Hour

An hour (symbol: h; also abbreviated hr) is a unit of time historically reckoned as 1⁄24 of a day and defined contemporarily as exactly 3,600 seconds. 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 hours, also known as temporal hours or unequal hours, varied by season and latitude. Equal hours 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 Earth's rotation, the hour was finally separated from the Earth's rotation and defined in terms of the atomic or physical second.