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A
day is a
unit of
time equal to 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 term comes from the
Old English dæg, with similar terms common in all other
Indo-European languages, such as
dies in
Latin and
dive in
Sanskrit.
Definitions
The day has several definitions.
International System of Units
A day is defined as 86,400 seconds. Those are currently defined as
… 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.
This makes the SI 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 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
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
1
solar day := a single rotation of a planet with respect to
Sun.
Colloquial
The word refers either to the period of light when the Sun is above the local
horizon or to the full day covering a dark and a light period. The latter is sometimes called a
nychthemeron in
English, from the Greek for
night-day.
Introduction
Different definitions of the day are based on the apparent motion of the Sun across the sky . 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 , 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 . The exact moment of, and the interval between, two sunrises or two sunsets depends on the geographical position , 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 or midnight . 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 . 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 .
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 . 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 . 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
railroads 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 .
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
In order 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.
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 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 .
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, and last a full twenty-four hours until the 24:00 .
In
ancient Egypt, the day was reckoned from
sunrise to sunrise.
Muslims fast from dawn 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 . 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 operates from e.g. 6:00 to 1:00 the next day , the last hour may well count as being part of the previous day . For services depending on the day 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 .
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 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 where are mentioned days in form of weeks and years.
List of famous days
See also List of commemorative days
See also
External links
