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The Encyclopedia of Horology

Tool · Flemish astrolabe

Astrolabe

Partially verified

This 1558 brass astrolabe, attributed to the maker Jacobus Valerius, measures 133 mm in diameter. The instrument survives only as the mater, with the limb, shackle, and suspension ring intact; latitude plates and the rete are missing. The interior of the mater features a complex radiating table of numbers designed to calculate astronomical events—specifically conjunctions, oppositions, and solar or lunar eclipses—for the city of Zaragosa (Caesar Augusta) between 1558 and 1570. The limb carries a 360-degree scale and a 24-hour scale, while the exterior rim displays the names of the 12 classical winds. The back of the instrument contains 13 concentric scales, including degree, zodiacal, and calendar divisions, alongside a central projection of the celestial sphere, a shadow square, and an unequal hour diagram.

The interior of the mater showing the radiating table of numbers for astronomical calculations and the limb scale.
The interior of the mater showing the radiating table of numbers for astronomical calculations and the limb scale. — 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 · Flemish astrolabe
Maker / attribution
Jacobus Valerius (attributed)
Reference
Not recorded
Catalogue / lot identifier
48892
Source-reported date
1558
Source-reported place
Flemish (attributed)

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

Construction and Mater

The astrolabe is constructed of brass and measures 133 mm in diameter. Currently, only the mater remains, with the limb soldered directly to the back plate. The suspension mechanism, including the shackle and ring, is extant. The interior surface of the mater is entirely occupied by a radiating table of numbers used to determine conjunctions, oppositions, and solar and lunar eclipses.

This table is specifically calculated for the Spanish town of Zaragosa, referred to in the instrument as Caesar Augusta, and covers the years 1558 to 1570. The limb is engraved with a degree scale of 0 to 360 and a scale of hours numbered I to XII twice. The outer rim is marked with the names of the 12 classical winds.

Evidence for this section
  • “The limb is soldered to the back plate, shackle and suspension ring.”
  • “The inside surface has a radiating table of numbers for calculating the conjunctions, oppositions and eclipses of the sun and moon, constructed for the Spanish town of Zaragosa and the dates 1558 to 1570”

The Reverse and Scales

The reverse of the astrolabe is decorated with 13 concentric scales. The outermost scale represents 90 to 0 to 90 degrees, followed by a zodiacal scale including names and symbols for each 30-degree section. A circle indicates the positions and magnitudes of certain stars, while additional circles contain planetary symbols and a calendar scale with Latin month names.

The central area of the back features a projection of the celestial sphere in the lower half. One of the upper quadrants contains a shadow square labeled 'VMBRA VERSA' and 'VMBRA RECTA', with scales 0 to 12. The opposing quadrant holds an unequal hour diagram and solar altitude lines every 5 degrees, marked with the inscription 'RECTIFICATA ? ANNO ? XPI ? 1558'.

Evidence for this section
  • “On the back are a set of 13 concentric scales.”
  • “In the central space, the lower half is occupied by a projection of the celestial sphere.”
  • “One of the upper quadrants has a shadow square with scales 0 to 12 to 0, divided to 3, subdivided to 1, numbered by 3, marked 'VMBRA VERSA' and 'VMBRA RECTA'.”

Attribution and Origin

The instrument is attributed to the maker Jacobus Valerius, based on stylistic parallels to an existing instrument in Brussels bearing his name. There is scholarly uncertainty regarding the place of manufacture; while the table of numbers specifically addresses the needs of Zaragosa, Spain, political ties between the Spanish crown and the Netherlands suggest that a Flemish origin is plausible.

The complexity of the calculations suggests the astrolabe was intended for a user with specific scholarly or specialist requirements. The absence of the rete and latitude plates limits its current utility, yet the existing engraving provides clear evidence of its original design and function.

Evidence for this section
  • “The ascription of this astrolabe to Jacobus Valerius is on the basis of its similarity to an instrument now in Brussels bearing this name.”
  • “It may have been made in Spain, but such were the political links between Spain and the Netherlands, that a Flemish origin is at least as likely.”

Open research questions

  • Can the specific engraving style of the mater be definitively linked to other instruments definitively signed by Jacobus Valerius?
  • Does the inscription 'RECTIFICATA' on the back suggest the instrument was modified or calibrated after its initial construction in 1558?

Source evidence

Recorded claims and source quotations for Astrolabe
FieldSource-reported valueSource quotation
MakerJacobus Valerius (attributed)“Attributed to Jacobus Valerius”
Date1558“Dated 1558”
Dimensions133 mm in diameter“133 mm in diameter”
MaterialBrass“Brass; 133 mm in diameter”
Inventory Number48892“Inventory number: 48892”
GeographyFlemish“Flemish”
Missing componentsLatitude plates and rete“For these reasons, it is a particular disappointment that the latitude plates and the rete are missing.”

Visible details

  • The limb is engraved with a degree scale 0 to 360 and a scale of hours numbered I to XII twice.
  • The central space on the back contains a projection of the celestial sphere.
  • The interior surface of the mater is densely engraved with a radiating table of numbers for astronomical calculations.
  • The limb is fixed to the back plate, shackle, and suspension ring.

Research qualifications

  • The ascription to Jacobus Valerius is based on similarities to another instrument, not a signature.
  • The origin is debated between a Spanish or Flemish manufacture, despite being attributed as Flemish.

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

Consulted source

Astrolabe

Reviewed 2026-10-26 · Record 530c5d7bc2

Terms in this record

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

Other flemish astrolabe records