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
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
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.
See also
Notes and references
External links