Day

The day (mhd. tag tac, asächs. dag, got. dags, urgerm. *dagaz) is used in different 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 broad daylight.
• Deviating from this, also the period of being awake and active of the people, which is considerably longer than the light day up to middle latitudes and especially in winter (northern hemisphere of the earth), is called day.
• In deviation from the light day, its sum with the night is also called day, more precisely full day or full day.
• Individual consecutive full days are called calendar days, which today usually last from midnight to midnight. The night thus belongs to two different calendar days.
• A solar day is the period 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 (true solar day) given by the course of the sun varies weakly over a year, a mean solar day (also called civil day) averaged from it is used generally and for the length of the calendar day.

The mean solar day was traditionally the basis for the unit second: . Since 1956, however, their length has been represented differently (atomic second). Currently, the mean solar day is 86,400.003 atomic seconds long. However, the calendar day continues to have a length of 86,400 s, except for a few days in which a leap second is inserted.

Basic terms

Variable daylight

Variable daylight is important at least for all those living beings that refer to light as a timer to coordinate internal and external processes and develop repeatable patterns of behaviour. Recurring bright phases of illumination during the day alternating with dark phases at night occur at locations on the earth's surface as a result of the rotation of the earth and its orbit around the sun.

However, these time spans are not of constant duration, but fluctuate in the course of the year, depending on the location to varying degrees. Thus, at 50° latitude, the span of broad daylight at 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 date of an equinox is then the same for the whole earth.

Time span day

The time period of day is also called daylight day. With daylight, it is day that lasts from sunrise to sunset. This span corresponds to the diurnal arc of the apparent course of the sun and is divided into halves of the day with the highest position of the sun in the middle of the day; it can also be divided into times of day and (often twelve) hours of the day.

The duration of each of these day segments depends on the length of the day and therefore varies more with increasing distance from the equator. In places with a 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 called a polar day. Days understood as (light) days are thus time spans of very different durations.

Time span from day and night

The period consisting of day and night is called a "full day" if the span illuminated by daylight is grasped together with the adjacent night between two comparable phases of exposure, for example outside the polar regions from one sunrise to the next sunrise or from one sunset to the next (nychthemeron). Since the sun rises earlier and sets later from day to day in the spring half-year and rises later and sets earlier from day to day 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 depends on the geographical latitude. A more useful reference point for defining the full day is noon, which lies almost symmetrically in the middle of the clear day, at which time the sun crosses the meridian at its upper culmination, the highest position above the horizon, and its hour 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 hour angle is equal to 12h = 180° (this position of the Sun is below the horizon all year round in the non-polar regions of the Earth). The full day, defined as the interval between two successive mid-days or mid-nights, is a solar day; its duration is the same for all places and varies only slightly even in the course of the year. In civil life, the solar day begins at midnight; in astronomy, the solar day beginning at noon is also common. The local solar time, defined as the hour angle of the sun ± 12 hours, is used for the temporal division of a solar day; thus it is 0h at midnight and 12h at noon.

Duration of the time intervals

The exact duration of the time interval between two consecutive middays or midnights changes in 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 according to Kepler's laws on an elliptical orbit around the sun, the annual revolution of the projection of the direction of the sun on the equator occurs with a variable angular velocity.

Therefore, the hour angle of the sun lying in the equatorial plane, even assuming a constant rotation speed of the earth, changes unevenly. So also the duration of a true solar day, in which the hour angle of the sun changes in 24h = 360°, i.e. approximately the duration from one lower culmination of the sun (= midnight) to the next, is variable. True solar days can differ by up to about one minute, the average value over the course of years for the time elapsing between the (lower) meridian passes is currently about 24 hours.

Average daily duration

The determined average value of the duration of true solar days, the mean solar day, provides the reference for the determination of the duration of the so-called civil day, which became the basis of the calendrical time reference. A time measure "day" of constant duration, the sequence of which is adapted to the current mean value by means of leap seconds, if any, is subordinated to this temporal reference scheme which is widespread today.

At the beginning of the 20th century, the time measures hour, minute, second were still defined as fractions of a mean solar day. However, when it became obvious that the Earth's rotation does not have a constant rotation period as assumed, the solar second, which is linked to the rotation, was initially replaced in the 1950s by the ephemeris second, which is based on the Earth's orbital motion, as the unit of time. Since the introduction of the atomic second at the end of the 1960s, the basic unit of this time measure is no longer determined astronomically, but is decoupled from the rotation and orbit of the Earth.

In the SI, the underlying unit of time is defined on the basis of atomic time, whose time standard atomic clocks set: the second. Its 86,400-fold is also called "day", indicated by the sign "d" (for Latin dies 'day'), and divided into 24 hours of 60 minutes of 60 seconds each.

Constant time dimension d

The usual constant time measure day (d) of 24 hours is 86,400 times the SI unit second. Its definition was chosen in such a way that the average solar day measured with it, rounded, now lasts 86,400 seconds (at present); any differences that occur are compensated for by leap seconds.

This time measure thus corresponds approximately to the present mean value, it does not indicate the true duration of a solar day. With the construction of consecutive constant time periods for the mean solar time, the true position of the sun at noon is regularly missed by a good quarter of an hour at the beginning of November at the most; the respective deviations can be calculated via the equation of time.

Actual daily duration

The actual duration of a true solar day varies somewhat from day to day; it is determined by the current orbital velocities during the orbit as well as by the rotational speed of the Earth, and deviates from 24 hours by up to about 30 seconds.

However, these time spans do not reflect the duration for a whole rotation of the earth. Already Copernicus knew, one day-night-cycle can not correspond to one whole rotation of earth, if earth is running around sun. Because without any self-rotation, already exactly one day-night-cycle results during one whole revolution.

Precise timing

If one measures the time span 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, the Earth's rotation 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 rotation duration of the earth of about 86,164 seconds.

Viewed against the background of the fixed star, the Earth completes exactly one rotation more than the number of (bright, full, solar or calendar) days related to the Sun. Since the rotation of the earth is in the same direction (prograde) as its course around the sun, its duration is about 24 h/366 or just under 4 minutes shorter than a mean solar day. At present, the Earth's period of revolution is about 23 hours 56 minutes and 4.10 seconds, with variations in the range of milliseconds and a long-term tendency to increase. An increasing period of revolution is reflected in increasing day lengths, both for the full day and the daylight day, under the same orbital conditions.

It is true that looking at the starry sky and viewing the position of the sun as a temporal reference in everyday life today is replaced for some by looking at the clock, sometimes with dual time for down under. But light is still time-giving for people, so also for the given date ("this given", Latin: datum) of today, on this day - which then is given subjective meaning, for example as birthday, on which someone "saw the light of day".

Day as time period

Starting from the basic concept - to indicate a temporal relationship in relation to the phase of exposure at a location - restricted or extended, special and general concepts of day have been developed:

• as an indefinite range
• day or day not more specifically defined or referred to than the time with light, the daylight
• as a rough guide
• Time of day such as morning, midday, afternoon or evening
• as a set period of time
• light day as the light time from sunrise to sunset
• full day consisting of the ranges at night and during the day between comparable sun positions, e.g. Nychthemeron
• solar day as time interval between midnight and midnight
• as measured time
• a day as a fixed 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 second
• the day as a period defined by a selectable number of time units, e.g. a collectively agreed working day
• as a lived period of time
• social day, how a daily routine is conveyed as usual for participants in society, culturally different
• subjective day, how someone organizes and experiences his time span from getting up to going to bed or getting up again
• as a successive period
• Day of the week, with countable place in the arrangement of a series of days
• Calendar day, summable in sequence, so colloquially "date".
• as a generalized time span
• terms analogous to day on earth, transferable to celestial bodies

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

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

The terms day and night can thus be grasped individually or together in different ways. The concept of daylight - during the day as opposed to at night - corresponds idealized to the astronomical concept of the day arc of the sun as a changed position of the sun.