hour, in timekeeping, 3,600 seconds, now defined in terms of radiation emitted from atoms of the element cesium under specified conditions. The hour was formerly defined as the 24th part of a mean solar day — i.e., of the average period of rotation of the Earth relative to the Sun. The hour of sidereal time, 1/24 of the Earth’s rotation period relative to the stars, was about 10 seconds shorter than the hour of mean solar time. World time and date for cities in all time zones. International time right now. Takes into account all DST clock changes. An hour is a unit of time — it's sixty minutes, 3,600 seconds, or the length it takes the long hand on your watch to move in a full circle. Learn about the Hour (h) unit of time: definition, symbol, history, common uses, and conversion options. Find Hour conversions to other time units.
