Glossary term: Greenwich Mean Time Zone (GMT)

Description: The time zone in which the historic Royal Observatory at Greenwich, Great Britain, is located is called the Greenwich Mean Time zone, or alternatively the Western European time zone. Historically, Greenwich Mean Time (GMT) was the mean solar time determined at the Royal Observatory and used as the reference point for naval chronometers carried on ships. Navigators would determine the time of their local noon (the highest point above the horizon reached by the Sun in a given day) by observations using a sextant or similar device and compare with the GMT shown by their chronometer; the difference allowed them to determine their geographic longitude. In the modern system, time in the GMT time zone corresponds to Universal Time Coordinated (UTC), written as "UTC + 0h".

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The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".