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mid-13c., from Old French hore "one-twelfth of a day" (sunrise to sunset), from Latin hora "hour, time, season," from Greek hora "any limited time," from PIE *yor-a-, from root *yer- "year, season" (see year). Greek hora was "a season; 'the season;'" in classical times, sometimes, "a part of the day," such as morning, evening, noon, night. The Greek astronomers apparently borrowed the notion of dividing the day into twelve parts (mentioned in Herodotus) from the Babylonians (night continued to be divided into four watches), but as the amount of daylight changed throughout the year, the hours were not fixed or of equal length. Equinoctal hours did not become established in Europe until the 4c., and as late as 16c. distinction sometimes was made between temporary (unequal) hours and sidereal (equal) ones. The h- has persisted in this word despite not being pronounced since Roman times. Replaced Old English tid, literally "time," and stund "period of time." As a measure of distance ("the distance that can be covered in an hour") it is recorded from 1785.
First found in Dan. 3:6; 4:19, 33;5:5. It is the rendering of the Chaldee shaah, meaning a "moment," a "look." It is used in the New Testament frequently to denote some determinate season (Matt. 8:13; Luke 12:39). With the ancient Hebrews the divisions of the day were "morning, evening, and noon-day" (Ps. 55:17, etc.). The Greeks, following the Babylonians, divided the day into twelve hours. The Jews, during the Captivity, learned also from the Babylonians this method of dividing time. When Judea became subject to the Romans, the Jews adopted the Roman mode of reckoning time. The night was divided into four watches (Luke 12:38; Matt. 14:25; 13:25). Frequent allusion is also made to hours (Matt. 25:13; 26:40, etc.). (See DAY.) An hour was the twelfth part of the day, reckoning from sunrise to sunset, and consequently it perpetually varied in length.
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, 124 of the Earth's rotation period relative to the stars, was about 10 seconds shorter than the hour of mean solar time.