seconds shorter than the solar day. In fact, the Earth spins 366 times about its axis during a 365-day year, because the Earth's revolution about the Sun removes one apparent turn of the Sun about the Earth.
ColloquialThe word refers to various relatedly defined ideas, including the following:
24 hours (exactly) the period of light when the Sun is above the local horizon (i.e., the time period from sunrise to sunset); the full day covering a dark and a light period, beginning from the beginning 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; the time period from 6:00 AM to 6:00 PM or 9:00 PM or some other fixed clock period overlapping or set off from other time periods such as "morning", "evening", or "night". Civil dayFor civil purposes a common clock time has been defined for an entire region based on the mean local solar time at some central meridian. Such time zones began to be adopted about the middle of the 19th century when railroads with regular schedules came into use, with most major countries having adopted them by 1929. For the whole world, 40 such time zones are now in use. The main one is "world time" or Coordinated Universal Time (UTC).
The present common convention has the civil day starting at midnight, which is near the time of the lower culmination of the mean Sun on the central meridian of the time zone. A day is commonly divided into 24 hours of 60 minutes of 60 seconds each.
Leap secondsTo keep the civil day aligned with the apparent movement of the Sun, positive or negative leap seconds may be inserted.
A civil clock day is typically 86,400 SI seconds long, but will be 86,401 s or 86,399 s long in the event of a leap second.
Leap seconds are announced in advance by the International Earth Rotation and Reference Systems Service which measures the Earth's rotation and determines whether a leap second is necessary. Leap seconds occur only at the end of a UTC month, and have only ever been inserted at the end of June 30 or December 31.
Boundaries of the dayFor most diurnal animals, the day naturally begins at dawn and ends at sunset. Humans, with our cultural norms and scientific knowledge, have supplanted Nature with several different conceptions of the day's boundaries. The Jewish day begins at either sunset or at nightfall (when three second-magnitude stars appear). Medieval Europe 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 the remnants of the older pattern when holidays began the evening before. Present common convention is for the civil day to begin at midnight, that is 00:00 (inclusive), and last a full twenty-four hours until 24:00 (exclusive).
In ancient Egypt, the day was reckoned from sunrise to sunrise. Muslims fast from daybreak to sunset each day of the month of Ramadan. The "Damascus Document", copies of which were also found among the Dead Sea scrolls, states regarding Sabbath observance 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.
In the United States, nights are named after the previous day, e.g. "Friday night" usually means the entire night between Friday and Saturday. This is the opposite of the Jewish pattern. 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.
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 e.g. 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 (also for the arrangement of the timetable). For services depending on the day ("closed on Sundays", "does not run on Fridays", etc.) there is a risk of ambiguity. As an example, for the Nederlandse Spoorwegen (Dutch Railways), a day ticket is valid 28 hours, from 0:00 to 28:00 (i.e. 4:00 the next day). To give another example, the validity of a pass on London Regional Transport 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 daytimeTo distinguish between a full day and daytime, the English word nychthemeron may be used 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, such as יממה in Hebrew, dygn in Swedish, etmaal in Dutch, doba in Polish and сутки in Russian. In Spanish, singladura is used, but only as a marine unit of length, being the distance covered in 24 hours.5
See also 104 s, times from 10 kiloseconds to 100 kiloseconds Calculating the day of the week Daylight Daylight saving time Meteorological day Season, for a discussion of daylight and darkness at various latitudes Synodic day Notes and references ^ NISI Guide to the SI ^ Non-SI units accepted for use with the SI, and units based on fundamental constants ^ Resolution 1 of the 13th meeting of the CGPM (1967/68) ^ P. Kenneth Seidelmann, ed., Explanatory Supplement to the Astronomical Almanac, (Mill Valley, CA: Uni versity Science Books, 1992) 696. ^ "singladura - Definición". WordReference.com. http://www.wordreference.com/definicion/singladura. Retrieved 2009-03-22. External links Look up day in Wiktionary, the free dictionary. Definitions of day, night, twilight (USA navy site) Formulas to calculate the length of day and night Sunrise and sunset, all year long, anywhere Show where it is daytime at the moment v • d • e Time Major conceptsTime · Eternity · Arguments for eternity · Immortality
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Theory of relativity · Time dilation · Gravitational time dilation · Coordinate time · Proper time Biology Chronobiology · Circadian rhythms Psychology Mental chronometry · Reaction time · Sense of time · Specious present Sociology and anthropology Futures studies · Long Now Foundation · Time discipline · Time use research Economics Newtonian time in economics · Time value of money · Time Banking · Time-based currency Related topics Space · Duration · Time capsule · Time travel · Time signature · System time · Metric time · Hexadecimal time · Carpe diem · Tempus fugit v • d • e Time measurement and standards Major subjects Time · Chronometry · Orders of magnitude · Metrology Time Portal International standards UTC · UT · TAI · ISO 31-1 · DUT1 · Leap second · IERS · Terrestrial Time · Geocentric Coordinate Time · Barycentric Coordinate Time · Civil time · 12-hour clock · 24-hour clock · ISO 8601 · International Date Line · Solar time · Time zone · Daylight saving time · Time offset Obsolete standards Ephemeris time · Barycentric Dynamical Time · Greenwich Mean Time · Prime Meridian Time in physics Spacetime · Chronon · Cosmological decade · Planck epoch · Planck time · T-symmetry · Theory of relativity · Time dilation · Gravitational time dilation · Coordinate time · Proper time · Time domain · Continuous time · Discrete time · Absolute time and space Horology Clock · Astrarium · Atomic clock · Hourglass · Marine chronometer · Radio clock · Sundial · Watch · Water clock · History of timekeeping devices · Equation of time · Complication · Marine sandglass Calendar Astronomical · Julian · Gregorian · Islamic · Lunisolar · Solar · Lunar · Epact · Intercalation · Leap year · Tropical year · Equinox · Solstice · Seven-day week · Week-day names · Calculating the day of the week · Dominical letter Archaeology & geology International Commission on Stratigraphy · Geologic Time · Archaeological dating Astronomical chronology Nuclear time scale · Precession · Sidereal time · Galactic year Units of time Second · Minute · Hour · Day · Week · Fortnight · Month · Year · Decade · Century · Millennium · Jiffy · Lustrum · Saeculum · Shake · Tide Related topics Chronology · Duration · System time · Metric time · Mental chronometry · Time value of money · Timekeeper v • d • e Parts of a day Dawn–Twilight–Sunrise • Morning • Noon • Afternoon • Evening • Sunset–Twilight–Dusk • Night • Midnight Related articles: Blue hour · Golden hour • Terminator · Daytime · Midnight sunday n. The period of light between dawn and nightfall; the interval from sunrise to sunset. The 24-hour period during which the earth completes one
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Callofdty4rlz Sniper Montage
One Day (Vintage Contemporaries Original) Vintage
D-day ( ) n. The unnamed day on which an operation or offensive is to be launched. The day on which the Allied forces invaded France during World War
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A day after Tropical Storm Earl passed near Connecticut, the National Weather Service has issued a dry weather alert for western and inland Connecticut.
so tomorrow is the first day of class and in an effort to make good on this note to self my 365 uploads will have to be scarcer P eventually
Wurdz word of the day!
The First Days of School: How to Be an Effective Teacher Harry K. Wong Publications
Day definition, the interval of light between two successive nights; the time between sunrise and sunset: See more.
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PGA Australia's Jason Day and American Zach Johnson shot eight-under 63s to take a one-stroke lead after the first round of the $7.5-million Deutsche Bank Championship yesterday, but Tiger Woods faces an uphill battle to stay in the tournament. The world No. 1 recovered from a dreadful start to shoot a one-over 72 and finish ahead of only five players in the 99-man field at Boston TPC, the ...
aaaaaaah quot The Green Leaves of Summer quot was the theme song of the 1960 epic movie quot The Alamo quot starring John Wayne Composed for the original soundtrack by Dimitri Tiomkin it s been recorded by many many artists including Sarah Vaughan Patti Page Hank Snow Frankie Laine Mahalia Jackson amp countless orchestras And was a 1 hit for The Brothers Four Mixed feelings about summer ending Hate to see it go yet impatient for what comes next once summer is over quot To everything there is a season Hello fall Happy last day folks <a href= http www youtube com watch v=KOVFbsHDgd0 target= blank ><b>original soundtrack green leaves of summer< b>< a> <a href= http bighugelabs com flickr onblack php id=2875102053 amp size=large >View Large On Black< a> explore 74
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In general, British planes bombed by night and American planes bombed by day. ... Nevertheless, by nightfall of the first day, large contingents of three British, one ...
worldwar2history.info/D-Day
Jason Day heads a strong Australian challenge, tied for the lead after the opening round at the Deutsche Bank Championship in Boston.
I have the <a href= http www flickr com groups sotts >SOTTS socks <a> This and the other shot I uploaded are merely precursors to something hopefully fabulous and with better lighting tomorrow I hope inspiration strikes I m a total blank at the moment I got much less done today than I had hoped but it was a lovely day so there you go < a>< a>
Chompster
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More than 5,000 Ships and 13,000 aircraft supported the D-Day invasion, and by day's end on June 6, the Allies gained a foot- hold in Normandy. ...
www.army.mil/D-day
Australian Jason Day and American Zach Johnson share the lead after the first round of the Deutsche Bank Championship in Boston.
<b>Waitin On A Sunny Day Thanks to all who visited Thanks to all who commented Thanks to all those favorite Obrigado a todos os que visitaram Obrigado a todos os que comentaram Obrigado a todos os que favoritaram Grazie a tutti coloro che hanno visitato Grazie a tutti coloro che hanno commentato Grazie a tutti coloro preferito <a href= http bighugelabs com flickr onblack php id=2058762803 rel= nofollow >View On Black< a> <a href= http www youtube com watch v=NXuuIu8zDOs rel= nofollow >www youtube com watch v=NXuuIu8zDOs< a> I m waitin waitin on a sunny day Gonna chase the clouds away Waitin on a sunny day Bruce Springsteen < b>
BAD HIAR DAY!
The First 90 Days: Critical Success Strategies for New Leaders at All Levels Harvard Business Press
All your favorite back in the day hits. 12:00am - 12:00am. Next Up: All your favorite back in the day hits. Advertisement: Copyright © 2010 935 KDAY and MediaSpan ...
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Australian Jason Day and American Zach Johnson beat the rain to share the first-round lead at the PGA Tour's Deutsche Bank Championship, with Tiger Woods nine shots back.
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Artisan Bread in Five Minutes a Day: The Discovery That Revolutionizes Home Baking Thomas Dunne Books
Day, Dorothy 1897-1980. American journalist and reformer who cofounded the Catholic Worker in 1933 to promote pacifism and social justice. ...
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Enhancing IT Agility with CRX 2.1. Day's standards-based ECM platform for rapid development, deployment and scalable hosting of composite content applications. ...
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MANILA, Philippines – The NU Bulldogs completed a day of upsets in the UAAP Season 73 men’s basketball on Saturday when they pulled off a come-from-behind 62-59 victory against the Adamson Falcons at the Araneta Coliseum.
Rosie picnics in New Buffalo-July 2010
Gang Leader for a Day: A Rogue Sociologist Takes to the Streets Penguin (Non-Classics)
Australia's Jason Day and American Zach Johnson shot eight-under 63s to take a one-stroke lead after the first round of the $7.5 million Deutsche Bank Championship on Friday, but Tiger Woods faces an uphill battle to stay in the tournament.
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The Last Days of Socrates CreateSpace
Zach Johnson and Jason Day beat the rain to share the clubhouse lead in the first round of the Deutsche Bank Championship.
Freestyle DaY