Time & mechanics · 1291 CE (690 AH)
A crowded disk that helped read the sky
What kind of instrument is this rotating, graduated disk?
Curious? Reveal the story
The reveal
Astrolabe made by a Yemeni prince
It is an astrolabe, translating relationships between the celestial sphere and the horizon onto a flat set of scales for astronomical measurement and time calculations. Its inscription attributes it to Prince Umar of Yemen’s Rasulid dynasty. The museum also identifies scientific writings that connect with this particular instrument.
Putting the sky on a flat surface
This disk does not run by clockwork. An astrolabe represents relationships between celestial bodies and an observer’s horizon on a plane. Its pierced star map rotates against scales below, allowing calculations after measurements such as the altitude of the sun or a star. It is an instrument to operate and read, not a self-running ancient clock.
Different pieces have different jobs
The case holds graduated plates, a pierced star map rotates above them, and a sighting rule on the back measures angles. Plates for particular geographic latitudes adapt the instrument to appropriate locations. The front photograph shows only part of this arrangement. Imagining the parts moving in relation to each other reveals how much information could fit into a hand-held metal disk.
The maker later became a ruler
The documentation is unusually revealing. The inscription associates the astrolabe with the Rasulid prince Umar and dates it to 1291, before his accession. The museum notes that he wrote scientific works. An autograph treatise on astrolabe construction preserved in Cairo includes both a teacher’s certification of his ability and a description of this instrument. Object and written record reinforce one another.
A photograph is not a working instrument
Dense scales do not make the instrument usable without adjustment everywhere. Readings depend on the selected plate, observing location, and method. This page explains a historical object and its basic principle; it does not turn the photograph into a navigation tool. Its fascination lies in the meeting of geometry, metalwork, and a documented historical education.
Follow the source
The Metropolitan Museum of Art · Edward C. Moore Collection, Bequest of Edward C. Moore, 1891 · 91.1.535a–h · Public domain
This is an independently edited story based on the original museum record.
View the museum record Image reuse policy University of Oxford: astrolabe structure and useTHE VISITOR BOOK
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