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Inventory no. 15363 - Former Display Label

?c. 1700

Brass and silvered brass. Unsigned and undated. An unusual instrument, superficially resembling an astrolabe, but without the stereographic projection characteristic of an astrolabe; it could, perhaps, be considered an elaboration of a mariner's astrolabe.

It consists of two flat circular plates riveted together, with a bracket and suspensory apparatus, as on an astrolabe. The centre of the upper plate is cut away to form a shallow circular opening, of about half the diameter of the whole instrument, in which a disc is free to revolve.

Near the edge of the upper plate are engraved the names of thirteen fixed stars and a circular scale of twenty-four hours, numbered 1-12 twice. Within the hour scale is a Zodiac/calendar scale (¤ 0°=191/2 March; eccentric type).

The central disc is engraved as a lunar volvelle and carries an index which, as the dic is turned, moves over the Zodiac/calendar scale. An alidade is attached to a thin plate, over the lunar volvelle, which has a circular opening to show pictorially the phase of the moon.

Each end of the alidade is shaped to form an index. At the end of one arm of the alidade is a sight, pierced by a hole and a slot; on the other arm is a small unpierced rectangle of metal, perhaps a support for another sight. One arm of the alidade is bent so as to avoid obscuring the circular opening in the disc of the lunar volvelle.

On the back of the instrument is a semicircle of degrees in two quadrants of 90° a diagram of unequal hours, and a shadow-square.

Billmeir Collection

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