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A '''[[sundial]]''' is a device that measures time by the [[Position of the sun|position]] of the [[Sun]].
A sundial measures local solar time. Before the coming of the railways in the 1830s and 1840s, local time was displayed on a sundial and was used by the government and commerce. Before the invention of the clock the sundial was the only source of time, after the invention, the sundial became more important as the clock needed to be reset regularly from a sundial- as its accuracy was poor. A clock and a dial were used together to measure longitude. Dials were laid out using straight edges and compasses. In the late nineteenth century sundials became objects of academic interest. The use of logarithms allowed algebraic methods of laying out dials to be employed and studied. No longer utilitarian, sundials remained as popular ornaments, and several popular books promoted that interest- and gave constructional details. Affordable scientific calculators made the algebraic methods as accessible as the geometric constructions- and the use of computers made dial plate design trivial. The heritage of sundials was recognised and Sundial Societies were set up worldwide, and certain legislations made studying sundials part of their national school curriculums. {{sfn|Jones|2005}} <!-- Crude start-->
==Ancient sundials==
[[File:Ancient-egyptian-sundial.jpg|thumb|World's oldest sundial, from Egypt's Valley of the Kings (c. 1500 BC), used to measure work hours.]]
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The Romans built a very large sundial in 10 BC, the [[Solarium Augusti]], which is a classic nodus-based obelisk casting a shadow on a planar ''pelekinon''.<ref>Edmund Buchner, "Solarium Augusti und [[Ara Pacis]]", ''Römische Mitteilungen'' '''83''' (1976:319-75); ''Die Sonnenuhr des Augustus: Kaiser Augustus und die verlorene Republik'' (Berlin) 1988.</ref>
The Greek dials were inherited and developed further by the [[Islamic astronomy|Islamic Caliphate cultures]] and the post-Renaissance Europeans. Since the Greek dials were nodus-based with straight hour-lines, they indicated unequal hours — also called temporary hours — that varied with the seasons, since every day was divided into twelve equal segments; thus, hours were shorter in winter and longer in summer. The idea of using hours of equal time length throughout the year was the innovation of Abu'l-Hasan [[Ibn al-Shatir]] in 1371, based on earlier developments in [[trigonometry]] by [[Muhammad ibn Jābir al-Harrānī al-Battānī]] (Albategni). Ibn al-Shatir was aware that "using a [[gnomon]] that is parallel to the Earth's axis will produce sundials whose hour lines indicate equal hours on any day of the year." His sundial is the oldest polar-axis sundial still in existence. The concept later appeared in Western sundials from at least 1446.<ref>{{cite web|title=History of the sundial|url=http://www.nmm.ac.uk/server/show/conWebDoc.353|publisher=[[National Maritime Museum]]|accessdate=2008-07-02}}</ref><ref>{{Cite journal|title=The Sundial And Geometry|first=Lawrence|last=Jones|journal=North American Sundial Society|volume=12|issue=4|date=December 2005|postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}</ref>▼
The custom of measuring time by one's shadow has persisted since ancient times. In [[Aristophanes]]' play, ''Assembly of Women'', Praxagora asks her husband to return when his shadow reaches {{convert|10|ft|m}}. The Venerable [[Bede]] is reported to have instructed his followers in the art of telling time by interpreting their shadow lengths.
==
▲The Greek dials were inherited and developed further by the [[Islamic astronomy|Islamic Caliphate cultures]] and the post-Renaissance Europeans. Since the Greek dials were nodus-based with straight hour-lines, they indicated unequal hours — also called temporary hours — that varied with the seasons, since every day was divided into twelve equal segments; thus, hours were shorter in winter and longer in summer. The idea of using hours of equal time length throughout the year was the innovation of Abu'l-Hasan [[Ibn al-Shatir]] in 1371, based on earlier developments in [[trigonometry]] by [[Muhammad ibn Jābir al-Harrānī al-Battānī]] (Albategni). Ibn al-Shatir was aware that "using a [[gnomon]] that is parallel to the Earth's axis will produce sundials whose hour lines indicate equal hours on any day of the year." His sundial is the oldest polar-axis sundial still in existence. The concept later appeared in Western sundials from at least 1446.<ref>{{cite web|title=History of the sundial|url=http://www.nmm.ac.uk/server/show/conWebDoc.353|publisher=[[National Maritime Museum]]|accessdate=2008-07-02}}</
The onset of the Renaissance saw an explosion of new designs. Italian astronomer [[Giovanni Padovani]] published a treatise on the sundial in 1570, in which he included instructions for the manufacture and laying out of mural (vertical) and horizontal sundials. [[Giuseppe Biancani|Giuseppe Biancani's]] ''Constructio instrumenti ad horologia solaria'' (ca. 1620) discusses how to make a perfect sundial, with accompanying illustrations.▼
The oldest [[Tide (time)|sundial]] in England is incorporated into the [[Bewcastle Cross]] ca. early 8th Century AD.
==Renaissance sundials==
▲The onset of the Renaissance saw an explosion of new designs. Italian astronomer [[Giovanni Padovani]] published a treatise on the sundial in 1570, in which he included instructions for the manufacture and laying out of mural (vertical) and horizontal sundials. [[Giuseppe Biancani|Giuseppe Biancani's]] ''Constructio instrumenti ad horologia solaria'' (ca. 1620) discusses how to make a perfect sundial, with accompanying illustrations.
==The dials of Zuphora (Provence, France)==
Painted vertical declining dials in villages around Briançon.
==Twentieth and Twenty-first century dialing==
Designers of the [[Taipei 101]], the first record-setting [[skyscraper]] of the 21st century, brought the ancient tradition forward. The tower, tallest in the world when it opened in [[Taiwan]] in 2004, stands over half a kilometer in height. The design of an adjoining park uses the tower as the style for a huge horizontal sundial.
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{{Reflist|2}}
;Bibliography
*{{Cite journal|title=The Sundial And Geometry|first=Lawrence|last=Jones|journal=North American Sundial Society|volume=12|issue=4|date=December 2005|ref=harv}}
* {{cite book | author = Earle AM | year = 1971 | title = Sundials and Roses of Yesterday | publisher = Charles E. Tuttle | location = Rutland, VT | isbn = 0-8048-0968-2|lccn=74142763}} Reprint of the 1902 book published by Macmillan (New York).
* A.P.Herbert, ''Sundials Old and New'',Methuen & Co. Ltd, 1967.
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