An hour (symbol: h; also abbreviated hr) is a unit of time conventionally reckoned as 1⁄24 of a day and scientifically reckoned between 3,599 and 3,601 seconds, depending on the speed of Earths rotation. There are 60 minutes in an hour, and 24 hours in a day. The hour was initially established in the ancient Near East as a variable measure of 1⁄12 of the night or daytime. Such seasonal, temporal, or unequal hours varied by season and latitude. Equal or equinoctial hours were taken as 1⁄24 of the day as measured from noon to noon; the minor seasonal variations of this unit were eventually smoothed by making it 1⁄24 of the mean solar day. Since this unit was not constant due to long term variations in the Earths rotation, the hour was finally separated from the Earths rotation and defined in terms of the atomic or physical second. In the modern metric system, hours are an accepted unit of time defined as 3,600 atomic seconds. However, on rare occasions an hour may incorporate a positive or negative leap second, making it last 3,599 or 3,601 seconds, in order to keep it within 0.9 seconds of UT1, which is based on measurements of the mean solar day. Name
Hour is a development of the Anglo-Norman houre and Middle English ure, first attested in the 13th century.It displaced tide tīd, "time" and stound stund, span of time.
Astrolabes, sundials, and astronomical clocks sometimes show the hour length and count using some of these older definitions and counting methods. Counting from dawn
In ancient and medieval cultures, the counting of hours generally started with sunrise. Before the widespread use of artificial light, societies were more concerned with the division between night and day, and daily routines often began when light was sufficient. "Babylonian hours" divide the day and night into 24 equal hours, reckoned from the time of sunrise. They are so named from the false belief of ancient authors that the Babylonians divided the day into 24 parts, beginning at sunrise. In fact, they divided the day into 12 parts (called kaspu or "double hours") or into 60 equal parts. Unequal hours
Sunrise marked the beginning of the first hour, the middle of the day was at the end of the sixth hour and sunset at the end of the twelfth hour. This meant that the duration of hours varied with the season. In the Northern hemisphere, particularly in the more northerly latitudes, summer daytime hours were longer than winter daytime hours, each being one twelfth of the time between sunrise and sunset. These variable-length hours were variously known as temporal, unequal, or seasonal hours and were in use until the appearance of the mechanical clock, which furthered the adoption of equal length hours.This is also the system used in Jewish law and frequently called "Talmudic hour" (Shaa Zemanit) in a variety of texts.