Hour

Hour, one twenty-fourth of a day. In one hour the earth rotates through 15°. Therefore an event such as sunrise occurs one hour later for every 15° west on the earth's surface. Each hour contains 60 minutes and 3,600 seconds. In astronomy, degrees of longitude are not used for measuring star positions. Astronomers instead talk of hours of right ascension. Each hour of right ascension corresponds to 15° on the celestial sphere. The hour as a unit of timekeeping has been in use only since the invention of clocks.