Thomas Gemini Astrolabe
Record status
Individual type · partial
Period
Mid 16th century
This mid-16th-century brass astrolabe, measuring 355 mm in diameter, is the work of maker Thomas Gemini. It features a mater constructed of three soldered and riveted brass layers, with a central wind rose for 32 directions labeled in English.
The interior includes a 'QVADRATVM NAVTICVM' with coordinates marked in Latin. The rete is characterized by 'wavy flame' star pointers and lacks an east-west line. Gemini signed the instrument on the bottom of the shadow square with the inscription 'Thomas II Anno 155', indicating an incomplete date. Historically, the instrument formed part of a bequest from Robert Dudley to Ferdinand II de' Medici, Grand Duke of Tuscany, in 1649.
The suspension system utilizes a sphere within a grooved throne, fixed by a pin and nut mechanism rather than a wedge. It serves as a navigational aid, containing markings for astronomical and stellar observation and various mathematical scales.
Evidence at a glance
- Linked sources
- 1
- Photograph views
- 1
- Catalogue references
- 0
Counts describe this entry’s documentation, not authenticity, rarity or completeness. Photographs may show one specimen; check each caption and licence.
Technical characteristics
- Date
- Mid 16th century
- Type
- Astrolabe
- Maker
- Thomas Gemini
- Material
- Brass
- Signature
- Thomas II Anno 155
- Provenance
- Robert Dudley to Ferdinand II de' Medici
- Place recorded
- England
- Maker / attribution
- Thomas Gemini
Design
- Visible features
- • Wavy flame style star pointers on the rete • Central wind rose with 32 directions • Circular throne decorated with foliate engraving • Signature 'Thomas II Anno 155' visible on the shadow square
- Rete and Ecliptic
- It includes 23 stellar pointers executed in a 'wavy flame' style, each terminating in a small star and labeled in Latin. A tulip-shaped design is positioned within the ecliptic circle, which is engraved with a scale of degrees and the symbols and pictorial representations of the zodiac signs. A meridian line is present exclusively on the lower portion of the rete, described as being elaborately detailed. Additional star positions are plotted directly onto the ecliptic ring, which features degrees numbered from 5 to 30 in increments of 5.
- Construction and Mater
- The astrolabe's mater is composed of three distinct brass layers, which have been soldered and riveted together to form the body of the instrument. The rim features two primary scales: an outer scale of hours, divided into intervals of 1/15 and numbered I to XII twice, and an inner scale of degrees numbered 10 to 360 in increments of 10. The interior space is dominated by a 'QVADRATVM NAVTICVM' (nautical square). This square is subdivided, with coordinates labeled 'Latitudo maior aut borealior', 'Latitudo minor uel Australior', 'Longitudo minor siue Occidentalior', and 'Longitudo maior seu Orientalior'. A 32-direction wind rose sits at the center, employing English nomenclature for the winds alongside Latin and Greek designations.
- Signature and Suspension
- The instrument is signed 'Thomas II Anno 155' on the base of the shadow square, signifying the maker, Thomas Gemini. This inscription provides an incomplete date. The throne is circular and adorned with foliate engraving, providing a base for the suspension assembly. The suspension mechanism is composed of a sphere that fits into a groove located on the throne, connected to a ring via a second ring. Unlike some contemporaneous instruments, this example does not utilize a wedge for securement, opting instead for a pin and nut system.
Identification
- Inventory Number
- 1093
- Source note
- Facts transcribed from the linked source listing or collection record. Attribution and specifications are source-reported and have not been independently authenticated.
Catalogue references
No catalogue numbers recorded. Numbers are only published once transcribed from a verified edition of a standard catalogue — they are never inferred.
Research required
- Research required: The date inscribed is incomplete, marked only as '155'
- Research required: The function of two symmetrical screw holes on the alidade is not identified