Day

A day is a unit of time. In common usage, it is an interval equal to 24 hours. It also can mean the consecutive period of time during which the Sun is above the horizon of a location, also known as daytime. The period of time measured from local noon to the following local noon is called a solar day.

Several definitions of this universal human concept are used according to context, need and convenience. In 1967, the second was redefined in terms of the wavelength of light, and it became the SI base unit of time. The unit of measurement for time called "day", redefined in 1967 as 86,400 SI seconds and symbolized d, is not an SI unit, but it is accepted for use with SI. A civil day is usually also 86,400 seconds, plus or minus a possible leap second in Coordinated Universal Time UTC, and, in some locations, occasionally plus or minus an hour when changing from or to daylight saving time. The word day may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?" Day also refers to the part of the day that is not night — also known as 'daytime'. The life patterns of humans and many other species are related to Earth's solar day and the cycle of day and night (see circadian rhythms).

The average length of a solar day on Earth is about 86,400 seconds (24 hours) and there are about 365.2422 solar days in one mean tropical year. Because celestial orbits are not perfectly circular, and thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. A day, understood as the span of time it takes for the Earth to make one entire rotation with respect to the celestial background or a distant star (assumed to be fixed), is called stellar day. This period of rotation is about 4 minutes less than 24 hours and there are about 366.2422 in one mean tropical year (one more stellar day than the number of solar days). Mainly due to tidal effects, the Earth's rotational period is not constant, resulting in further minor variations for both solar days and stellar 'days'. Other planets and moons also have stellar and solar days.

Read more about Day:  Introduction, Etymology, International System of Units (SI), Astronomy, Colloquial, Civil Day, Leap Seconds, Boundaries of The Day, 24 Hours Vs Daytime

Other articles related to "day, days":

All Souls' Day
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24 Hours Vs Daytime
... To distinguish between a full day and daytime, the word nychthemeron (from Greek for a night and a day) may be used in English for the former, or more colloquially the term 24 hours ... languages also have a separate word for a full day, such as vuorokausi in Finnish, ööpäev in Estonian, dygn in Swedish, døgn in Danish, døgn in Norwegian, sólarhringur ... In Italian, giorno is used to indicate a full day, while dì means daytime ...
February 24
... February 24 is the 55th day of the year in the Gregorian calendar ... There are 310 days remaining until the end of the year (311 in leap years) ... By Roman custom, February 24 is the day added to a leap year in the Julian calendar ...

Famous quotes containing the word day:

    Our day you will find that you have stopped regarding your baby as a totally unpredictable and therefore rather alarming novelty, and have begun instead to think of him as a person with tastes, preferences and characteristics of his own. When that happens you will know that he has moved on from being a “newborn” and has got himself settled into life.
    Penelope Leach (20th century)

    All my life I believed I knew something. But then one strange day came when I realized that I knew nothing, yes, I knew nothing. And so words became void of meaning ... I have arrived too late at ultimate uncertainty.
    Ezra Pound (1885–1972)

    The Christian religion not only was at first attended with miracles, but even at this day cannot be believed by any reasonable person without one.

    David Hume (1711–1776)