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

Famous quotes containing the word day:

    “Suppose they had saved up all my punishments?” she went on, talking more to herself than to the kitten. “What would they do at the end of a year? I should be sent to prison, I suppose, when the day came. Or—let me see—suppose each punishment was to be going without a dinner: then, when the miserable day came, I should have to go without fifty dinners at once! Well, I shouldn’t mind that much! I’d far rather go without them than eat them!”
    Lewis Carroll [Charles Lutwidge Dodgson] (1832–1898)

    To get time for civic work, for exercise, for neighborhood projects, reading or meditation, or just plain time to themselves, mothers need to hold out against the fairly recent but surprisingly entrenched myth that “good mothers” are constantly with their children. They will have to speak out at last about the demoralizing effect of spending day after day with small children, no matter how much they love them.
    —Wendy Coppedge Sanford. Ourselves and Our Children, by Boston Women’s Health Book Collective, introduction (1978)

    The arbitrary division of one’s life into weeks and days and hours seemed, on the whole, useless. There was but one day for the men, and that was pay day, and one for the women, and that was rent day. As for the children, every day was theirs, just as it should be in every corner of the world.
    Alice Caldwell Rice (1870–1942)