Diptych dial

Diptych dial for latitudes 45°-54° North. This dial consists of two leaves that fold flat when not in use. Leaf Ia displays a windrose with the compass points named in German, surrounded by a circle with eight winds named in Italian. Inscribed around the windrose is the phrase, 'MONSTRO VIAM PERGE SECVRVS' [I show the way, proceed securely]. On leaf Ib is a dial for length of day, incorporating a vertical dial with holes for the adjustment of the string gnomon. Below is a table of latitudes for towns in Europe. Leaf IIa has four hour-circles around the compass and the string gnomon is missing.

The compass is marked with the cardinal points in Latin and is offset 9° East of North to allow for magnetic variation. There is a single horizontal dial in the southern part of the leaf for Italian and Babylonian hours with a brass gnomon. The solar declination is also shown. Leaf IIb features a lunar volvelle surrounded by circular epact tables with an aspectarium in the centre. The inner one is marked for Gregorian epacts and the outer one for Julian epacts. On leaf Ib, it is signed 'HANNS TROSCHEL NOREBERGE FACIEBAT 1624', and Troshel's makers mark - a star - appears twice.

This dial includes brief instructions on its use and is similar in style to that in the History of Science Museum, Oxford (inventory no. 55659). The repetition of the zodiac signs on the Italian and Babylonian hours dial of the lower leaf is unusual.

For more information regarding this dial please refer to the OUP & NMM catalogue, 'Sundials at Greenwich'.

Object Details

ID: AST0227
Collection: Astronomical and navigational instruments
Type: Diptych dial
Display location: Display - ROG
Creator: the Younger, Hans Troschel,
Date made: 1624
Credit: National Maritime Museum, Greenwich, London
Measurements: Overall: 14 x 72 x 114.5 mm

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