The Tusi couple in Greek: the Schemata of the Stars.
A Greek astronomical text carries a Persian lunar model and, with it, the same numerical error. Learn why a shared error is a stronger fingerprint than a shared diagram.
Start with the shared error
Ragep's study of the Tusi couple finds that the Greek Schemata of the Stars is largely derived from Tusi's earlier Persian Mu'iniyya and Hall tradition. Both preserve the same six-orb lunar model and the same inclined-orb value of 13;11 degrees per day, which Tusi later corrected to 13;14 in the Tadhkira. The research case rates their shared ancestry a verified fact at 99. Max Planck Institute for the History of Science ↗
Why the error matters
A shared design can arise independently, since astronomers solved the same problems. A shared mistake is much harder to explain without copying. That is why the research record prizes rare shared errors as transmission fingerprints, and why it states plainly that none yet links Ibn al-Shatir's Mercury model to Copernicus.
A partial carrier, not the whole route
The research case rates at 98 that the Vatican Greek manuscript carrying the Schemata is a verified partial carrier of Tusi's lunar and couple material, and adds that this does not by itself show the full Ibn al-Shatir Mercury packet reached Copernicus. Evidence for one model's journey is not evidence for every model's journey. Max Planck Institute for the History of Science ↗
How a model is used and passed on
A 1995 study by Di Bono follows the use and transmission of the Tusi device among Copernicus and the Italian astronomers Amico and Fracastoro. It is a reminder that the couple was a tool people picked up and used, not only a document that was copied. Journal for the History of Astronomy ↗
A question to carry forward
If you found a Latin text with the same unusual numerical slip as a Persian original, what else would you need before calling it a copy, and what would make you suspect a common source instead?