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The Encyclopedia of Horology

Tool · Italian quadrant

Giovanni Antonio Magini Planispheric Quadrant

Partially verified

This 297 mm radius brass quadrant, signed by Giovanni Antonio Magini, functions as a model of the heavens and a calculating device. It serves as a planisphere, incorporating a stereographic projection of the northern hemisphere on one side and a projection of the heavens on the other, functioning essentially as a folded astrolabe. A radial rule, constructed from two soldered brass components, allows for the instrument's use on both sides. The device features highly complex engravings, including celestial latitude and longitude scales, ecliptic divisions, astrological houses, almucantars, and 31 named stars. Additional decorative elements include a coat-of-arms in the centre and geometric motifs along the edges. The instrument is equipped with a ring at its apex for suspension. It is held in the collection of the Museum of the History of Science, Oxford.

Front view of the brass quadrant showing the radial rule and intricate celestial projections.
Front view of the brass quadrant showing the radial rule and intricate celestial projections. — Museum of the History of Science, Oxford. Rights pending — permission not yet requested. Original source

Catalogue identity

Source
www.mhs.ox.ac.uk
Category
Tool · Italian quadrant
Maker / attribution
Giovanni Antonio Magini
Reference
Not recorded
Catalogue / lot identifier
31514
Source-reported date
Late 16th century
Source-reported place
Italy

Source-reported attribution and specifications are not independent authentication. A listing does not establish present ownership, current display, availability or permission to reproduce.

Object description

Structural Design and Components

The instrument is a large, brass-constructed quadrant with a radius of 297 mm, featuring a suspension ring at its apex. It incorporates a dual-sided radial rule, designed by soldering two separate brass components together, enabling its application across both faces of the instrument. The edges and limb are finished with geometric decorative motifs.

This quadrant is identified as a functional model of the heavens rather than an observational tool. Its design is derived from 13th-century principles established by Profatius Judaeus, essentially functioning as a folded astrolabe capable of complex astronomical calculations.

Evidence for this section
  • “The quadrant is rather big, and has at the apex a ring for suspension.”
  • “A radial rule is made of two brass parts soldered together at the ends, so that the same rule is used for both sides of the instrument.”
  • “The instrument was probably not used for observation, but as a model of the heavens and as a calculating device for the same tasks as an astrolabe”

Celestial Projections and Engravings

One face displays a polar projection of the celestial sphere with lines marking degrees, celestial latitude, and longitude. It includes an ecliptic divided into degrees with associated zodiacal symbols and astrological houses. The sliding rule here features unequal scales of latitude.

The reverse side features a geocentric projection of the heavens. Engravings include the meridian, the horizon, equinoctial and tropic lines, and almucantars spaced every 3 degrees. Zodiacal signs and the names of 31 stars are present, alongside scales for right ascension and elevation.

Evidence for this section
  • “On one side is a projection of the heavens, in fact a folded astrolabe , indicating the equator , the tropics, the ecliptic and 37 stars.”
  • “On the back of the quadrant is a stereographic projection of the northern hemisphere.”
  • “The names of 31 stars and their constellations are engraved.”

Inscriptions and Markings

The instrument is explicitly signed by the maker, Giovanni Antonio Magini, on the edge of the quadrant. Various scales are numbered in complex, often bustrophedic arrangements to accommodate the geometry of the device. Specific lines, such as those for celestial latitude and declination, are marked with Latin inscriptions.

At the central point of the instrument, a distinctive coat-of-arms is engraved, featuring a pyramid atop a rectangular base, positioned between two stars.

Evidence for this section
  • “On this edge is also engraved the inscription 'QVADRANS PLANISPHAERII GENERALIS IO ANTONII MAGINI'.”
  • “In the centre of the quadrant is engraved a coat-of-arms with a pyramid, standing on a rectangular base, between two stars.”
  • “The longitude lines are engraved by 1, and are numbered on the other edge in bustrophedic way”

Open research questions

  • What is the specific historical significance of the pyramid coat-of-arms in relation to Magini's patronage?
  • How do the discrepancies between the star count in the overview (37) and the detail section (31) resolve upon physical inspection?

Source evidence

Recorded claims and source quotations for Giovanni Antonio Magini Planispheric Quadrant
FieldSource-reported valueSource quotation
MakerGiovanni Antonio Magini“Signed by Giovanni Antonio Magini”
MaterialBrass“Brass; 297 mm in radius”
Inventory number31514“Inventory number: 31514”
Radius297 mm“297 mm in radius”
OriginItalian“Italian”
DateLate 16th century“Late 16th century”
Star count31“The names of 31 stars and their constellations are engraved.”

Visible details

  • Radial rule constructed of two brass parts
  • Apex ring for suspension
  • Engraved coat-of-arms with a pyramid and two stars in the centre
  • Decorated edges with geometric motifs

Research qualifications

  • The source notes the instrument was 'probably' not used for observation
  • It is uncertain if the full sequence of numbers is visible due to the sliding rule covering parts of the instrument

Photograph identity and source facts were screened before import; rights clearance remains pending.

Consulted source

Quadrant

Reviewed 2026-10-26 · Record 008d6d7eb9

Terms in this record

Glossary explanations for terms named in this object’s record.

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