Editing Solar time (section)

==History==
{{See also|History of timekeeping devices}}
[[File:Sun and Moon Nuremberg chronicle.jpg|176px|thumbnail|right|[[Sun]] and [[Moon]], [[Nuremberg Chronicle]], 1493]]
The sun has always been visible in the sky, and its position forms the basis of apparent solar time, the timekeeping method used in antiquity. An Egyptian [[obelisk]] constructed c. 3500 BC,<ref>{{cite web |title=A Walk Through Time - Early Clocks |url=https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time/walk-through-time-early-clocks |work=A Walk Through Time - The Evolution of Time Measurement through the Ages|publisher=[[National Institute of Standards and Technology]] |language=en |date=August 12, 2009}}</ref> a [[gnomon]] in China dated 2300 BC,<ref>{{cite book |last1=Li |first1=Geng |chapter=Gnomons in Ancient China |editor-last=Ruggles |editor-first=C. |title=Handbook of Archaeoastronomy and Ethnoastronomy |date=2015 |pages=2095–2104 |doi=10.1007/978-1-4614-6141-8_219|bibcode=2015hae..book.2095L |isbn=978-1-4614-6140-1 }}</ref> and an Egyptian [[sundial]] dated 1500 BC<ref>{{cite journal|url=http://aaatec.org/documents/article/vl4.pdf|last=Vodolazhskaya|first=L.N.|title=Reconstruction of ancient Egyptian sundials|journal=Archaeoastronomy and Ancient Technologies|date=2014|volume=2|issue=2|pages=1–18|arxiv=1408.0987 }}</ref> are some of the earliest methods for measuring the sun's position.

[[Babylonia]]n astronomers knew that the hours of daylight varied throughout the year. A tablet from 649 BC shows that they used a 2:1 ratio for the longest day to the shortest day, and estimated the variation using a linear zigzag function.<ref>{{cite journal |last1=Pingree |first1=David |last2=Reiner |first2=Erica |title=A Neo-Babylonian Report on Seasonal Hours |journal=Archiv für Orientforschung |date=1974 |volume=25 |pages=50–55 |jstor=41636303 |url=https://www.jstor.org/stable/41636303 |issn=0066-6440}}</ref> It is not clear if they knew of the variation in the length of the solar day and the corresponding [[equation of time]]. [[Ptolemy]] clearly distinguishes the mean solar day and apparent solar day in his ''[[Almagest]]'' (2nd century), and he tabulated the equation of time in his ''Handy Tables''.<ref>{{Citation | last = Neugebauer | first = Otto | author-link = Otto Neugebauer | date = 1975 | title = A History of Ancient Mathematical Astronomy | publisher = Springer-Verlag | location = New York / Heidelberg / Berlin | pages = 984–986 | isbn = 978-0-387-06995-1}}</ref> 

Apparent solar time grew less useful as commerce increased and mechanical clocks improved. Mean solar time was introduced in almanacs in England in 1834 and in France in 1835. Because the sun was difficult to observe directly due to its large size in the sky, mean solar time was determined as a fixed ratio of time as observed by the stars, which used point-like observations. A specific standard for measuring "mean solar time" from midnight came to be called Universal Time.<ref name="time-from-earth-rotation-to-atomic-physics" />{{rp|9–11}}

Conceptually [[Universal Time]] is the rotation of the Earth with respect to the sun and hence is mean solar time. However, UT1, the version in common use since 1955, uses a slightly different definition of rotation that corrects for the motion of Earth's poles as it rotates. The difference between this corrected mean solar time and [[Coordinated Universal Time]] (UTC) determines whether a [[leap second]] is needed. (Since 1972 the UTC time scale has run on [[second|SI seconds]], and the SI second, when adopted, was already a little shorter than the current value of the second of mean solar time.<ref>:(1) In "The Physical Basis of the Leap Second", by D D McCarthy, C Hackman and R A Nelson, in Astronomical Journal, vol.136 (2008), pages 1906-1908, it is stated (page 1908), that "the SI second is equivalent to an older measure of the second of UT1, which was too small to start with and further, as the duration of the UT1 second increases, the discrepancy widens." :(2) In the late 1950s, the cesium standard was used to measure both the current mean length of the second of mean solar time (UT2) (result: 9192631830 cycles) and also the second of ephemeris time (ET) (result:9192631770 ± 20 cycles), see [http://www.leapsecond.com/history/1968-Metrologia-v4-n4-Essen.pdf "Time Scales", by L. Essen] {{webarchive|url=https://web.archive.org/web/20081019014533/http://www.leapsecond.com/history/1968-Metrologia-v4-n4-Essen.pdf |date=October 19, 2008 }}, in Metrologia, vol.4 (1968), pp.161-165, on p.162. As is well known, the 9192631770 figure was chosen for the [[second|SI second]].  L Essen in the same 1968 article (p.162) stated that this "seemed reasonable in view of the variations in UT2".</ref><ref name="time-from-earth-rotation-to-atomic-physics" />{{rp|227–231}})