Double Quadrant by Thomas Gemini
Record status
Individual type · partial
Period
mid 16th century
This double quadrant, signed by Thomas Gemini, dates to the mid-16th century. It is constructed from two brass plates and a central wooden core, with an overall radius of 270 mm.
The assembly is held together by three iron bolts with square shanks and flattened pyramidal ends. Designed for astronomical and trigonometric operations, the instrument includes varied scales for calculating moveable feasts, measuring elevation, and performing astrological predictions. One side features a stereographic projection of celestial coordinates, including the ecliptic and various stars, while others include a zodiacal calendar, an hour scale, and a shadow square.
The current mounting arrangement, featuring a wooden core, is documented in the 1654 Medici inventories, though it is considered a non-original modification to the plates, potentially performed within a Medici workshop.
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
- English quadrant
- Maker
- Thomas Gemini
- Materials
- Brass, wood, iron
- Dimensions
- 270 mm radius, 8 mm core thickness
- Place recorded
- England
- Attributed Origin
- English
- Maker / attribution
- Thomas Gemini
Design
- Visible features
- • Central wooden core measuring 8 mm in thickness • Brass plates with engraved scales and star names • Iron bolts with flattened pyramidal ends • Sighting vanes with open and pinnule sights on a radial side
- Construction and Assembly
- The instrument functions as a double quadrant, composed of two brass plates positioned on either side of a central wooden core. These components are secured at the apex pivot point, though they are further held in their present form by three square-shanked iron bolts that penetrate the assembly. The wooden core is 8 mm thick and features a non-functional hole at its centre, suggesting it was adapted for the plates rather than created specifically for them. The presence of a screw hole that obscures part of the original engraving on the horizontal quadrant plate indicates that the current mounting is not original. This configuration was already in place by the time the instrument appeared in the 1654 Medici inventories. It is proposed that this structural modification was undertaken in a Medici workshop prior to the mid-17th century.
- Signatures and Attribution
- The authorship of the instrument is established through the specific engraving of the symbol 'II' upon the brass plates. This mark represents the sign of Gemini, which serves as a signature for the maker Thomas Gemini. The provenance of the object is linked to the historical Medici collection, as the instrument is documented in their inventories as early as 1654. The inventory entry provides a description of the instrument's components, specifically noting the brass plates, the wooden core, and the iron screws, confirming that the transformation of the piece was finalized at least by the mid-17th century.
- Astronomical and Calendrical Features
- The quadrant is engraved with a variety of mathematical and astronomical scales across its multiple faces. Side one of the first quadrant contains a doubled zodiacal calendar, a scale of 90 degrees, and a list of seventeen festivals related to the liturgical year. These are accompanied by concentric semi-circular scales for leap year calculations, the solar cycle, and the dominical letter. Side two of the first quadrant presents a stereographic projection of celestial coordinates, functioning similarly to an astrolabe plate. This surface displays the ecliptic, the horizon, and the meridian, alongside the positions of specific stars such as Caput Algol and Hircus. The second quadrant provides a 90-degree limb scale and a shadow square, enabling calculations related to sines and versed sines.
Identification
- Inventory Number
- 2509
- Medici Inventory Reference
- 1654
- 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 mounting of the plates onto the wooden core is considered a non-original transformation
- Research required: The exact date of the instrument's transformation remains unverified