Day

Day ( MHG tag tac, asächs. Dag, Goth, dags, PGmc. * Dagaz ) may have different concepts of time designate, which date back to a basic concept that allows people to respond to the perceived light and the periodic changes in lighting their environment by the sun relate to specify temporal relationships and to establish comparable in different forms spans of time.

In relation to the current day today is the past and the future yesterday morning, on the day as bright contrast at night time follows day. The daily period of light from sunrise to sunset, taken by itself is lighter day whose length of day location dependent to varying degrees seasonal variations. An exposure phase taken together with the dark interval is the concept of time full day, complement each other in the ( lights ) day and night as part of and contrary to a common range.

The time of the full day there corresponds a day-night cycle, its duration depends on the current passages of the day - night boundary of a geographic location. Away from the poles currently pass on average about 24 hours from a daily high of the sun to the following, depending on rotation speed and train speed of the earth, with a coefficient of variation of about one minute in the course of the year. The actual duration of a true solar day is measured in each case for the period of one midnight (lower meridian passage of the sun) to the next. In between there is a lunch (upper meridian passage, 12 clock real time), which may differ from the time of the solar maximum object ( upper culmination) slightly.

Based on the average duration of sunny days, the mean solar day, the civil day is designed to a consistent measure of time is taken as a basis, which is adapted in today's calendar calculations by leap seconds. This constant tempo " day " ( d ) is the 86.400fache the time unit second, which is based since the introduction of the SI system on the atomic second and is thus decoupled from rotation and revolution of the Earth. The unit "d" 24 hours to 60 minutes of 60 seconds each are not the actual duration of a true solar day on even the time for a whole rotation of the earth again, which presents itself as a sidereal day before the fixed star background.

5.1 Special terms of the length of the day
5.2 extraterrestrials days
5.3 Earlier Earth days

Basic concepts

Day as the time

Starting from the basic concept - to provide with respect to the phase of exposure at a staging a temporal Verhalt - have been developed limited or extended, special and general terms of the day:

As indeterminate range day or day failed to identify, or related, as the time of light, the daylight

Time of day as in the morning, in the morning, noon, afternoon or evening

Lights bright clear day as the time from sunrise to sunset
Full day consisting of the tensioning night and day between comparable positions of the sun, eg Nychthemeron
Sunny day as the time interval between midnight and midnight

One days as a fixed number of uniform time intervals, such as 24 hours in WOZ
A standard tag as a measure of time with a fixed sum of SI units seconds
The day as defined by a selectable number of time units of time, eg a tariff working

Social day, like a daily routine for participants is mediated in society than usual, culturally diverse
Subjective day, like someone 's time of getting up to going to bed or getting up again organized and experienced

Weekday, with a countable point in the arrangement of a number of days
Calendar, summable in rows, so colloquially as " Date"

The day on Earth analog terms, be transferred to the heavenly bodies

The name tag is therefore used for both time periods as for units of measurement.

Different definitions of the daily limits - if this is the real, apparent or medium rise, destruction or passage of the edge or center of the sun as an observed, Calculated, fixed or Announced date is now - as well as different for precise time determinations factors to be considered - such as the equation of time, time zones, leap day, leap seconds, reference points and reference systems - cause, for example, also the beginning of a calendar day can be set depending on the cultural context different.

The terms between day and night so that individually or together depending adopted different. The concept of the clear day - during the day compared to at night - corresponds to a modified Sun idealizes the astronomical term diurnal arc of the sun.

Day as a measure of time

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

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

The unit " day " is not part of the International System of Units ( SI), but is approved for use with the SI. This is a legal unit. The definition is chosen so that "d" is approximately equal to the average duration of the sun on the earth based days.

Since the days of sun naturally occurring as a result of periodic fluctuations and also due to non-periodic shifts last much different length, arise because differences to a reference pattern, the unit "d" is the standard for the day used as the basis. Only with a reference system in the background can be constructed on a repeatable measure of time then the equation of time for the various periods of actual days are based on a unit and adjusted by the switching seconds if necessary.

Similarly, the Coordinated Universal Time ( UTC) is formed today.

Calendar

The calendar is the calendar account as the time of the calendar year and sometimes the calendar month, the basic size.

Gregorian calendar

In the world today commonly used Gregorian calendar, a day is the period of one midnight to the next midnight.

A combination as May 5 therefore determined by month and day number, but no year is called a calendar.

The 8601 calendar days within a month of "1 " starting consecutively numbered calendar date according to the ISO and set out in writing in a date format. In addition, they will, regardless of month and year, given a week in a fixed order. This describes the date when a continuous time scale (linear time), in distinction to the weekday, regularly in his sequence is repeated ( cyclical time ).

Beginning and end of such a day depend on the time zone to which the claim relates.

Days in another calendar and time systems

The daily beginning at midnight is an agreement similar to the conventions of astronomy. Other calendar systems set the start of the day to the sunrise. In the Jewish and Islamic calendar day will be the period of one sunset to the next sunset. This view was prevalent in the European- Near East Total long. The Roman census night hours ( Vigiliae ) and certain elements of the Christian rite may be mentioned as examples. The best known example is likely to already be the start of Christmas (December 25 ) on its eve, the bill for modern yet for December 24 belongs ( Christmas Eve). The reduction of the days beginning on the sunset is useful especially in combination with moon calendar, the month in which also the evening begins with the then visible new crescent moon.

Even today, many holidays are celebrated on the eve, such as Christmas Eve or Nicholas Eve, because in some previous calendars in Europe the new day began, similar to Jewish and Islamic calendars not only at midnight, but already with the local dusk, and so a holiday with the celebration evening.

Astronomical days

Specific terms of the length of the day

Conventionally, the length of a day is defined as the amount of time that the Earth or a celestial body needs to make a single rotation with respect to a star, measured precisely by a culmination to the next, or between one meridian passage and the subsequent similar. With respect to a distant star, as assumed fixed, this is a Sidereal day and equivalent to one complete rotation of the body about itself in view of the sun, based as the central star, a solar day is such not equal to a whole period of rotation of the body about its axis - for the run around the sun brings with it by themselves during the annual round already a day - night cycle out. There are various the calendar similar sizes, which have their origin in the complex movements of the heavenly bodies and the various reference points sky mechanical calculations:

For the determination of the world or time for finding Sternörtern the solar time or the sidereal time is given in reference to the zero meridian.

Extraterrestrial days

In more general terms is meant to a celestial body at one day, the time interval between two successive, identical or comparable exposure phases. In relation to its exposure through the central star umlaufene a day will when the rotational movement of the body and its orbital motion can be a ratio is according to their duration, level and direction.

Thus, besides the day on Earth for example, a " Martian " ( " Sol ") and a " Merkurtag "; measured in terrestrial time standards - d as the unit of day based on the SI second - is a day on Mars for about 24 hours and 40 minutes and on Mercury about 176 days d The " lunar day " as a day on the earth- moon is on average about 29.53 days long d; this corresponds to a period of the moon phases, when viewed from Earth - from one new moon to the next new moon which is equal to one synodic month.

Earlier Earth days

Since the earth's rotation is slowed down over time - especially by tidal effects of the moon - are future Earth days tend to last longer; vice versa took a day on Earth before not so long today. Before about 600 million years ago, Earth underwent a full rotation around itself today in about 22 hours. Since the orbit around the sun about the same time took like today, a year had at that time almost 400 days of sunshine. Evidence can be found among others in the cyclically deposited sediments ( varves ) Precambrian rocks.

For the very young Earth about 4.5 billion years, numerical simulations showed one days duration of about 6 hours. The ratios even earlier times before the formation of the Moon and a presumed previous collision of the hypothetical protoplanet Theia with the proto- Earth are difficult to reconstruct.

Subjective and social day

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

Biological rhythms occur with different period on - for several years, about a year or a month or a day or even shorter ultradian periods of time - and can be understood as repeated patterns of adaptation of internal states to external circumstances. The change in the internal process readiness of an organism is organized as an endogenous rhythm and via certain signals to the temporal variations in the sequence of changes in its environment. The environment changes little or lack appropriate external signals, the endogenous rhythm runs freely with its own period length. If the approximately one day, is spoken by Circadianem rhythm. Generated this endogenous circadian rhythm in an organism - it is found in plants and animals, such as man - by an oscillating subsystem oscillator or internal clock called the acting as a pacemaker now pretends possible cycles as phase whose length or interval then outer stimuli, called timer is fine tuned. This allows internal and external relationships to be reconciled in terms of their temporal structure and be as synchronous as internal vibrations altered external fluctuations were aligned ( entrainment ).

Most chronobiologically investigated beings construct the matching actual daily rhythm with light as the most important timer; for the organization of appropriate natural emoluments thus acts the light of day time of print.

Therefore the light of day is also the basis of the social and subjective daytime terms: Until the introduction of artificial lighting had to be exploited for almost all work the natural light - in many industries and regions of the world to this day. Internally experienced subjective day and outwardly been requested "objective" time reference fall apart a little for the vast majority of people. In temperate latitudes, however, no longer corresponds to the daily routine at present mostly with the daylight; in the summer months you wake up generally until long after daybreak, in the winter you will already awake - and while the sun is above the horizon, many people do not keep on outdoors. This is seen as one of the causes of seasonal depression ( seasonal affective disorder ); the strength of artificial lighting is only a fraction of the luminance of a naturally bright day.

Less easy is the situation for people whose subjective day often or regularly not the bourgeois daily routine ( social tag ) follows, such as shift work. Such people describe intuitively the time after midnight as the previous day belonging. The shift to calendar them then drops to about while writing written dates, since the communication to - sleeping - rest of society is limited. More problematic, however, is the shift of the sleep schedule against the daylight, which can also lead to health problems ( shift worker syndrome) or causes the dreaded dog guard at the guard service. In some Circadian sleep-wake rhythm disorders, the personal day shifts so far that it overlaps or intersects with the next social day. People whose personal day seems to move as individual lifestyle permanently against the daylight, are called night owls.

Another problem arises from the possible time difference compared to other time zones. When communicating with people whose local time differs considerably from one's own, is a clarification as to what day it is there today, indispensable. In modern computer- based life that is supported by time zone watches, or the function that emails dated in UTC and converted only locally. When traveling across time zones occurs due to the desynchronization of the internal clock to various local bright days of jet lag.

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