Glossary term: Universal Time (UT)
Description: Universal Time, abbreviated UT, is an umbrella term for several ways of defining the measurement of time. UT1 is the mean solar time at longitude zero (the historical location of the Royal Greenwich Observatory). Mean solar time defines the length of a day as the average duration between one noon (highest position of the Sun in the sky) and the next. In practice, the necessary measurements are made not with the Sun, but with distant astronomical objects such as quasars at night. UTC, Universal Time Coordinated, is a time standard based on the timekeeping of a specific large set of atomic clocks ("international atomic time" or TAI), but with occasional extra seconds ("leap seconds") added to some days to ensure UTC and UT1 never diverge by more than 0.9 seconds.
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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 العالمي
- German: Universalzeit (UT)
- French: Temps Universel (UT)
- Italian: Tempo Universale (UT)
- Japanese: 世界時 (external link)
- Simplified Chinese: 世界时（UT）
- Traditional Chinese: 世界時（UT）
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
Related Diagrams
International Date Line
Caption: The International Date Line (shown here as a solid line) is a specific meridian which roughly follows the meridian of longitude 180°. The International Date Line passes north–south between Russia and Alaska, through the Pacific Ocean including parts of Micronesia and Polynesia, and to the east of Australia/New Zealand before reaching the South Pole on Antarctica. The International Date Line marks the boundary where calendar dates change by one. Therefore, regions to the west of the International Date line are one calendar day ahead of regions to the east.
The International Date Line does not always exactly follow the meridian of longitude 180°. This is partly to stop some territory of a country falling on the other side of the International Date Line than the rest of that country (the Aleutian Islands in the United States being a good example). Some countries also choose which side of the International Date Line they lie on. Samoa changed which side of the date line it lay on in 2011. At this point the International Date Line was moved to lie to the east of Samoa having previously passed to the west of Samoa.
On the opposite side of the world the dashed line marks the prime meridian (the meridian of longitude 0°). This is a line that passes through the Royal Greenwich Observatory in the UK. All time zones are measure relative to Universal Time which is the zero point for all timezones. The time zones marked at the top of the diagram are the timezones in the shaded regions excluding the effects of daylight savings time.
Credit: Maria Cristina Fortuna/IAU OAE
License: CC-BY-4.0 Creative Commons Attribution 4.0 International (CC BY 4.0) icons
