Hour:

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. Learn what an hour is, how to measure it, and how to convert it to other units of time. Find out the history, tools, and applications of the 24-hour day and the 12-hour cycle. Learn the meaning and usage of the word hour, a unit of time equal to sixty minutes or one twenty-fourth of a day. Find out the origin, pronunciation, idioms, and related terms of hour in British and American English. Definition of Hour explained with real-life illustrated examples. Also, learn the facts about hours in a day and its history at SplashLearn.