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Signed by Thomas Gemini
Mid 16th century; English
Brass; 270 mm in radius

The instrument is composed of two quadrants each engraved on both sides now mounted within a quadrantal frame pinned to the edges of a central wood core 8 mm thick by three square shanked, screw fastening, iron bolts with flattened pyramidal ends. Since the wood core has a non-functional hole at its centres, and one of the screw holes breaks the engraving on the edge of the horizontal quadrant plate, it seems probable that the mounting arrangement is not original but was carried out before the first mention of the instrument, when it was already in its present form, in the Medici inventories in 1654. It is perhaps not unlikely that the transformation was carried out in the Medici workshop. The edging strips are decorated by parallel lines inscribed along the edges of each strip and two vanes each with an upper open and lower pinnule sight are set on one of the radial sides.

Quadrant I, side 1: The limb is engraved with a doubled zodiacal calendar (with 0? Aries at 10.5 March) reading respectively to 1? and 1 day by scales with alternated hatched and unhatched divisions. The two outer arcs read from the apex, the two inner from the limb. Above the upper arc (reading towards the apex) is a scale of 90?, and above this the names of seventeen festivals are marked although sometimes at some distance from the arc so as to avoid confusion with the lines and numbers of the horary diagram drawn above by right-curving arc towards the centre of the instrument at the apex. The festivals named are written sometimes in the direction of the limb, sometimes in the direction of the apex of the instrument, presumably so that they may be readily associated with the appropriate half year when their dates are read off. The festivals are, Winter half year: 'Na{tus} D{omi}ni', 'Epiph{an}ia Fest{us} om{ni} sanctoru{m}', 'Purificatio Ma{rie}', 'S. Matthias', 'Annu{nciato} Marie', 'Marc{us} Eva{n}gel{is}', 'Phi{lippus} Iacobi'; Summer half year: 'Andreas Simo{nus}', 'Iude Michaelis', 'Math{ias} Apo{stolicus}', 'Bartolome', 'Assumptio Marie', 'Mari{e} Magda{lena}', 'Pet{rus} et Pauli', and 'Nat{us} Iohannes Baptus'.

To the left of the hour arcs are two concentric semi-circular scales for leap year ('Bisextilis'), the dominical letter and the solar circle; the epact and the lunar cycle. The scales close against a vertical Easter scale giving the dates of Easter and the dominical letter for one complete lunar cycle. At the centre of the inner semi-circle are the legends: 'A car{e} mispr{emium} ad Pasc. 47 dieb' ('From Shrove Tuesday to Easter, 47 days'); 'A quadra ad Pasc. 6 Hebd.' ('From quadragesima to Easter, 6 weeks'); 'A pasch, ad ascen d{omi}ni, 39 dieb' ('From Easter to the ascension of our Lord, 39 days'); and 'A pasch ad Pente 7 Hebdo' ('From Easter to Pentecost, 7 weeks').

Quadrant I, side 2: The entire plate is filled with the diagram of an horizontal quadrant, that is a stereographic projection of the celestial co-ordinates onto the plane of the observer's horizon. The diagram is in effect that found on an astrolabe plate, with the addition of the path of the ecliptic, when this is twice folded on itself to produce a quadrant. Along the limb of the instrument is an hour scale 1 to 6 / 6 to 12 with undivided numbered double divisions although the scale could be read to 4 minutes against the divisions of the fourfold concentric degree scale drawn immediately next to it. The degree scale 0? to 360? reads to one degree and within it is the bounding arc of the projection marked 'Tropicus Capricornus'. The left hand edge of the quadrant represents the meridian, the right hand edge the horizon, both being so labelled. The meridian line is engraved between the equator and the ecliptic line with a scale of 231/2? for the declination of the ecliptic, and from the equator to the apex with a scale of degrees for the almucantars which are drawn as arcs across the instrument for every 10?. The two arcs of the ecliptic are drawn with the names of the signs, each sign divided to 30?, and the arc for the equinoctial is also divided to 90?. A segment of the tropic of Cancer arc is drawn below the apex and the names and positions of stars are marked, viz.: 'Canis maior', 'canis minor', 'Lucida hydr?', 'Aguila [sic]', 'Humerus dexter Orion', 'Hircus', 'Caput Algol'.

Quadrant II, side 1: The limb is divided with a 90? scale numbered in groups of five and reading to 30' of arc. Within this is a shadow square marked 'Umbra recta', 'umbra versa' each divided to 60 parts, crossed by intersecting semi-circles for sines and versed sines drawn with the radius of the instrument as their diameter and each divided to 60. All the scales have alternating hatched and unhatched divisions for ease of reading.

This instrument is mentioned in its present form in the Medici Inventories for 1654: 'Un quadrante composto di due lastre d'ottone, e anima di legno e due traguardi, ferme le piastre con tre viti di ferro, serve a molte operazioni astronomiche'.

Anthony J. Turner

Istituto e Museo di Storia della Scienza, Firenze
Inventory no. 2509

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