The Astrolabe
The Islamic Golden AgeA flat sky you can hold
Hold one and it is smaller than you expect: a brass disc about the width of a palm, with a ring at the top so it can hang plumb. The hollow body holds a stack of engraved plates, each cut for a single latitude — Cordoba, Toledo, Baghdad — because the geometry of the sky changes as you move north or south. Over them turns the rete, a pierced brass spider whose curling points, on many instruments twenty to thirty of them, each mark one bright star. On the back pivots a sighting bar, the alidade. You hang the disc and swing the alidade: for a star you sight it through the two pinholes; for the sun you never look at all, but tilt the bar until a pinpoint of light through the upper vane falls on the hole in the lower one. Either way you read the altitude off the rim, then rotate the rete until the flat sky matches the real one. Everything else is read off.
Inherited before it was improved
It was not invented in Baghdad. The whole instrument rests on stereographic projection — the trick of flattening a sphere onto a disc without distorting angles — which Ptolemy set out in his Planisphere around 150 CE, and which some scholars trace back to Hipparchus. The earliest treatises on the instrument that survive are Greek and Syriac: John Philoponus in Alexandria, and Severus Sebokht, a bishop at Qenneshrin who died in 667. Both drew on a lost work by Theon of Alexandria. Arabic bibliographers say al-Fazari was the first Muslim to build an astrolabe, in eighth-century Baghdad — but that is their claim, recorded long after the fact, and no instrument survives to test it.
Prayer, shadow, and an honest limit
What Islam added was relentless demand. Five daily prayers are timed by the sun's altitude and the length of shadows, so plates were engraved with curves marking the afternoon and evening prayers, and by the fourteenth century large mosques kept a muwaqqit, a salaried timekeeper — Ibn al-Shatir held the post at the Umayyad Mosque in Damascus. The hours it gave were "unequal": a daylight hour was a twelfth of that day's daylight, stretching in summer and shrinking in winter. The qibla, the direction of Mecca, the instrument did not really compute; direction to a distant point is a harder problem. Makers engraved the answers instead, as gazetteers of towns. One Nastulus astrolabe in Cairo carries the earliest such list known on any instrument before about 1100.
The maker with one line to her name
The earliest surviving dated Islamic astrolabe is inscribed "made by Nastulus in the year 315" — 927 or 928 — and is now in Kuwait. A generation later, Ibn al-Nadim's Fihrist, a Baghdad bookseller's survey of everything then written in Arabic, lists among working instrument-makers a woman: al-'Ijliyya, daughter of al-'Ijli, employed at Aleppo by the emir Sayf al-Dawla, who ruled from 944 to 967. She and her father, it says, trained under Nastulus. That is the entire record: one entry, no anecdote, no surviving instrument. Modern retellings call her Mariam al-Asturlabi, but "Mariam" appears nowhere in the source. The bare listing may be the stronger evidence — she is named among the men without comment.
The brass travelled, and so did the names
In 2024 Federica Gigante, a historian of scientific instruments, noticed an overlooked astrolabe on a Verona museum's website and found a palimpsest. Made in eleventh-century al-Andalus, its plates read "for the latitude of Cordoba, 38° 30′" and "for the latitude of Toledo, 40°"; a signature names Ishaq and Yunus; later owners added Hebrew, and later still someone scratched Western numerals beside the Arabic figures — making things worse, not better. Against the Arabic reading for Medinaceli, 41° 30′, one later hand added '42' and another '40'; the true value is 41° 15′, so both corrections moved further from it than the figure they were correcting. Star names drifted west the same way. Of the 254 traditional star names catalogued by Paul Kunitzsch and Tim Smart, the large majority — most counts put it at around two-thirds — are Arabic in origin. Their share of the IAU's official register is falling, though, and not because anything was taken away: the list simply keeps growing, past 600 names, with new entries drawn from Chinese, Sanskrit, Sumerian and Indigenous star lore. And many of the Arabic names are themselves Arabic renderings of Ptolemy's Greek descriptions: an inheritance passed along, not simply coined.
Key Dates to Remember
- c. 150 CE — Ptolemy's Planisphere sets out the stereographic projection the astrolabe depends on. Whether he also describes a working instrument is still argued.
- before 667 — Severus Sebokht, a Syriac bishop at Qenneshrin, writes a treatise on the astrolabe drawn from Greek sources — among the earliest to survive, alongside John Philoponus's.
- 927/8 (315 AH) — Nastulus of Baghdad signs the earliest surviving dated Islamic astrolabe, now in Kuwait. Earlier Abbasid instruments survive, but undated.
- 944–967 — Sayf al-Dawla rules Aleppo. A single line in Ibn al-Nadim's Fihrist places the astrolabe-maker al-'Ijliyya at his court — the whole of what is known about her.
- 11th century — The Verona astrolabe is made in al-Andalus, with plates for Cordoba and Toledo. Over the following centuries owners add Hebrew and then Latin inscriptions to the same disc.