Universal Time (UT1)

Universal Time (UT1)
Name	Universal Time (UT1)
Abbreviation	UT1
Introduced	19th century
Region	Worldwide
Based on	Earth's rotation
Precision	millisecond-level

Universal Time (UT1) Universal Time (UT1) is an astronomical time standard based on the rotation of the Earth as measured by the position of distant celestial objects, used for civil, navigational, and astronomical purposes. It provides a realization of mean solar time at the prime meridian in Greenwich, connects to historical systems developed by institutions such as the Royal Observatory, Greenwich and the International Astronomical Union, and interfaces with atomic time scales maintained by organizations including the International Bureau of Weights and Measures and the International Telecommunication Union.

Definition and relationship to other time scales

UT1 is defined by the orientation of the Earth in space, specifically by the hour angle of the mean Sun relative to the Prime meridian, and is distinct from time scales derived from atomic frequency standards such as International Atomic Time and Coordinated Universal Time. The concept traces to mean solar time measured in places like the Royal Observatory, Greenwich and relates to sidereal measures used by observatories such as the Yerkes Observatory and the Mount Wilson Observatory; it contrasts with ephemeris time developments involving the Jet Propulsion Laboratory and the Bureau International de l'Heure. UT1 underpins astronomical quantities used in products from the United States Naval Observatory, the European Space Agency, and the National Aeronautics and Space Administration.

Historical development and adoption

The origins of UT1 lie in the 17th and 18th centuries when astronomers at institutions like the Royal Observatory, Greenwich, the Paris Observatory, and the Uppsala Observatory established mean solar time for navigation and cartography during events such as the Age of Discovery. The 19th century saw standardization through efforts by the International Meridian Conference and national services like the Ordnance Survey and the United States Naval Observatory, while the 20th century introduced precise definitions adopted by bodies such as the International Astronomical Union and the International Union of Geodesy and Geophysics. Developments in the mid-20th century connected UT1 to ephemeris and atomic standards via work at the Bureau International de l'Heure, the International Bureau of Weights and Measures, and the International Telecommunication Union.

Measurement and determination

UT1 is determined from observations of celestial objects using techniques developed at observatories including the Greenwich Observatory, Royal Greenwich Observatory, Flagstaff Station, and facilities operated by the United States Naval Observatory and the European Southern Observatory. Determination combines astrometric measurements, very long baseline interferometry performed by arrays such as the Very Long Baseline Array and the European VLBI Network, and reductions modeled by groups like the International Earth Rotation and Reference Systems Service, the International Astronomical Union, and the International Association of Geodesy. Data from spacecraft tracking conducted by agencies including the Jet Propulsion Laboratory and the European Space Agency also inform Earth orientation parameters that yield UT1.

Applications and significance

UT1 is crucial for precise pointing of telescopes at institutions like the Hubble Space Telescope operations teams and the Arecibo Observatory records, for navigation systems used by the Royal Navy, the United States Navy, and civil aviation authorities like ICAO, and for geodetic tasks carried out by the International Association of Geodesy and the National Geospatial-Intelligence Agency. It supports spacecraft operations by agencies such as NASA and the European Space Agency, underpins predictions used by the United States Naval Observatory and the International Earth Rotation and Reference Systems Service, and is essential to timing in radio astronomy networks like the Very Large Array and the European VLBI Network.

Variations and irregularities

UT1 exhibits variations due to tidal friction related to the Moon, angular momentum exchange between the Earth's mantle and core as studied by researchers at institutions like the Scripps Institution of Oceanography and the Geophysical Fluid Dynamics Laboratory, and atmospheric and oceanic processes monitored by agencies such as NOAA and the European Centre for Medium-Range Weather Forecasts. Long-term secular slowing from tidal torques associated with the Lunar recession and shorter-term fluctuations linked to events such as large earthquakes recorded by the United States Geological Survey produce measurable changes that differentiate UT1 from atomic time maintained by the International Bureau of Weights and Measures.

Coordination with UTC and leap seconds

Coordinated Universal Time (UTC) is kept within specified bounds of UT1 by adjustments coordinated by the International Telecommunication Union and the International Bureau of Weights and Measures, historically through the insertion of leap seconds as decided by the International Telecommunication Union Radiocommunication Sector and informed by the International Earth Rotation and Reference Systems Service. Policy discussions about leap seconds have involved organizations such as the International Astronomical Union, International Telecommunication Union, United Nations bodies, and national timing agencies including the National Institute of Standards and Technology and the National Physical Laboratory, reflecting the operational needs of satellite navigation systems like GPS and telecommunication infrastructures maintained by carriers and agencies worldwide.

Category:Timekeeping