A day is a unit of time, commonly defined as 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun. The period of time measured from local noon to the following local noon, is called a ''solar day''.
Several definitions of this universal human concept are used according to context, need and convenience. In 1967, the second was redefined in terms of the wavelength of light, and it became the SI base unit of time. The unit of measurement for time called "day", redefined in 1967 as 86,400 SI seconds and symbolized d, is not an SI unit, but it is accepted for use with SI. A civil day is usually also 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time UTC, and, in some locations, occasionally plus or minus an hour when changing 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?" ''Day'' also refers to the part of the day that is not night — also known as 'daytime'. The life patterns of humans and many other species are related to Earth's solar day and the cycle of day and night, when timing (See circadian rhythms.)
The average length of a solar day on Earth is about 86,400 seconds (24 hours) and there are about 365.2422 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
with respect to the celestial background or a distant star (assumed, to be fixed), is called ''stellar day''. This period of rotation is about 4 minutes less than 24 hours and there are about 366.2422 in one mean tropical year. 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 also have stellar and solar days.
Introduction
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 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) 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. 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 one day was about 21.9 hours 620 million years ago as recorded by rhythmites (alternating layers in sandstone); when the Earth was new about 4.5 billion years ago, it was probably around six hours as determined by computer simulations. The length of day for the Earth or Proto-Earth before the event which created our moon by an impact is yet unknown.
Etymology
The term comes from the
Old English ''dæg'', with its cognates such as ''Tag'' in
German, and ''dag'' in
Norwegian,
Danish,
Swedish and
Dutch.
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.
This makes the SI-based day last exactly 794,243,384,928,000 of those periods.
Decimal and metric time
In the 19th century it had also been suggested to make a decimal fraction ( or ) of an astronomic day the base unit of time. This was an afterglow of
decimal time and
calendar, which had been given up already for its difficulty to comply with familiar units. The still most successful candidate is the ''
centiday'' = 14.4 minutes, as a shorter
quarter of an hour and also close to the SI target
kilosecond and old Chinese
ke.
Astronomy
A day of exactly 86,400 SI seconds is the astronomical unit of time (the second is not preferred in astronomy).
For a given planet, there are three types of day defined in astronomy:
stellar day - an entire rotation of a planet with respect to the distant stars
sidereal day - a single rotation of a planet with respect to the vernal equinox
mean solar day - average time of a single rotation of a planet with respect to the sun as the central star
For Earth, the stellar day and the sidereal day are nearly of the same length and 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.
Colloquial
The 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 (that is, 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 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 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 seconds
To 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 day
For 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 24 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 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 (also for the arrangement of the timetable). For services depending on the day ("closed on Sundays", "does not run on Fridays", and so on) 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 (that is, 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 daytime
To distinguish between a full day and daytime, the word ''
nychthemeron'' may be used in English for the former, or more colloquially the term . 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, ''
??тки'' (''sutki'') in Russian, ''
??ткі'' (''sutki'') in Belarusian, ''
døgn'' in Danish and ''
доба?'' (''doba'') in Ukrainian. In Spanish, ''singladura'' is used, but only as a marine unit of length, being the distance covered in 24 hours.
See also
10 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
External links
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
Category:Orders of magnitude (time)
Category:Units of time
af:Dag
als:Tag
ang:Dæȝ
ar:ي?م
an:Día
arc:ܝ?ܡ?
ast:Día
ay:Uru
az:Sutka
bn:দিন
zh-min-nan:Kang
ba:Тәүлек
be:Дзень
be-x-old:Дзень
bh:दिन
bcl:Aldaw
bs:Dan
br:Deiz
bg:Ден
ca:Dia
cv:К?н
cs:Den
tum:Dazi
cy:Diwrnod
da:Dag
de:Tag
et:Ööpäev
el:Ημέ?α
eml:Giōren
myv:Чи-ве
es:Día
eo:Tago
ext:Dia
eu:Egun
fa:ر?ز
hif:Din
fr:Jour
fy:Dei
fur:Dì
ga:Lá
gv:Laa
gd:Là
gl:Día
gan:日
gu:દિવસ
hak:Ngit
xal:Хонг
ko:날
hr:Dan
ilo:Aldaw
id:Hari
ia:Die
iu:??/qau
os:Бон
is:Dagur (tímatal)
it:Giorno
he:יממה
jv:Dina
kn:ದಿನ
krc:С?тка
ka:???-?????
kk:Тә?лік
kw:Dydh
rw:Umunsi
sw:Siku
ht:Jou (Tan)
ku:Roj (dem)
mrj:Кечӹ (календарь)
lad:Dia
lo:ມື້
la:Dies
lv:Diena
lt:Para
ln:Mokɔlɔ
lmo:Dì
hu:Nap (időegység)
mk:Ден
ml:ദിവസം
mr:दिवस
arz:ي?م
ms:Hari
mdf:Ши (пингонь вал)
mn:Өдөр
nah:Tōnalli
nl:Dag
nds-nl:Dag
ne:दिन
ja:日
no:Dag
nn:Dag
nrm:Jouo
oc:Jorn
uz:Sutka
pa:ਦਿਨ
pnb:دن
pap:Dia
tpi:De
nds:Dag
pl:Doba
pt:Dia
kaa:Ku'n (waqıt)
ro:Zi
qu:P'unchaw
rue:День
ru:С?тки
sq:Dita
scn:Jornu
simple:Day
sk:De?
sl:Dan
so:Maalin
sr:Дан
sh:Dan
fi:Vuorokausi
sv:Dygn
tl:Araw (panahon)
ta:நாள்
tt:Kön
th:วัน
tg:Рӯз
tr:Gün
uk:Доба
ur:دن
vi:Ngày
vo:Del
fiu-vro:Päiv (aomõõt)
war:Adlaw (oras)
wo:Bés
yi:??ג
yo:Ọjọ?
zh-yue:一日
bat-smg:Para
zh:日