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Attributed to Charles Whitwell
Late 16th century; English
Brass; 673 mm in diameter

A very large brass disc with a raised limb to form a mater, has a suspension ring connected by a chain to two eyes attached to the limb. The limb is blank on the recto, but the inside of the mater has two half projections of the sphere, separated by a vertical division, which is divided into degrees with alternate hatching. The right hand hemisphere has the saphea projection, the left the Rojas. The equator scale on the saphea side is numbered four times, so as to cover the full range from 0 to 360; on the Rojas side it is numbered only once, 0 to 90. Both projections have double ecliptic scales to cover the whole zodiac, marked with zodiacal symbols for each sign, each with a 30-degree scale, divided to 10, subdivided to 1 with alternate hatching, numbered by 10. The tropics are marked with degree scales with alternate hatching, by straight lines on the Rojas projection and arcs on the saphea, and both have the hours indicated in Roman numerals. Meridians and parallels are engraved for every degree on both projections, except for the extremes of the Rojas: every fifth and tenth degrees are indicated by one-sided and two-sided arrows respectively, and whole hour divisions by wavy lines. A double zodiacal index can pivot at the centre, with a skeletal grid covering 6 degrees on either side of the ecliptic, one arm being appropriate to each projection. The central band, representing the ecliptic, is marked with the zodiacal symbols. The pattern of the grid is engraved on the saphea projection on the base plate.

Two semicircular plates can be placed in the mater. One has the northern or southern half of the saphea projection, with a grid similar to the hemisphere on the base plate, but without the tropic or ecliptic being indicated. The equator is numbered every 10 degrees from the centre 0 to 90 to 270 to 360. The other side of this semicircle has a polar projection, with a similar repetition of parallels and meridians, the parallels numbered every 10 degrees, and the hour lines every hour 1 to 12 and 12 to 1, and also 1 to 12 in Roman numerals. The equator is numbered every tenth degree, 270 to 360, 0 to 90. The other semicircle has a form of nautical square on one side and a proportional scale on the other.

On the verso the inner edge of the limb has a degree scale 0 to 360, divided with alternate hatching to 20 minutes of arc, numbered by 10 degrees, while outside this are a number of scales and stars. On the plate inside the limb are a lunar scale, a zodiacal scale for the sun's motion, marked with the names and symbols of the zodiac, and within these is a diagram for tides. The central area includes a sinecal quadrant, a line of meridional parts, and the positions of a number of stars.

Jim Bennett

Istituto e Museo di Storia della Scienza, Firenze
Inventory nos 1123, 1124, 1127

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