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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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher
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".
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In Other Languages
- Arabic: التوقيت المتوسط لجرينتش
- German: Greenwich Mean Time (GMT)
- Spanish: Zona horaria de Greenwich (GMT)
- French: Fuseau horaire de Greenwich (GMT)
- Hindi: ग्रीनविच मीन टाइम ज़ोन (जीएमटी)
- Italian: Fuso orario medio di Greenwich (GMT)
- Marathi: ग्रीनविच मीन टाइम झोन (GMT)
- Simplified Chinese: 格林尼治标准时区（GMT）
- Traditional Chinese: 格林尼治標準時區（GMT）
Related Media
Map of de-facto time zones on Earth
Caption: Due to the Earth's spherical shape and its rotation around its own axis, local noon – the moment in time when, for an observer at a specific geographic location, the Sun reaches its highest position in the sky varies with longitude. That is why a time coordinate, such as Universal Time (UT or UTC), might be of advantage for specifying moments in time in a unified way. But a global time coordinate will be out of synch with the local day-night rhythm in most locations on Earth. A compromise is to divide Earth into zones each comprising 15 degrees of longitude, and in each zone define time as UTC plus or minus an integer number of hours, with the offset chosen so as to make time match as closely with local time at the middle longitude of the time zone. In practice, political considerations have altered the time zone boundaries somewhat. When a country straddles several time zones, it is not uncommon for the country's government to choose one of those time zones to define the country's official time.
This map, originally created by the CIA and updated by several Wikimedia Commons users, shows the current definitions of the world's time zones. The IAU OAE is not the original author of this map. The designations employed and the presentation of the material on this maps do not imply the expression of any opinion whatsoever on the part of the IAU or the IAU OAE concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.
Credit: Wikimedia Users UnaitxuGV, Heitordp and others based on a map created by the U.S. Central Intelligence Agency (CIA)
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License: PD Public Domain icons
