   #copyright

Day

2007 Schools Wikipedia Selection. Related subjects: Everyday life

   Water, Rabbit, and Deer: three of the 20 day symbols in the Aztec
   calendar, from the Aztec Sun Stone.
   Enlarge
   Water, Rabbit, and Deer: three of the 20 day symbols in the Aztec
   calendar, from the Aztec Sun Stone.

   A day (symbol: d) 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 (SI)

   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 (E−4 or E−5) 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
          (for Earth it is 23.934 solar hours or 24 sidereal hours)

   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.
   Dagr, the Norse god of the day, rides his horse in this 19th century
   painting by Peter Nicolai Arbo.
   Enlarge
   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 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 2700
   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 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
   (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

   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 (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,
   and last a full twenty-four hours until the 24:00 (also known as 00:00
   of the next day).

   In ancient Egypt, the day was reckoned from sunrise to sunrise. Muslims
   fast from dawn (traditionally when a white thread can be distinguished
   from a black thread) 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 Dutch Railways, a day ticket is valid 28 hours, from
   0:00 to 28:00 (i.e. 4:00 the next day).

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.

   Retrieved from " http://en.wikipedia.org/wiki/Day"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
