SUNDIALS

 

a sundial that was off by an hour

 

Standard Green Sundial

 

 

A sundial is a device that measures time by the position of the sun. The most commonly seen designs, such as the 'ordinary' or standard 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. However, sundials can be designed for any surface where a fixed object casts a predictable shadow. Sundials in the form of obelisks (3500 BC) and shadow clocks (1500 BC) are known from ancient Egypt, and were developed further by other cultures, including the Chinese, Greek, and Roman cultures. A type of sundial without a gnomon is described in the Old Testament (Isaiah 38:8).

 

 

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 French astronomer Oronce Finé constructed a sundial of ivory in 1524. The 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” discusses how to make a perfect sundial, with accompanying illustrations.

The oldest sundial in Britain is incorporated into the famous Bewcastle Cross. The dial is divided into four tides, covering the parts of the working day in medieval times.

 

 

The famous Bewcastle Cross, pictured above.

 

 

Many ornamental sundials are designed to be used at 45 degrees north. By tilting such a sundial, it may be installed so that it will keep time. However, some mass-produced garden sundials are inaccurate because of poor design and cannot be corrected. A sundial designed for one latitude can be adjusted for use at another latitude by tilting its base so that its style, or gnomon, is parallel to the Earth's axis of rotation and pointing in the direction of the north celestial pole in the northern hemisphere, or the south celestial pole in the southern hemisphere. The Equation of Time above the axis the dial will appear fast, and below the dial will appear slow.

 

 

The human sundial is an interesting device in which a person is used in place of the gnomon.

 

Image:Analemmatic (Human Sundial).png

 

A Human Sundial

 

 

 

 

Another type of sundial is called an Equinoctial sundial, and is in the form of a disk mounted on a bar. The bar must be parallel to the Earth's axis of rotation. The disk forms a plane parallel to the plane of the Earth's equator. The disk is marked so that one edge of the shadow of the bar shows the time as the Earth rotates. Usually noon will be at the bottom of the disk, 6AM on the western edge, and 6PM on the eastern edge. In the winter, the north side of the disk will be shaded, and hard to read. In the summer, the south side will be shaded.

 

An Equinoctial sundial in the Forbidden City, Beijing.

 

An Equinoctial sundial in the Forbidden City, Beijing.

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Ordinary sundials do not correct apparent solar time to clock time. There is a 15 minute variation through the year, known as the equation of time, because the Earth's orbit is slightly elliptical and its axis is tilted relative to the plane of its orbit. A quality sundial will include a permanently-mounted table or graph giving this correction for at least each month of the year. Some more complex sundials have curved hour-lines, curved gnomons or other arrangements to directly display the clock time.

 

 

http://en.wikipedia.org/wiki/Sundials