# Day

The day ( mhd. Tag tac , asächs. Dag , got. Dags , urgerm. * Dagaz ) is used in various ways as a concept of time determined by the apparent course of the sun around the earth .

• The day is the time from sunrise to sunset , also known as the light day .
• Deviating from this, the period of wakefulness and activity of people, which is much longer than the clear day in mid-latitudes and especially in winter (northern hemisphere), is called day.
• Unlike in the daylight and the sum of which is night than day, more than full day or Full-time designated.
• Single consecutive full days are called calendar days , which today usually last from midnight to midnight. The night therefore belongs to two different calendar days.
• A sunny day is the length of time between two upper meridian passages of the sun. These are the times when the sun is exactly in the south (northern hemisphere of the earth) or exactly in the north (southern hemisphere of the earth). Because the duration of the full day given by the course of the sun ( true sunny day ) varies slightly over a year, an average sunny day (also called civil day ) is used in general and for the length of the calendar day.

The mean solar day has traditionally been the basis for the unity second: . Since 1956 their length has been shown differently ( atomic second ). Currently, the mean solar day is 86,400.003 atomic seconds long. However, apart from a few days in which a leap second is inserted, the calendar day still has a length of 86,400 s. ${\ displaystyle 1 \, \ mathrm {s} = {\ tfrac {1} {24 \ cdot 60 \ cdot 60}} \, \ mathrm {d} = {\ tfrac {1} {86,400}} \, \ mathrm {d}}$

## Basic terms

Variable daylight
The changeable daylight is important at least for all those living beings who relate to light as a timer to coordinate internal and external processes and develop repeatable behavior patterns. Recurring bright phases of exposure during the day alternating with dark ones at night occur in places on the earth's surface as a result of the earth's rotation and its orbit around the sun.
However, these periods of time are not of constant duration , but fluctuate over the course of the year, depending on the location. At 50 ° latitude, the span between broad daylight and the solstice ( solstice ) lasts about twice as long in summer as in winter. Twice a year, at the equinox , day and night are of equal length. This equinox date is then the same for the whole earth.
Time span of the day
The daytime period is also called the light day . With daylight, it is day that lasts from sunrise to sunset . This range corresponds to the diurnal curve of the apparent course of the sun and is divided into halves of the day with the highest position of the sun at midday; It can also be divided into times of day and divided into (often twelve) hours of the day .
The duration of these day segments depends on the length of the day and therefore fluctuates more strongly with increasing distance from the equator. In places with a geographical latitude of the polar circles or higher, in the polar regions , every rotation of the earth is no longer linked to the rising and setting of the sun, which is known as polar day . Tags understood as (clear) days are time spans of very different lengths.
Period of time from day and night
The time span of day and night is called a "full day" if the span illuminated by daylight is taken together with the adjacent night between two comparable exposure phases, for example outside the polar regions from a rising of the sun to the next sunrise or from a sunset to to the next ( Nychthemeron ) . Since the sun rises earlier and later from day to day in the spring half-year and rises later and sets earlier in the autumn half-year, the duration of a full day related to the rising or setting of the sun is subject to strong fluctuations, the extent of which is the latitude dependent. A more sensible point of reference for the definition of the full day is midday , which is almost symmetrically in the middle of the clear day, when the sun crosses the meridian at its upper culmination , the highest point above the horizon , and its hourly angle is zero. An equivalent alternative is the opposite position of midnight , in which the sun crosses the meridian in its lower culmination and its hourly angle is equal to 12 h = 180 ° (this position of the sun lies below the horizon all year round in the non-polar regions of the earth). The full day, defined as the time between two consecutive noon or midnight, is a sunny day ; its duration is the same for all locations and fluctuates only slightly over the course of the year. In civil life the sunny day begins at midnight; In astronomy, the sunny day beginning at noon is also common. The local solar time , defined as the hour angle of the sun ± 12 hours, is used to schedule a sunny day ; so it's midnight 0 h and at noon 12 h solar time.
Duration of the time intervals
The exact duration of the time interval between two consecutive noon or midnight changes over the course of the year . Since the earth does not move in the equatorial plane and not on a circular orbit with constant angular velocity, but in the ecliptic in accordance with Kepler's laws on an elliptical orbit around the sun, the annual orbit of the projection of the direction of the sun onto the equator takes place with a variable angular velocity .
Therefore, the hour angle of the sun lying in the equatorial plane changes unevenly , even if the earth's rotation speed is assumed to be constant . The duration of a true sunny day in which the hour angle of the sun changes in 24 h = 360 °, i.e. the duration from one lower culmination of the sun (=  midnight ) to the next, is variable. True sunny days can differ by up to about a minute; the average value over the course of years for the time that elapses between the (lower) meridian passages is currently about 24 hours.
Average duration of the day
The determined average value of the duration of true sunny days, the mean sunny day , gives the clue for determining the duration of the so-called civil day , which became the basis of the calendar time reference. This temporal reference scheme, which is widespread today, is based on a time measure “day” of constant duration, the sequence of which is adapted to the current mean value by means of any leap seconds .
At the beginning of the 20th century the time measures of hours, minutes and seconds were still defined as fractions of an average sunny day. However, when it became apparent that the earth's rotation did not have a constant period of rotation as assumed , the solar second linked to the rotation was initially replaced in the 1950s by the ephemeris second based on the orbital motion of the earth as a unit of time. Since the introduction of the atomic second at the end of the 1960s, the basic unit of this measure of time is no longer determined astronomically, but decoupled from the rotation and orbit of the earth.
In the SI, the underlying unit of measurement of time is defined on the basis of atomic time , the time standard of which is set by atomic clocks : the second . Their 86,400-fold is also referred to as “day”, indicated by the character “d” (for Latin this is “ day” ), and divided into 24 hours of 60 minutes of 60 seconds each.
Constant time measure d
The usual constant time measure day (d) of 24 hours is 86,400 times the SI unit second . Their definition was chosen so that the mean sunny day measured with it now lasts (currently) 86,400 seconds; Any differences that occur are compensated for by leap seconds.
This time measurement corresponds roughly to the current mean value, it does not indicate the true duration of a sunny day. With the construction of consecutive constant time segments for the mean solar time, the true position of the sun at noon is regularly missed by a maximum of a quarter of an hour at the beginning of November; the respective deviations can be calculated using the time equation .
Actual duration of the day
The actual duration of a true sunny day changes slightly from day to day; it is determined by the current orbital speeds during the orbit as well as the rotation speed of the earth and deviates by up to 30 seconds from 24 hours.
However, these periods of time do not give the duration for a complete revolution of the earth. It was already known to Copernicus that a day-night cycle cannot correspond to a whole rotation of the earth when the earth revolves around the sun. Because without any self-rotation, there is exactly one day-night cycle during an entire cycle.
Precise time information
If one measures the period from culmination to culmination not for the star sun, which is orbited as the central star, but in relation to the light of other very distant stars, then the rotation of the earth against the background of the fixed stars is represented by the duration of the sidereal day . One observes the apparent rotation of the night sky and thus obtains an approximate value for the duration of the earth's rotation of approximately 86.164 seconds.
When viewed against the fixed star background, the earth rotates exactly one more rotation than the number of days related to the sun (clear, full, solar or calendar days). Since the rotation of the earth is in the same direction ( prograd ) as the course around the sun, its duration is around 24 h / 366 or just under 4 minutes shorter than a mean sunny day. The Earth's rotation time is currently approximately 23 hours 56 minutes and 4.10 seconds with fluctuations in the millisecond range and a long-term tendency to increase. An increasing duration of rotation is reflected in increasing day lengths under the same conditions of rotation, both for the full and the clear day.
To be sure, looking at the starry sky and looking at the position of the sun as a reference to time in everyday life is replaced for some by looking at the clock, sometimes with dual time for down under . But light is still time-giving for people, also for the given date (“this given”, Latin : date ) of today , on this day - which then has subjective meaning, for example as a birthday on which someone “sees the light of day " Has.

## Day as a period of time

On the basis of the basic concept - to specify a temporal relationship with regard to the phase of exposure at a location - restricted or expanded, special and general terms of the day have been developed:

• as an indefinite range
• day or day not more precisely determined or related than the time with light, the daylight
• as an approximate range
• Time of day such as morning, morning, noon, afternoon or evening
• as a certain period of time
• as a measured time span
• a day as a specified number of uniform time periods, for example 24 hours in WOZ
• a standard day as a measure of time with a fixed sum of SI units seconds
• the day as a period defined by a selectable number of time units, e.g. B. a collectively agreed working day
• as a lived period of time
• social day , how a daily routine is conveyed to participants in society as usual, culturally different
• subjective day , how someone organizes and experiences the time span from getting up to going to bed or getting up again
• as a successive period of time
• as a generalized time span

The term day is used both for periods of time and for units of measurement .

Different definitions of the day limits - whether this is the true, apparent or mean rise, set or passage from the edge or center of the sun as an observed, calculated, fixed or announced date - as well as various circumstances to be taken into account for precise time determination - such as the equation of time , time zones , leap day , leap seconds , reference points and reference systems - mean that, for example, also the beginning of a calendar day can be set differently depending on the cultural context.

The terms day and night can be used individually or together in different ways. The concept of clear day - daytime versus night - equivalent to a modified Sun idealized the astronomical term diurnal arc of the sun .

## Day as a measure of time

Physical unit
Unit name Day
Unit symbol ${\ displaystyle \ mathrm {d}}$ [1]
Physical quantity (s) Time , span of time
Formula symbol ${\ displaystyle t; \, T; \; \, \ tau, \, \ mathrm {T}}$
dimension ${\ displaystyle {\ mathsf {T}}}$
system Approved for use with the SI
In SI units ${\ displaystyle \ mathrm {1 \, d = 86 \, 400 \; s}}$

In metrology , a unit of measurement “day” of the physical quantity time (duration) is defined as a certain multiple of the base unit second of the International System of Units (SI). The unit symbol is the small letter "d", after the Latin word this for day.

1 d = 24  h = 1440  min = 86 400  s

Time can be specified in days with hours and subdivisions, or in fractions of a day .

The unit “day” does not belong to the International System of Units (SI), but is approved for use with the SI. Like the hour and minute, it is also the legal unit of measurement in accordance with EU directive 80/181 / EEC (units directive ) as well as in accordance with the German and Swiss unit regulations. The definition is chosen so that "d" corresponds approximately to the mean duration of sun-related days on earth.

Since the naturally occurring sunny days last different lengths of time as a result of the periodic fluctuations and also because of non-periodic shifts, there are differences to a reference pattern, which is based on the unit of measurement “d” as the standard for day. Only with a reference system in the background can a repeatable time measure be constructed using the equation of time for the different time periods of actual days , related to a unit of measurement and, if necessary, adjusted using leap seconds .

Coordinated Universal Time ( UTC ) is formed in a similar way today .

## Calendar day

The calendar is the calendar account as a period of time in addition to the calendar year , and sometimes to the calendar month , the basic size.

### Gregorian calendar day

In the Gregorian calendar in use around the world today, a day is the time span from one midnight to the next midnight.

A 24- hour period that begins at 12:00 AM and ends at 12:00 AM. Midnight coincides with the beginning of the next day ( ISO 8601 )

A combination like May 5th, i.e. determined by month and day number, but without a year, is called a calendar day .

According to ISO 8601, the calendar days are numbered consecutively within a month starting with "1" as the calendar date and fixed in writing in a date format . In addition, they are assigned a day of the week in a fixed order, regardless of the month and year . The date of the day describes a continuous time scale ( linear time ), in contrast to the day of the week, which is repeated regularly in its sequence ( cyclical time ).

The beginning and end of such a day depend on the time zone to which the information relates.

### Days in other calendar and time systems

The start of the day at “midnight” is based on an agreement based on astronomical conventions. Other calendar systems set the start of the day on the " sunrise ". In the Jewish and Islamic calendars , the day includes the time from one “ sunset ” to the next sunset. This view was prevalent for a long time in the whole of Europe and the Middle East. The Roman counting of the night hours (vigiliae) and certain elements of the Christian rite can be given as examples. The best-known example is probably the beginning of Christmas (December 25th) on the previous evening, which according to modern calculations still belongs to December 24th ( Christmas Eve ). Setting the start of the day at sunset is particularly useful in combination with lunar calendars , in which the month also begins in the evening with the new crescent moon that is then visible.

Even today, many public holidays are celebrated the evening before, for example as Christmas Eve or St. a holiday with the end of the day.

A special feature is the synodic lunar days Tithi the Vedic calendar, which vary in duration from 19 to 26 hours with 1 masa ( lunar month ) = 30 tithi

## Astronomical days

### Special terms for the duration of a day

Conventionally, the duration of a day is defined as the amount of time that the earth or a celestial body needs to complete a single rotation in relation to a star, measured precisely from one culmination to the next or between one meridian passage and the next similar one. With regard to a distant star, assumed to be fixed , this is a sidereal day and is equivalent to a complete rotation of the body around itself. With regard to the sun, referred to as the central star, such a solar day is not equal to a complete rotation period of the body its axis - because the course around the sun in itself produces a day-night cycle during the annual cycle.

There are various quantities similar to the calendar day, which have their origin in the complex movements of the celestial bodies and the various reference points in celestial mechanical calculations:

True sunny day
The time span from one sun high to the next sun high or from one sun low to the next (mostly from midnight (lower meridian passage) to midnight). The true solar time , the true local time (WOZ), is based on the sunny day . Duration: around 24 hours plus / minus around 30 seconds
Mean sunny day
Mean sunny day is the average duration of true sunny days averaged over the years, which was initially only calculated for astronomical purposes.
Duration: around 24 hours
Civil day
In an effort a temporal reference scheme to construct a constant amount of time, the (full) on each day is applicable, the mean solar day as a reference value for the period of the so-called bourgeois day was (English civil day ) used and equal to 24 x 60 x 60 shares divided up. In today's calendar calculation, a day, the calendar day, is usually based on this civil day . Occasionally, the duration of a calendar day is changed by inserting a leap second in order to compensate for deviations from the average value of sunny days that occur because the earth's rotation is not constant. With these adjustments, the universal time ( UTC ) is on the one hand matched to the universal time ( UT1 ) - except for a small difference ( dUT1 ) - and on the other hand it is coordinated with the atomic time .
Duration: 24 hours (plus / minus 1 leap second)
Sidereal day
The time of revolution of the earth in relation to the fixed stars . The sidereal day (in English "stellar day") does not refer to the exposure by the sun, but to the light of other distant stars that are assumed to be fixed.
Duration: around 23 hours 56 minutes 4.10 seconds
Sidereal day
The rotation time of the earth in relation to the culmination of the vernal equinox is - not quite accurately - as sidereal called (in English "sidereal day"). Sidereal time is based on sidereal day ; its duration differs from the Sidereal Day by about 8 thousandths of a second, but is important for more precise astronomical calculations.
Duration: around 23 hours 56 minutes 4.09 seconds
Ephemeris day
The day that is based on the ephemeris second or ephemeris time is called ephemeris day .
Duration: around 24 hours

To determine the world time or to find star locations , the solar time or the sidereal time is given in reference to the prime meridian .

### Alien days

In a more general form, a day is understood to mean the time span between two successive identical or comparable exposure phases on a celestial body . In relation to its exposure through the orbiting central star, a day results from the fact that the rotational movement of the body and its orbital movement are related to each other according to their duration, level and direction.

For example, in addition to the day on earth , there is also a “ Martian day ” (called “ Sol ”) and a “ Mercury day ” in relation to the sun; measured in terrestrial time norms - d as the unit of measure day based on the SI second - a day on Mars lasts about 1 d and 40 minutes and a day on Mercury about 176 d. The " lunar day " as a day on the earth's moon is about 29.53 d long on average; this corresponds to a period of the phases of the moon if they are viewed from the earth - from one new moon to the next new moon this is equal to a synodic month .

### Earlier earthly days

Since the rotation of the earth is slowed down in the course of time - especially by the tidal effects of the moon - future earth days tend to be longer; conversely, a day on earth did not last as long as it does today. About 600 million years ago, the earth completed a full rotation around itself in about 22 hours today. Since the orbit around the sun took about as long as it does today, a year back then had almost 400 days of sunshine. Evidence for this can be found in the cyclically deposited sediments ( varvae ) of Precambrian rocks.

For the very young Earth around 4.5 billion years ago, numerical simulations resulted in a day duration of around 6 hours. The conditions of even earlier times before the formation of the moon and a presumably previous collision of the hypothetical protoplanet Theia with the proto-earth are difficult to reconstruct.

## Subjective and social day

Allegory Day by Ludwig Brunow , 1880.

In daily life, the "subjective day," English also awake time period determined by the rhythm of getting up and going to bed. The day is often divided into the sections morning , morning , noon , afternoon , evening and night .

Biological rhythms occur with different period durations - several years, for example a year or a month or a day or even shorter, ultradian periods of time - and can be understood as repeated patterns of the adaptation of internal states to external circumstances. The change in the internal process readiness of an organism is organized as an endogenous rhythm and linked via certain signals to the temporal fluctuations in the course of changes in its environment. If the environment hardly changes or if there are no corresponding external signals, the endogenous rhythm runs freely with its own period length. If this is about a day, it is referred to as the circadian rhythm . This endogenous circadian rhythm is generated in an organism - it is found in plants and animals such as humans - by a vibrating subsystem, called an oscillator or internal clock , which acts as a pacemaker and now specifies possible clocks as a phase, the length or interval of which is then external Stimuli, called timers , are more finely tuned. As a result, internal and external conditions can be reconciled with regard to their temporal structure and be as synchronous as internal vibrations have been adjusted to changed external fluctuations ( entrainment ).

Most of the chronobiologically examined living things construct the appropriate actual daily rhythm with light as the most important timer; for the organization of suitable natural references, the light of the day sets the time.

Therefore, the bright day also forms the basis of the social and subjective terms of the day: Until the introduction of artificial lighting, natural light had to be used for almost all work - in many industries and parts of the world to this day. For the overwhelming majority of people, the internally experienced subjective day and the externally required “objective” reference to time hardly differ. In the moderate latitudes, however, the daily routine mostly no longer corresponds to the clear day; In the summer months you generally only wake up long after daybreak, in winter you wake up before that - and while the sun is over the horizon, many people are not even outside. This is seen as one of the causes of seasonal (winter) depression ; the strength of artificial lighting is only a fraction of the luminance of a naturally bright day.

The situation is less easy for people whose subjective day often or regularly does not follow the normal daily routine (social day). After shift work until, above or after midnight, some of these people intuitively perceive the following time as belonging to the previous day. They then notice the shift to the calendar day, for example when writing written dates . More problematic, however, is the shift in sleep rhythm towards the clear day, which can also lead to health disorders ( shift worker syndrome). In some circadian sleep-wake arrhythmias , the personal day shifts so much that it overlaps with the next social day. People whose personal day as an individual lifestyle seems permanently shifted compared to the bright day are called night owls .

Another problem arises from the possible time difference compared to other time zones . In modern everyday life, time zone clocks help to clarify which day is today in another location , or e-mails are dated in UTC and only converted on site. When traveling long-distance to other time zones, jet lag can occur due to the lack of entrainment of the internal clock .

Commons : Day  - collection of images, videos and audio files
Wiktionary: Tag  - explanations of meanings, word origins, synonyms, translations
Wikiquote: Tag  - Quotes

## Individual evidence

1. day, m. . In: Jacob Grimm , Wilhelm Grimm : German Dictionary . Hirzel, Leipzig 1854–1961 ( woerterbuchnetz.de , University of Trier).
2. The history of the unit of time. PTB , accessed on March 1, 2019 .
3. In the geocentric picture of the day arc of the apparent is the sun's course considered path of the sun around the earth. In the heliocentric picture this is understood as the movement of the earth, broken down into its rotation and its orbit . So set out in 1543 in De revolutionibus orbium coelestium by Nicolaus Copernicus ; later understood as the " Copernican turn ". Subsequently, Johannes Kepler describes the orbit in Astronomia Nova in 1609 - instead of circling on spheres - as a movement on an elliptical orbit and formulates Kepler's laws .
4. The International System of Units (SI) . German translation of the BIPM brochure "Le Système international d'unités / The International System of Units (8e édition, 2006)". In: PTB-Mitteilungen . tape 117 , no. 2 , 2007, p. 18–21 ( Online [PDF; 1.4 MB ]).
5. Directive 80/181 / EEC , Chapter 1 of the Appendix, No. 2.
6. Unit Ordinance , Art. 15.
7. Researchers study the human body clock. In: CORDIS. Publications Office of the EC, January 23, 2007, accessed on September 22, 2009 (on study T. Roenneberg, et al .: The human circadian clock entrains to sun time. Current Biology, 2007, 17: R44-R45.). see. EUCLOCK: Humans , EU research project on chronobiology, subproject human research at the University of Basel.