A day (symbol: d) is a unit of time equivalent to 24 hours. For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of The hour (symbol h) is a unit of Time. It is not an SI unit but is accepted for use with the SI It is not an SI unit but it is accepted for use with SI.  The term comes from the Old English dæg. The word is also used to mean daytime, the period of daylight experienced once per day and alternating with night. On Earth, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or (especially direct Daylight or the light of day is the combination of all direct and indirect Sunlight outdoors during the daytime (and perhaps Twilight) Night or nighttime is the period of Time when the Sun is below the Horizon.
The day has several definitions.
A day contains 86,400 SI 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.
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 the decimal time used with the French Republican Calendar, which had already been given up. Decimal time is the representation of the Time of Day using units which are Decimally related The French Republican Calendar or French Revolutionary Calendar was a Calendar proposed during the French Revolution, and used by the French government
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:
The word refers to various relatedly defined ideas, including the following:
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). Solar times are measures of the apparent position of the Sun on the Celestial sphere. 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 rotation is a movement of an object in a circular motion A two- Dimensional object rotates around a center (or point) of rotation In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star
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. Night or nighttime is the period of Time when the Sun is below the Horizon. 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. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. 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. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Refraction is the change in direction of a Wave due to a change in its Speed. The horizon ( Ancient Greek ὁ ὁρίζων, /ho horídzôn/ from ὁρίζειν, "to limit" is the apparent line that separates 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. Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the
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. Sunrise is the instant at which the upper edge of the Sun appears above the Horizon in the East. Sunset, also called sundown in some American English Dialects is the instant when the trailing edge of the Sun 's disk disappears below Longitude (ˈlɒndʒɪˌtjuːd or ˈlɒŋgɪˌtjuːd symbolized by the Greek character Lambda (λ is the east-west Geographic coordinate measurement A year (from Old English gēr) is the time between two recurrences of an event related to the Orbit of the Earth around the Sun This is the time as indicated by ancient hemispherical sundials. A sundial is a device that measures time by the position of the Sun.
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). This article is about the geographical concept For other uses of the word see Meridian. Noon (also midday) is the hour of 1200 in an observer's local time zone or more loosely a time near the middle of the day when workers in many countries take a meal break In astronomy the culmination, at a given point of a Planet, Star, Constellation, etc Midnight is literally "the middle of the night" In most systems it is when one day ends and the next begins when the date changes 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 celestial equator is a Great circle on the imaginary Celestial sphere, in the same plane as the Earth 's Equator.
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. Characteristics A tide is a repeated cycle of sea level changes in the following stages Over several hours the water rises or advances up a beach in the flood Because of the way the second is defined, the mean length of a day is now about 86,400. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units 002 seconds, and is increasing by about 1. 7 milliseconds per century (an average over the last 2700 years). See tidal acceleration for details. Tidal acceleration is an effect of the Tidal forces between an orbiting Natural satellite ( i
During the biblical Creation week, the day appears in several forms: As the seven days in the Creation week ("the evening and the morning", a nychthemeron or 24-hour day), as the light created during the first day ("Let there be light … and God called the light Day" (daylight, not night, Genesis 1:3-5), as periods of time delimited by the lights created during the fourth day ("for seasons, and for days, and years", Genesis 1:14), and for the Sun created during the fourth day to rule ("the greater light to rule the day", daylight, Genesis 1:16). Creation according to Genesis refers to the Hebrew narrative of the creation of the heavens and the earth as told in chapters 1 and 2 of Genesis Nychthemeron or nycthemeron or nuchthemeron (from Greek nykt- "night" (hemera "day" is a period of 24 consecutive hours
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. The 19th century of the Common Era began on January 1, 1801 and ended on December 31, 1900, according to the Gregorian calendar 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. In astronomy the culmination, at a given point of a Planet, Star, Constellation, etc A day is commonly divided into 24 hours of 60 minutes of 60 seconds each.
The actual mean period of rotation of the earth with respect to the sun is slightly longer than the SI day of 86,400 seconds. It is more nearly 86,400. 002 seconds. This additional time accumulates to about 0. 7 s per year or about seven seconds every ten years, necessitating the addition of an extra second to the civil clock occasionally to retard it and keep it more closely synchronized to the apparent movement of the sun. By the middle of this century the amount of time to be added to the clock will increase to one second every year. This additional second is called a leap second. A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time. 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. "IERS" redirects here for other uses see IERS (disambiguation The International Earth Rotation and Reference Systems Service is 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. Events 350 - Roman usurper Nepotianus, of the Constantinian dynasty, is defeated and killed by troops of the Usurper Events 406 – Vandals, Alans and Suebians cross the Rhine, beginning an invasion of Gallia.
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. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Sidereal time is a measure of the position of the Earth in its rotation around its axis or time measured by the apparent Diurnal motion of the Vernal equinox 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.
For most diurnal animals, including Homo sapiens, the day naturally begins at dawn and ends at sunset. Dawn refers to the Twilight before Sunrise. It is recognized by the presence of weak sunlight while the sun itself is still below the horizon Sunset, also called sundown in some American English Dialects is the instant when the trailing edge of the Sun 's disk disappears below Humans, with their 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). Judaism (from the Greek Ioudaïsmos, derived from the Hebrew יהודה Yehudah, " Judah " in Hebrew יַהֲדוּת Yahedut Sunset, also called sundown in some American English Dialects is the instant when the trailing edge of the Sun 's disk disappears below 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. Florence ( Italian: Firenze Florentia and Fiorenza) is the Capital City of the Italian region of Tuscany 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. Christmas Eve, December 24, is the day before Christmas Day, the celebrated birthday of Jesus. Halloween, or Hallowe’en, is a Holiday celebrated on the night of October 31. Saint Agnes (291–304 feast day January 21) is a Virgin Martyr Saint of the Roman Catholic Church and Eastern Catholic Churches Lists of holidays The words holiday or vacation have related meanings in different English-speaking countries and continents but will usually refer to one of 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). Midnight is literally "the middle of the night" In most systems it is when one day ends and the next begins when the date changes The hour (symbol h) is a unit of Time. It is not an SI unit but is accepted for use with the SI
In ancient Egypt, the day was reckoned from sunrise to sunrise. Ancient Egypt was an Ancient Civilization in eastern North Africa, concentrated along the lower reaches of the Nile River in what is now Sunrise is the instant at which the upper edge of the Sun appears above the Horizon in the East. Muslims fast from daybreak to sunset each day of the month of Ramadan. A Muslim (مسلم pronounced Muslim, not Muzlim) is an adherent of the Religion Ramadan or Ramazan ( Arabic: رمضان Ramaḍān) is a Muslim religious observance that takes place during the ninth month of the Islamic 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. The Damascus Document is the name given to one of the works found in multiple fragments and copies in the caves at Qumran, and as such is counted amongst the Dead Sea Scrolls The Dead Sea Scrolls consist of roughly 1000 documents including texts from the Hebrew Bible, discovered between 1947 and 1979 in eleven Caves Shabbat or Shabbos ( Hebrew: שַׁבָּת, shabbāt, shabbes, "rest/inactivity" is the Weekly Sabbath
In the United States, nights are named after the previous day, e. The United States of America —commonly referred to as the g. "Friday night" usually means the entire night between Friday and Saturday. Saturday is the day of the week between Friday and Sunday. It is either the sixth or seventh Day of the week as discussed below. This is the opposite of the Jewish pattern. 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"). PLEASE BEAR IN MIND THAT THIS IS A GENERAL ARTICLE ABOUT VCRs/VIDEOCASSETTE RECORDERS Expressions like "today", "yesterday" and "tomorrow" become ambiguous during the night.
Validity of tickets, passes, etc. A ticket is a Voucher to indicate that one has paid for Admission to an event or establishment such as a Theatre, Movie theater, Amusement , 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. Nederlandse Spoorwegen ( Dutch Railways) or NS, is the principal passenger Railway operating company in the Netherlands. 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.
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. God is the principal or sole Deity in Religions and other belief systems that worship one deity. 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.