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Attributed to Gualterus Arsenius
circa 1570; Flemish
Brass; 297 mm in diameter

The throne of the astrolabe displays two reclining counterpoised satyrs, who lean on the moulded block to which the suspension ring of the instrument is pivoted.

The limb is divided into four 90? quadrants and in divisions from I to XII repeated twice.

The rete bears 54 stars, represented by protruding points accompanied by their names. The zodiac circle is divided 12 times into 3 parts, and bears the names of the signs in Latin, accompanied by their symbols.

The pointer traverses the whole face, and is divided into two parts by the central pivot. One part is graduated 70 to 80 to 90 towards the external point, with the inscription 'Declin: meridi:'; the other is graduated 40 to 50 to 60 to 70 to 80 to 90, with the inscription 'Declinacio septen:'.

The back of the astrolabe has its limb divided into four 90? quadrants. It then bears the names of the signs of the zodiac in Latin, accompanied by their symbols and by the numbers I to XII (with Aries as I). These are followed by 12 divisions into 30, then by the divisions of the months and the names of the months in Latin, accompanied by the numbers I to XII (with 'IANVUARIUS' as I).

The central part of the back of the astrolabe is divided into two parts by the diameter. The upper part is traced with slightly curved lines, running towards the diameter from the top. These cover its entire surface. The lines are numbered at the top, from the diameter, 1 to 12, 12 to 1. Along the diameter, the base of the lines is numbered from 1 to 12, beginning from the outside. A semicircle, which bounds this region, stands on the diameter. The arc of the semicircle is divided into parts numbered from 1 to 12, with the inscription, in the centre of the upper part: 'Hore aequales'.

Underneath the diameter, the following inscriptions, referring to the numbering of the base of the hour lines can be read: on the left hand side: 'Hore ante meridiem et aequales', on the right hand side: 'Hore post meridiem et aequales'.

The lower part of the back, under the diameter, bears the semi shadow square, in which one can read the inscription 'Scalae altimetrae'. The sides of the half square bear a double division from 0 to 60 and 1 to 12, and the inscription 'UMBRA RECTA' and 'VMBRA UERSA'. The use of different and alternated forms of the letter 'U' is worthy of note.

The alidade is graduated from 1 to 12 in both directions, with the inscriptions 'Hore occasus' on one side and 'Hore ortus' on the other. It is provided with pierced holes at its ends.

The mater contains eight plates, calibrated for the latitudes 24 and 27?, 30 and 33?, 36 and 38?, 40 and 42?, 44 and 46?, 48 and 51?, 54 and 57?, and 60? with the tablet of horizons.

The instrument is unsigned, but its decorative characteristics and the style in which it is made suggest its attribution to Gualterus Arsenius, nephew of Gemma Frisius.

See M. Miniati, Museo di Storia della scienza: Catalago (Florence, 1991), p. 34.

Mara Miniati

Istituto e Museo di Storia della Scienza, Firenze
Inventory no. 1094

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