Sundials

A sundial 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. more...

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.

Sundials are known from ancient Egypt, and were developed further by other cultures, including the Greeks and Romans. 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 Fine 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.

Installation of standard sundials

Tilting the style or gnomon of a standard sundial is the only practical way to install a mass-produced garden sundial so that it will keep time. Some mass-produced garden sundials are improperly designed, and unable to keep time. Many sundials are made to be used at 45 degrees north.

A sundial can be adjusted to another latitude by tilting it so its style or gnomon(s) is (are) parallel the Earth's axis of rotation. That is, the end of a gnomon should point at the north celestial pole in the northern hemisphere, or the south celestial pole in the southern hemisphere.

A sundial can be rotated around its style or gnomon (which must still point at the celestial pole) a maximum of 7.5 degrees to the east or west to adjust to the local standard time zone (time zones are 360 degrees/24 hours = 15 degrees wide). Tilt the sundial so that it is oriented as if it were at the longitude of the center of your local time zone.

To correct for daylight saving time, a face needs two sets of numerals or a correction table, and must be adjusted for longitude from the center of the time zone. The admittedly informal standard is to have numerals in hot colors for summer, and in cool colors for winter. Twisting the face of the sundial will not work because sundials (except at the north and south pole) do not have equal hour angles.

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.

Design & principles of operation

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