Day

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Day by fluentwater.jpg

Origin

Middle English, from Old English dæg; akin to Old High German tag day

Definitions

  • 1:a. a period of twenty-four hours as a unit of time, reckoned from one midnight to the next, corresponding to a rotation of the earth on its axis.
b. the part of a day when it is light; the time between sunrise and sunset: she sleeps all day and goes out at night | the animals hunt by day.
c. the part of a day spent working: he works an eight-hour day.
d. Astronomy a single rotation of a planet in relation to its primary.
e. Astronomy the period on a planet when its primary star is above the horizon.
f. archaic daylight: by the time they had all gone it was broad day.
  • 2: a. a particular period of the past; an era: the laws were very strict in those days.
b. (the day) the present time: the political issues of the day.
c. [ with adj. ] a day associated with a particular event or purpose: graduation day | Christmas Day.
d. a day's endeavor, or the period of an endeavor, especially as bringing success: speed and surprise would win the day.
e. [ usu. with modifier ] (days) a particular period in a person's life or career: my student days.
f. (one's day) the successful, fortunate, or influential period of a person's life or career: he had been a matinée idol in his day.
g. (one's days) the span of someone's life: she cared for him for the rest of his days

Description

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 during which the Earth completes one rotation with respect to the sun 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).

In recent decades the average length of a solar day on Earth has been about 86 400.002 seconds[4] (24.0000006 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 a stellar day. This period of rotation is about 4 minutes less than 24 hours (23 hours 56 minutes and 4.1 seconds) 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.[1]