Asundial is a device that indicates time by using a light spot or shadow cast by theposition of the Sun on a reference scale.[4] As the Earth turns on itspolar axis, the sun appears to cross the sky from east to west, rising at sun-rise from beneath the horizon to a zenith at mid-day and falling again behind the horizon at sunset. Both the azimuth (direction) and the altitude (height) can be used to create time measuring devices. Sundials have been invented independently in every major culture and became more accurate and sophisticated as the culture developed.[5]
A sundial useslocal time. Before the coming of the railways in the 1840s, local time was displayed on a sundial and was used by the government and commerce. Before theinvention of the clock the sundial was the only way to measure time. After the invention of the clock, the sundial maintained its importance, as clocks needed to be reset regularly from a sundial, because the accuracy of early clocks was poor. A clock and a sundial were used together to measurelongitude. Dials were laid out using straightedges and compasses. In the late nineteenth century sundials became objects of academic interest. The use oflogarithms allowedalgebraic 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. Affordablescientific calculators made the algebraic methods as accessible as thegeometric 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 theirnational school curriculums.[5]
The earliest household clocks known, from the archaeological finds, are thesundials (1500 BCE) inAncient Egypt and ancientBabylonian astronomy. Ancientanalemmatic sundials of the same era (about 1500 BCE) and their prototype have been discovered on the territory of modern Russia.[6][7][8][9] Much earlierobelisks, once thought to have been used also as sundials, placed at temples built in honor of a pharaoh, are now thought to serve only as a memorial.[10] Presumably, humans were telling time from shadow-lengths at an even earlier date, but this is hard to verify. In roughly 700 BCE, theOld Testament describes a sundial – the "dial of Ahaz" mentioned inIsaiah 38:8 and2 Kings 20:9 (possibly the earliest account of a sundial that is anywhere to be found in history)—which was likely of Egyptian or Babylonian design. Sundials were also developed inKush.[11][12] Sundials existed in China since ancient times, but very little is known of their history. It is known that the ancient Chinese developed a form of sundials c. 800 BCE, and the sundials eventually evolved to very sophisticated water clocks by 1000 CE, and sometime in theSong dynasty (1000–1400 CE), acompass would sometimes also be constructed on the sundial.[13]
An early reference to sundials from 104 BCE is in an assembly of calendar experts.[14]
The ancient Greeks developed many of the principles and forms of the sundial. Sundials are believed to have been introduced into Greece byAnaximander of Miletus,c. 560 BCE. According toHerodotus, Greek sundials were initially derived from their Babylonian counterparts. The Greeks were well-positioned to develop the science of sundials, having developed the science ofgeometry, and in particular discovering theconic sections that are traced by a sundial nodus. The mathematician and astronomerTheodosius of Bithynia (c. 160 BCE toc. 100 BCE) is said to have invented a universal sundial that could be used anywhere on Earth.[15]
The Romans adopted the Greek sundials, and the first record of a sundial in Rome is in 293 BCE according toPliny.[16] A comic character in a play byPlautus complained about his day being "chopped into pieces" by the ubiquitous sundials. Writing inc. 25 BCE, the Roman authorVitruvius listed all the known types of dials in Book IX of hisDe Architectura, together with their Greek inventors.[17] All of these are believed to be nodus-type sundials, differing mainly in the surface that receives the shadow of the nodus.[citation needed]
The Romans built a very large sundial inc. 10 BCE, theSolarium Augusti, which is a classic nodus-based obelisk casting a shadow on a planarpelekinon.[18] TheGlobe of Matelica is felt to have been part of an Ancient Roman sundial from the 1st or 2nd century.[citation needed]
The custom of measuring time by one's shadow has persisted since ancient times. InAristophanes' playAssembly of Women, Praxagora asks her husband to return when his shadow reaches 10 feet (3.0 m).
The Greek dials were inherited and developed further by astronomers of theIslamic Caliphates and post-Renaissance Europeans.
The VenerableBede is reported to have instructed his followers in the art of telling time by interpreting their shadow lengths, however, Bede's most important association with sundials is that he encouraged the use ofcanonical sundials to fix the times of prayers.[citation needed] The oldest sundial in England is atide dial incorporated into theBewcastle Cross, Cumbria, and dates from the 7th or early 8th century.[19]
Since the Greek dials were nodus-based with straight hour-lines, they indicatedunequal hours that varied with the seasons, since every day was divided into twelve equal segments; hours were shorter in winter and longer in summer. Sundials with gnomons oriented towards the celestial pole are able to exploit the fact that the markings for theequinoctial hours are a family of straight-line segments, thereby allowing for the measure of equal hours. It is unclear when such sundials were first built. A polar-axis sundial was constructed by the 14th century Arabic engineerIbn al-Shatir, a replica of which still exists today, though he was not the inventor of the device. The concept may have been known for centuries before his time, and appeared in Western sundials from at least 1446.[20][21][5]
Europe then saw an explosion of new designs. Italian astronomerGiovanni 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'sConstructio instrumenti ad horologia solaria (ca. 1620) discusses how to make a perfect sundial, with accompanying illustrations.[citation needed]
The villages aroundBriançon, Hautes-Alpes, France were a major site of sundial production in the 18th and 19th centuries, with at least 400 painted dials in this one French department. Among the most famous sundial makers of this era wasGiovanni Francesco Zarbula, who created a hundred of them between 1833 and 1881.[22]
Designers of theTaipei 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 500 metres (1,600 ft) in height. The design of an adjoining park uses the tower as the style for a huge horizontal sundial.[citation needed]
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