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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. 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?".

The 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 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.

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.

Astronomy

A day of exactly 86,400 SI seconds is used in astronomy as a unit of time.

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.

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 (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".


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). 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 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.

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 railroad 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 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 Statesmarker, 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 ticket, 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.

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 (Psalms 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.

24 hours vs daytime

To distinguish between a full day and daytime, the English word nychthemeron may be used 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 and сутки in Russian. In Spanish, singladura is used, but only as a marine unit of length, being the distance covered in 24 hours.

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