Hour

An hour is a unit of time equal to 60 minutes, or 3,600 seconds. In modern civil time, an hour is 1/24 of a mean solar day, and the day is divided into 24 hours, counted from midnight to midnight.

The word hour comes from Old French heure, from Latin hora, from Greek hora. Historically, many cultures divided the day into 12 daylight hours and 12 night hours, so hours varied with the seasons. With the invention of mechanical clocks in medieval Europe, hours became equal-length divisions of time.

Hours are subdivided into 60 minutes; minutes into 60 seconds. The second is the base unit of time in the International System of Units; an hour is defined as 3,600 seconds. Civil time uses 24-hour notation, and Coordinated Universal Time (UTC) serves as a global reference; time zones offset from UTC by whole hours or occasionally minutes, with daylight saving adjustments.

In astronomy, hour-related concepts include the hour angle and sidereal time, which use the Earth's rotation relative to celestial reference objects. Historically there were solar hours (unequal with seasons) before standardization.

In everyday life, hours organize work shifts, travel, and sport; in science and industry, hours quantify durations and rates. The hour remains a fundamental unit in most calendars and timekeeping systems worldwide.