Hour

The hour (symbol: h) is a unit of time. It is not an SI unit but is accepted for use with the SI.

History

The hour was originally defined in Egypt as 1/24 of a day, based on their duo-decimal numbering system (which counted finger joints on each hand).

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In modern usage, an hour was redefined as a unit of time 60 minutes, or 3600 seconds in length. It is approximately 1/24 of a median Earth day.

Related Topics:
Unit - Time - Minute - Second - Earth - Day

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There is also the hour of right ascension, a unit of both time and angle.

Related Topics:
Right ascension - Angle

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Earlier definitions of the hour:

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• One twelfth of the time from sunrise to sunset. As a consequence, hours on summer days are longer than on winter days, their length varies with latitude and even, to a small extent, with the local weather (since it affects the atmosphere's index of refraction). For this reason, these hours are sometimes called temporal or unequal hours. Romans and Greeks used this definition and divided the night into three or four night watches. Later, the night (i.e., the time between sunset and sunrise) was also divided into twelve hours. When a clock showed these hours, its period had to be changed every morning and evening (for example by changing the length of its pendulum), or it had to keep position of the Sun on the ecliptic (see Prague Astronomical Clock).
• One twenty-fourth of the apparent solar day (between one noon and the next, or between one sunset and the next). As a consequence, hours vary a little, as the length of an apparent solar day varies throughout the year. When a clock showed these hours, it had to be adjusted a few times in a month.
• One twenty-fourth of the mean solar day. See mean sun for more information on the difference to the apparent solar day. When an exact clock showed these hours, it had to be adjusted virtually never. However, as Earth rotation slows down, this definition has been abandoned. See UTC.