Sundial

:This article pertains to the astronomical instrument. For the psychedelic rock band, see Sun Dial.

Portable sundials, for navigation and time

During the middle ages advanced yet portable astronomical instruments were developed.

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Diptych sundial

One popular portable sundial design was called a diptych. It consisted of two small flat faces, joined by a hinge. Diptychs usually folded into little flat boxes suitable for a pocket. The gnomon was a string between the two faces. When the string was tight, the two faces formed both a vertical and horizontal sundial. The best material was white ivory, inlaid with black lacquer markings. The best gnomons were black braided silk, linen or hemp.

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By making the two sundials have different angles to the string (and thus different projections), a diptych can be self-aligning. When both faces show the same time, the diptych shows the local apparent solar time. Additionally, the hinge will be level, and point north (in the northern hemisphere), and the diptych will be angled so the gnomon is parallel to the Earth's axis of rotation. At solar noon, sunrise and sunset, the latitude adjustment of the diptych can't affect the time of either sundial, but at 9am and 3pm, each degree of latitude error (from holding the sundial at the wrong angle) creates four minutes of difference between the two faces.

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This means that a diptych can also act as a compass and even measure latitude. Some diptychs included a small scale and a plumb-bob to read the latitude. Some others included a compass rose to measure angles to geographic features. Large (meter-sized) diptychs may have been used for navigation in ancient times.

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Early 18th Century Irish portable sundials

This form of sundial was about 8 cm diameter and made of brass. It had a brass lid, not shown, to protect it when travelling. Several features enabled precision to be achieved. It had an iron compass needle so that North could be accurately set. The scale division is to 5 minutes.

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This dial was made in Dublin in 1742 by Gabriel Stokes a mathematical instrument maker.

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Elevation sundial

Astrolabes were used as sundials, as well as for calendrical observations, navigation and astronomy.

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An even smaller design was the ring. It had a small handle, or was a fob or the decoration of a necklace. When held by its handle, a hole would cast a shadow on the inside of the ring, telling the time by markings on the inside. The user had to know if it was morning or evening. Usually the hole was mounted in a sliding lockable piece of metal, which was adjusted to correct date.

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In recent times, U.S. Special Forces have taken to engraving a simple sundial on their knife-blade. It works even when a watch fails.

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