International System of Units (SI)
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.
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:
1 apparent sidereal day
= a single rotation of a planet with respect to the distant stars
(for Earth it is 23.934 solar hours)
The word refers to various relatedly defined ideas, including the following:
The period of light when the Sun is above the local horizon (i.e., the period from sunrise to sunset), opposed to night. See Daytime (astronomy).
The full day covering a dark and a light period, beginning from the beginning of the dark period or from a point near the middle of the dark period.
A full dark and light period, sometimes called a nychthemeron in English, from the Greek for night-day.
The period from 06:00 to 18:00 or 21:00 or some other fixed clock period overlapping or set off from other periods such as "morning", "evening", or "night".
The mostly regular interval of one awaking, usually in the morning (personal day).
Dagr, the Norse god of the day, rides his horse in this 19th century painting by Peter Nicolai Arbo.
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). The reason for this... [continues]
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