Day 8376 223295769 2008-07-03T13:07:07Z Lightbot 7178666 Units/dates/other {{otheruses}} [[Image:Water,Rabbit,Deer.jpg|right|thumb|Water, Rabbit, and Deer: three of the 20 day symbols in the [[Aztec calendar|Aztec]] [[calendar]], from the [[Aztec sun stone|Aztec Sun Stone]].]] A '''day''' (symbol: d) is a [[units of measurement|unit]] of [[time]] equivalent to 24 [[hour]]s. It is not an [[SI]] unit but it is accepted for use with SI.<ref name=SI>[http://www.bipm.org/en/si/si_brochure/chapter4/table6.html SI brochure: Table 6: Non-SI units accepted for use with the International System of Units]</ref> The term comes from the [[Old English]] ''dæg''. The word is also used to mean [[daytime (astronomy)|daytime]], the period of [[daylight]] experienced once per day and alternating with [[night]]. == Definitions == The day has several definitions. === International System of Units (SI) === A day contains 86,400 SI seconds.<ref name=SI/> Each second is currently defined as <blockquote>… the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.</blockquote> In the 19th century it had also been suggested to make a decimal fraction ({{frac|1|10,000}} or {{frac|1|100,000}}) of an astronomic day the base unit of time. This was an afterglow of the [[decimal time]] used with the [[French Republican Calendar]], which had already been given up.{{Fact|date=June 2008}} === Astronomy === A day of exactly 86,400 SI seconds is the fundamental unit of time in astronomy. For a given planet, there are two types of day defined in [[astronomy]]: ; 1 apparent [[sidereal day]] := a single rotation of a planet with respect to the distant stars : (for [[Earth]] it is 23.934 solar hours) === Colloquial === The word refers to various relatedly defined ideas, including the following: *The period of light when the Sun is above the local [[horizon]] (i.e., the period from sunrise to sunset), opposed to [[night]]. See [[Daytime (astronomy)]]. *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 language|English]], from the [[Greek language|Greek]] for ''night-day''. *The period from 06:00 to 18:00 or 21:00 or some other fixed clock period overlapping or set off from other periods such as "[[morning]]", "[[evening]]", or "[[night]]". *The mostly regular interval of one awaking, usually in the morning (''personal day''). [[Image:Dagr by Arbo.jpg|thumb|right|[[Dagr]], the [[Norse god]] of the day, rides his horse in this 19th century painting by [[Peter Nicolai Arbo]].]] == Introduction == The word ''day'' is used for several different units of time based on the rotation of the Earth around its axis. The most important one follows the apparent motion of the Sun across the sky (solar day; see [[solar time]]). The reason for this apparent motion is the [[rotation]] of the Earth around its axis, as well as the revolution of the Earth in its [[orbit]] around the Sun. A day, as opposed to [[night]], is commonly defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles. Two effects make days on average longer than nights. The Sun is not a point, but has an apparent size of about 32 [[minute of arc|minutes of arc]]. Additionally, the [[Earth's atmosphere|atmosphere]] [[refraction|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 [[sunrise]]s or two [[sunset]]s depends on the geographical position ([[longitude]] as well as latitude), and the time of [[year]]. This is the time as indicated by ancient hemispherical [[sundial]]s. A more constant day can be defined by the Sun passing through the local [[meridian (geography)|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.{{Fact|date=May 2008}} It was 21.9 hours 620 million years ago as recorded by rhythmites (alternating layers in sandstone). This phenomenon is due to [[tide]]s 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 2700 years). See [[tidal acceleration]] for details. During the biblical [[Creation week]], the day appears in several forms: As the seven days in the Creation week ("the evening and the morning", a [[nychthemeron]] or 24-hour day), as the light created during the first day ("Let there be light … and God called the light Day" (daylight, not night, {{bibleverse||Genesis|1:3-5|9}}), as periods of time delimited by the lights created during the fourth day ("for seasons, and for days, and years", {{bibleverse||Genesis|1:14|9}}), and for the Sun created during the fourth day to rule ("the greater light to rule the day", daylight, {{bibleverse||Genesis|1:16|9}}). == Civil day == For 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 zone]]s began to be adopted about the middle of the 19th century when [[railroad]]s with regular schedules came into use, with most major countries having adopted them by 1929. For the whole world, 39 such time zones are now in use. The main one is "world time" or [[UTC]] (Coordinated Universal Time). 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 seconds == The actual mean period of rotation of the earth with respect to the sun is slightly longer than the SI day of 86,400 seconds. It is more nearly 86,400.002 seconds. This additional time accumulates to about 0.7 s per year or about seven seconds every ten years, necessitating the addition of an extra second to the civil clock occasionally to retard it and keep it more closely synchronized to the apparent movement of the sun. By the middle of this century the amount of time to be added to the clock will increase to one second every year. This additional second is called a [[leap second]]. 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]]. == Astronomy == In [[astronomy]], the [[sidereal day]] is also used; it is about 3 minutes 56 seconds shorter than the solar day, and close to the actual rotation period of the Earth, as opposed to the Sun's apparent motion. 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. == Boundaries of the day == For most diurnal animals, including ''Homo sapiens'', the day naturally begins at [[dawn]] and ends at [[sunset]]. Humans, with their cultural norms and scientific knowledge, have supplanted Nature with several different conceptions of the day's boundaries. The [[Judaism|Jewish]] day begins at either [[sunset]] or at nightfall (when three second-magnitude stars appear). [[Middle Ages|Medieval]] [[Europe]] followed this tradition, known as [[Florence|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 [[holiday]]s 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 [[hour]]s until 24:00 (exclusive). In [[ancient Egypt]], the day was reckoned from [[sunrise]] to sunrise. [[Muslim]]s 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 [[Shabbat|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. <!--There are presumably other traditions to present here.--> In the [[United States]]<!--and elsewhere perhaps-->, 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. 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 [[Videocassette recorder|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 [[Ticket (admission)|ticket]]s, 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 [[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. [[Image:A day panorama - digital art.jpg|thumb|center|750px|A digital composite of several panoramas of the same location on the same day show the passing time - on the northern hemisphere, on the left is west and sunset, on the right is east and dawn]] == Metaphorical days == In the Bible, as a way to describe that time is immaterial to [[God]], one day is described as being like one thousand years (Psalm 90:4, 2 Peter 3:8) to him. Also in 2 Peter 3:8, one thousand years is described as being like one day. However, some Bible "experts" interpret this more literally as a way to understand some prophecies like those in Book of Daniel and others (like the Book of Revelation) where are mentioned days in form of weeks and years.{{Fact|date=December 2007}} == References == {{reflist}} {{wiktionarypar|day}} == See also == * [[Day length]] * [[Daylight]] * [[Earth clock]] * [[Season]] == External links == *[http://www.fourmilab.ch/cgi-bin/uncgi/Earth/action?opt=-p&img=learth.evif Show where it is daytime at the moment] *[http://ptaff.ca/soleil/?lang=en_CA Sunrise and sunset, all year long, anywhere] *[http://aa.usno.navy.mil/faq/docs/RST_defs.php Definitions of day, night, twilight (USA navy site)] *[http://herbert.gandraxa.com/herbert/lod.asp Formulas to calculate the length of day and night] *[http://www.calendaryear.net Dates by each calendar year] {{Time Topics}} {{Time measurement and standards}} [[Category:Units of time]] [[Category:Parts of a day| ]] [[af:Dag]] [[als:Tag]] [[ang:Dæg]] [[ar:يوم]] [[arc:ܝܘܡܐ]] [[ast:Día]] [[ay:Uru]] [[zh-min-nan:Kang]] [[be-x-old:Дзень]] [[bg:Ден]] [[ca:Dia]] [[cv:Кун]] [[cs:Den]] [[cy:Diwrnod]] [[da:Dag]] [[de:Tag]] [[et:Ööpäev]] [[eml:Dè]] [[es:Día]] [[eo:Tago]] [[eu:Egun]] [[fa:روز]] [[ext:Dia]] [[fr:Jour]] [[fy:Dei]] [[fur:Dì]] [[ga:Lá]] [[gv:Laa]] [[gd:Là]] [[gl:Día]] [[ko:날]] [[hr:Dan]] [[ilo:Aldaw]] [[id:Hari]] [[iu:ᖃᐅ/qau]] [[is:Sólarhringur]] [[it:Giorno]] [[he:יממה]] [[jv:Dina]] [[ka:დღე]] [[kaa:Ku'n (waqıt)]] [[kk:Тәулік]] [[sw:Siku]] [[ht:Jou]] [[ku:Roj (dem)]] [[lad:Dia]] [[lo:ມື້]] [[la:Dies]] [[lv:Diena]] [[lt:Para]] [[ln:Mokɔlɔ]] [[lmo:Dí]] [[hu:Nap (időegység)]] [[mk:Ден]] [[ms:Hari]] [[mn:Өдөр]] [[nah:Tōnalli]] [[myv:Чи (шкань вал)]] [[nl:Dag]] [[nds-nl:Dag]] [[ja:日]] [[no:Dag]] [[nn:Dag]] [[nrm:Jouo]] [[oc:Jorn]] [[nds:Dag]] [[pl:Dzień]] [[pt:Dia]] [[ro:Zi]] [[qu:P'unchaw]] [[ru:Сутки]] [[sq:Dita]] [[scn:Jornu]] [[simple:Day]] [[sl:Dan]] [[so:Maalin]] [[sr:Дан]] [[fi:Vuorokausi]] [[sv:Dygn]] [[tl:Araw (panahon)]] [[ta:நாள்]] [[tt:Kön]] [[th:วัน]] [[tg:Рӯз]] [[tr:Gün]] [[uk:Доба]] [[vo:Del]] [[fiu-vro:Päiv (aomõõt)]] [[yi:טאג]] [[yo:Ọjọ́]] [[zh-yue:一日]] [[bat-smg:Dėina]] [[zh:日]]