Redesigning the astrolabe

Name the limit of the earlier instrument, then ask what the new design changed to remove it. Note that a change is not always an improvement in every respect.

List what each design gained and what it lost, using the records' own statements, and decide which gains matter for which kind of user.

Read one report about the universal plate and notice the route by which it reaches us. Roser Puig describes al-Zarqali as a Toledan instrument maker who taught himself astronomy and worked with Said al-Andalusi under the city's ruler al-Mamun ². This was the 11th-century ruler of Toledo, not the Abbasid caliph who lived centuries earlier ². Puig reports a 14th-century Egyptian witness to a lost work by Said: al-Zarqali constructed an instrument called al-zarqala for al-Mamun in 1048/1049 and wrote 100 chapters on its use ². This is a report transmitted through a later witness, not a foundation document independently read by this museum ².

The museum's records hold no statement in which al-Zarqali says why he designed the plate, and they do not supply a motive for him. What they show is the problem his design addressed: an ordinary astrolabe's horizon was drawn on a plate associated with a particular latitude ³. For al-Tusi's staff, Ozel describes simplicity, low cost and fewer defects as intentions behind the instrument, and the record calls these design aims, not measured superiority ^4 5. The translated uses include altitude and time, and relate the afternoon prayer-time problem to shadows and altitude ⁶.

"Universal" means applicable across latitudes, not a machine that discovers location and time without knowledge or interpretation ⁷. A second confusion concerns al-Tusi's name. This is Sharaf al-Din al-Muzaffar al-Tusi, not Nasir al-Din al-Tusi of the Maragha observatory, and the 1895 editor explicitly corrects an earlier addition of Nasir's name ^8 9. A shared geographical surname is not an identity record ^8 9.

The detailed witness to al-Tusi's staff is later: Carra de Vaux's 1895 edition of passages from al-Marrakushi, with French translation, whose editor reports a missing ending in his manuscript ¹⁰. The record read selected pages of that publication, not Sharaf's autograph, and did not collate its Arabic text against manuscripts ¹⁰. The 1895 editor notes that al-Marrakushi sometimes depends on an unnamed author and does not confidently identify that author; the museum does not fill the gap with a guess ^4 11. For a physical object, the record on al-Zarqali lists a brass saphea at Louvre Abu Dhabi, ca. 1224, and says its catalogue does not establish that al-Zarqali made it or that its manufacture dates the invention ¹². Ask which witness, the printed text or the displayed object, stands closer to the maker's hand.

The translated witness explicitly calls the staff's azimuth method incomplete and discusses difficulties near the equinoxes and in nighttime meridian determination ^13 14. Ozel describes simplicity, low cost and fewer defects as intentions behind the instrument ⁴. Take a position on whether such a design is an improvement, and answer the strongest objection to it: is a tool that cannot do some of what its predecessor did not a substitute rather than an improvement?

• Al-Zarqali's universal plate: what the design changed, and what survives of it in translations and objects.
• Al-Tusi's linear astrolabe: the staff and its threads, and the limits the witness states.
• Al-Zarqali: the record of the person to whom the plate is attributed.
• Sharaf al-Din al-Tusi: the record of the person to whom the staff is attributed.