Hour

An hour (also called saacad) is a unit of time used to measure duration. An hour comprises 60 minutes, and each minute comprises 60 seconds. Therefore one hour equals 3,600 seconds.

A full day, using standard measurement, comprises 24 hours. Hour systems are divided into two types: the 24-hour system, and the 12-hour system, which divides the day into two parts, morning and afternoon/night.

Relationship to the international system of units

In the International System of Units (SI), the fundamental unit of time is the second. The hour and minute are accepted units for use with SI, but are not fundamental units. The second is defined on the basis of changes within the caesium atom, which atomic clocks use to measure time with precision.

History and origin of division

The division of the day into 24 parts traces its origin to ancient Egyptian and Mesopotamian practice. The Babylonian sexagesimal system led to the division of minutes and seconds into 60. In early times, the length of an hour varied depending on the time of year, as both day and night were each divided into 12 parts. Only after mechanical clocks were invented did the hour become a fixed duration.

Time zones

The world is divided into time zones, most of which differ by whole hours or half-hours from Coordinated Universal Time (UTC). Some countries also use daylight saving time, which shifts the hour forward by one hour for part of the year. Somalia is in the UTC+3 zone.