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What (who) is ΔT (timekeeping) - definition
![The [[Tower of the Winds]] in Athens (1st century BC)](https://commons.wikimedia.org/wiki/Special:FilePath/Aerides Athens 2.jpg?width=200 "The [[Tower of the Winds]] in Athens (1st century BC)")
![Allegory of Good Government]]'' ({{circa}} 1338)](https://commons.wikimedia.org/wiki/Special:FilePath/Ambrogio Lorenzetti 003-detail-Temperance.jpg?width=200 "Allegory of Good Government]]'' ({{circa}} 1338)")
![National Physical Laboratory]] in London](https://commons.wikimedia.org/wiki/Special:FilePath/Atomic Clock-Louis Essen.jpg?width=200 "National Physical Laboratory]] in London")
![An [[incense clock]]; time was measured by means of powdered incense burnt along a pre-measured path](https://commons.wikimedia.org/wiki/Special:FilePath/Chinese incense clock.jpg?width=200 "An [[incense clock]]; time was measured by means of powdered incense burnt along a pre-measured path")
![Detail from the face of an [[equation clock]] made by [[Ferdinand Berthoud]], {{circa}}1752 ([[Metropolitan Museum of Art]])](https://commons.wikimedia.org/wiki/Special:FilePath/French longcase equation regulator (c.1752) - detail.jpg?width=200 "Detail from the face of an [[equation clock]] made by [[Ferdinand Berthoud]], {{circa}}1752 ([[Metropolitan Museum of Art]])")
![Treatise of statics and mechanics]]'](https://commons.wikimedia.org/wiki/Special:FilePath/Fusee for clocks (Leonardo da Vinci).png?width=200 "Treatise of statics and mechanics]]'")
![chronometer]]](https://commons.wikimedia.org/wiki/Special:FilePath/H4 timekeeper.jpg?width=200 "chronometer]]")
![[[Engraving]] of [[John Harrison]]—with his [[gridiron pendulum]] shown in the background (1768), [[Science Museum, London]]](https://commons.wikimedia.org/wiki/Special:FilePath/John Harrison's portrait.jpg?width=200 "[[Engraving]] of [[John Harrison]]—with his [[gridiron pendulum]] shown in the background (1768), [[Science Museum, London]]")
![An Ancient Egyptian [[sundial]] ([[Rijksmuseum van Oudheden]])](https://commons.wikimedia.org/wiki/Special:FilePath/Leiden - Rijksmuseum van Oudheden - Egyptian antiquities - Ancient Egyptian sundial.jpg?width=200 "An Ancient Egyptian [[sundial]] ([[Rijksmuseum van Oudheden]])")
![A detail of the [[Salisbury Cathedral clock]], showing the [[verge and foliot]]](https://commons.wikimedia.org/wiki/Special:FilePath/Salisbury Cathedral 2012 12.jpg?width=200 "A detail of the [[Salisbury Cathedral clock]], showing the [[verge and foliot]]")
.png?width=200 "Water clock (representing a clock at the royal court in Paris, {{circa}}1250)")
Wikipedia
In precise timekeeping, ΔT (Delta T, delta-T, deltaT, or DT) is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of International Atomic Time (86,400 seconds). Formally, ΔT is the time difference ΔT = TT − UT between Universal Time (UT, defined by Earth's rotation) and Terrestrial Time (TT, independent of Earth's rotation). The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day (Δτ) which are dealt with separately.
Since 2017, the length of the day has happened to be very close to the conventional value, and ΔT has remained within a second of 69 seconds
