hour

(h or hr)

   1. a traditional unit of time, equal to 60 minutes, or 3600 seconds, or 1/24 day². The custom of dividing the daylight into 12 hours goes back at least as far as the Babylonians, who liked to divide units by 12 because groups of 12 are easily divided into halves, thirds, or fourths. Originally an hour was 1/12 of the time between sunrise and sunset, so summer hours were longer than winter hours. Later, when people wanted to express times at night, it was natural to divide the night into 12 hours as well, making 24 hours in the day. Only after the invention of mechanical clocks, around 1300, did hours became equal intervals marked by clocks. The word comes from an ancient Greek word hora which originally meant a season, especially a religious season, and hence a "defined" period of time. In the Christian church hora came to mean one of the services held at seven specific times during the day, thus establishing the word as marking subdivisions of the day.

   2. a unit of angular measure used in astronomy, equal to 1/24 circle or 15°. Objects can be located in the sky in a coordinate system in which the equatorial plane is the same as that of the Earth. In this system, the latitude coordinate is called declination and is measured in degrees from the Equator to the poles, just as latitude is measured on the surface of the Earth. The longitude coordinate, called right ascension, is measured in hours from the longitude, traditionally known as the First Point of Aries, at which the Sun appears to cross the Equator on its northward journey in the spring.

   3. a unit of sidereal time in astronomy; see sidereal day.