# Day

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A day is traditionally defined as the span of time it takes for the Earth or a celestial body (such as an other planet or a moon) to make a single rotation with respect to a star, measured most accurately from local noon to local noon. With a view to a far star assumed as fixed, this stellar day could be equivalent to one entire rotation of the body upon itself; in view of the Sun being the central star, this solar day does not equal a whole rotation period around the body's axis (obversely, the motion of revolving around the Sun would perform one day within an orbital period by its own without any spinning). 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 of the same length of time throughout the orbital period, in the course of a year. An additional disparity derives from the fact that the rotation periods, for instance of Earth's spin, are not exactly the same, mainly as a result of tidal effects.
The average length of a solar day on Earth is about 86,400 seconds (24 hours), and, until 1967, other smaller units of time (hour, minute, and second) were defined as specific fractions of the solar year for 1900. In 1967, the second was redefined in terms of the wavelength of light, and it became the SI base unit of time. The measurement unit of time called "day", defined as 86,400 SI seconds and symbolized d is not an SI unit, but it is accepted for use with SI.[1] It equates to the civil day, plus or minus a possible leap second in Coordinated Universal Time UTC.
However, the solar day on Earth is a natural phenomenon a lot of species refer to, when they constitute patterns of proceeding and the like circadian rhythms.

The word day can also refer to the (roughly) half of the day that is not night, also known as 'daytime'. Within these meanings, several definitions can be distinguished. 'Day' may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?".