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Instrument type

Polyhedral Dial
Late 16th century; German
Gilt and silvered brass; 63 mm in diameter

The instrument consists of a plain octagonal cup screwed to a stand and a loose polyhedral lid with a baluster finial screwed in. The stand is very poorly executed and may well be a later replacement. It has a domed circular foot engraved with roundels supporting an urn-shaped stem with alternating raised flutes and straps terminating in a turned ringed capital.

Both the outside of the lid and the inside of the scaphe bear markings.

The lid: Four sides of the polyhedral are marked as sundials, the rim bears the inscription 'SEP{TENTRIO}' on one side and 'MER{IDIES}' on the other side indicating north and south. On every dial face is a vertical line indicating the length of the gnomon.

The north slope is marked with an equinoctial pin gnomon dial for the latitude 47? N with hour lines labelled 4 to 12 to 8.

The south slope is marked with a pin gnomon reclining horizontal dial with hour lines labelled 8 to 12 to 4 and the curves for the tropic of Capricorn and Cancer (labelled with the zodiacal signs and 'TRO{PICUS}' at the Capricorn curve) and the equator line (labelled with the zodiacal signs for Aries and Libra).

The east and west slopes are marked as pin gnomon reclining horizontal dials, the curves for the tropics and the lines for the equator labelled with the zodiacal signs, the hour lines labelled 4 to 12 to 1 and 11 to 12 to 8 respectively.

The inside of the scaphe has markings for an azimuth dial with a sloping gnomon equal to the latitude circa 47? N. The shadow of the gnomon falls on a straight-line pattern of hour lines for the equal hours, labelled 4 to 12 to 8, radiating from the point where the gnomon is fixed to the north reclining face of the dial (marked as 'S' for Septentrio, the opposite face is marked 'M' for Meridies, the east 'OR' for Oriens and the West 'Oc' for Occidens). The southern face bears a scale with lines for the equator, the tropics and the declination lines for the zodiacal signs. Two further sets of lines for the houses radiate from the north and south points of the rim (i.e. the horizon), labelled in Roman numerals I to VI and in Arabic numerals 7 to 12. The words 'Do' and 'mus' (meaning house or houses) are inscribed near the rim. The division of the houses is done in the manner of Regiomontanus.

The pin gnomon is not long enough to serve its purpose and is certainly a later replacement.

A compass well set for a magnetic deviation of circa 9? E in the base is now empty.

With the help of the now missing compass, the dial could be oriented in North-South direction. The true solar time could then be read from one of the four dials on the polyhedral lid. When used as an azimuth dial the lid is taken off and the scaphe turned until the sloping gnomon on the inside points at the celestial pole. The shadow of the whole length of the gnomon falling on the hour markings then indicates true solar time in equal hours.

The instrument was bequeathed by Maurice Rosenheim in 1922 and is described in F. A. B. Ward, A Catalogue of European Scientific Instruments in the Department of Medieval and Later Antiquities of the British Museum (London, 1981), p. 33 no. 67.

Silke Ackermann

British Museum, London
Registration no. MLA 1922,7-5.6

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