Glossary term: Time Zone

Description: A time zone is a region where a uniform standard time is used. Our measures of time are linked to Earth's rotation. In particular, the moment of noon is linked to the Sun's highest position in the sky. Local noon strongly depends on longitude: An observer located eastward of another observer will see the Sun rise, and reach noon, earlier. All locations at the same longitude have the same moment of local noon, so in principle, we could define a common measure of time for all those locations. In practice, a common time is defined for a segment of longitude or for a country or a region of a country. For most locations, local noon will be close to, but slightly before or after the time zone's official noon. In this scheme, differences in official time almost always amount to an integer number of hours (although some defined time zones deviate from this scheme by, say, half an hour).

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