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Astrolabe
Signed by Anthoine Mestrel
Dated 1551; Paris
Gilt brass; 256 mm in diameter

The mater is made from a single brass plate and has an elaborately shaped, pierced and decorated throne comprising on both sides a head and ornamental scrolls, with a shackle and suspension ring. A plate with a zodiacal calendar can be removed and the inside of the mater is blank

One side has a universal projection of the Rojas type. On outer degree scale 0 to 90 to 0 to 90 to 0, divided to 5, subdivided to 1 with alternate hatching, numbered by 5. Right ascension lines are drawn for every 12 minutes between the arctic and antarctic circles, beyond which are 'POLVS ARCTICVS' and 'POLVS ANTACTICVS [sic]' respectively. Between the tropics these lines are crossed by declination parallels drawn for every 2 degrees of the sun's position in the ecliptic, i.e. 15 per zodiacal sign. On the whole hour lines of right ascension these divisions of the zodiac are marked to single degrees. The zodiacal symbols are marked on the central vertical diameter and the ecliptic diameter is drawn. On the lines marking the divisions between the signs, right ascension is subdivided to 4 minutes. The hours are marked above and below the tropics, 'HOR? ANTE MERIDIEM', 1 to 12 and 'HOR? POST MERIDIEM', 12 to 1 respectively. Positions are marked for a number of named stars.

The other side has a scale of hours on the limb, XII to I twice, and within this a degree scale 90 to 0 to 90 to 0 to 90, divided to 5, subdivided to 1 with alternate hatching, numbered by 5. A rotatable and removable plate has an outermost zodiacal scale with the names and symbols of the zodiacal signs, each with a scale of 30 degrees, divided to 5, subdivided to 1 with alternate hatching, numbered by 5. Within this is a calendar scale with the names of the months in Latin and the number of days in each month, each with a scale of days divided to 5, subdivided to 1 with alternate hatching, numbered by 5 as appropriate. Within this named stars are arranged according to position. The upper half of the central space has a diagram for conversion between equal and unequal hours, 'HORE IN?QVALES' on two quadrant scales 6 to 0 and 12 to 6, divided to 1, subdivided to 20 minutes and to 4 minutes, numbered by 1 hour, and lines for 'HORE ?QVALES', numbered 12 to 1 twice, with half-hour lines interposed. One quadrant is marked 'HORE ANTE MERIDIEM', the other 'HOR? POST MERIDIEM'. The lower half of the central space has a double shadow square, marked 'VMBRAVERSA' / 'VMBRARECTA' twice and with scales 0 to 12 to 0 to 12 to 0, divided to 3, subdivided to 1 with alternate hatching, numbered by 3. Signed '+AN+THOIT+NE+' / 'MES+TREL APARIS' and dated '1551'.

Rule with shaped ends, with scale for degrees, 0 (at the centre) to 360, with 90 at each end, divided to 10, subdivided to 5, numbered by 10. Alidade with double pinhole sights, shaped and decorated ends, decorated centre and scales of either arm: 'HOR? ORTVS' 1 to 10, divided to 1, subdivided to 1/2 with alternate hatching, numbered by 1, and on the other arm 'HOR? OCCASVS' 3 to 12, similarly divided and numbered.

See Juan de Rojas, Commentariorum in astrolabium (Paris, 1551) and J.Bennett and S. Johnston, The Geometry of War 1500-1750 (Oxford, 1996), p. 54.

Jim Bennett

Museum of the History of Science, Oxford
Inventory number 32378

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