Sundials

A sundial is a device that measures time by the position of the Sun. The most commonly seen designs, such as the horizontal or garden sundial, cast a shadow on a flat surface marked with the hours of the day. As the position of the sun changes, the time indicated by the shadow changes. more...

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However, sundials can be designed for any surface where a fixed object casts a predictable shadow.

Most sundial designs indicate apparent solar time. Minor design variations can measure standard and daylight saving time, as well.

History

Sundials in the form of obelisks (3500 BC) and shadow clocks (1500 BC) are known from ancient Egypt and Babylon. A type of sundial is described in the Old Testament (Isaiah 38:8).(ca.700 BC) is thought to have been of an Egyptian or Babylonian design. Sundials of a wide variety of types were developed by other cultures, including the Greeks (scaphion, hemyspherium, pelekinon), Romans, and Islamic cultures. Although sundials are believed to have existed in China since ancient times, very little is known of their history.

The mathematician and astronomer Theodosius of Bithynia (ca. 160 BC-ca. 100 BC) is said to have invented a universal sundial that could be used anywhere on Earth. The Romans built the largest sundial the world has known, the Solarium Augusti in 10 BC, although many of their earlier sundials were captured from other cultures. Marcus Vitruvius Pollio, the Roman author of De Architectura wrote on various types of dials in ca. 25 BC.

The oldest sundial in Britain is incorporated into the Bewcastle Cross ca. 800 AD. The dial is divided into four tides, covering the parts of the working day in areas influenced by the Vikings, a maritime culture which noted the passage of time in the progression of the two high and two low tides each day.

Early sundials elsewhere divided the daylight into 12 equal parts, and thus the hours were shorter in winter and longer in summer. Islamic astronomers are credited with the concept of equal hours (for the purposes of astronomy) ca. 1200 AD, and the concept is known in western sundials at least from the year 1400.

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's Constructio instrumenti ad horologia solaria (ca. 1620) discusses how to make a perfect sundial, with accompanying illustrations.

Terminology

The 'shadow-maker' of the sundial is called a gnomon. The linear feature that casts the shadow from which the time can be read is often called a style. On a standard garden sundial, this line is the top edge of the gnomon. The style should be parallel to the Earth's axis of rotation, and point to the celestial pole.

Read more at Wikipedia.org