A day (symbol: d) is a unit of time equivalent to 24 hours. It is not an
SI unit but it is accepted for use with SI.[1] The SI unit of time is the second. The term comes from the
Old English dæg.
Definitions
The day has several definitions.
International System of Units (SI)
A day is defined as 86,400 seconds. Each second is 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 (1⁄10,000 or 1⁄100,000) of an astronomic day the base unit of time. This was an
afterglow of decimal time and calendar,
which had already been given up.
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)
- 1 solar day
- = a single rotation of a planet with respect to its star.
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 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 06:00 to 18:00 or 21:00 or some other fixed clock period overlapping or set off from other time periods
such as "morning", "evening", or "night".
- The mostly regular interval of one awaking, usually in the morning (personal day).
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 (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 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). 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 (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.
References
- ^ http://physics.nist.gov/Pubs/SP811/sec05.html
See also
External links
bat-smg:Dėina
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