Originally posted at ouhos.org, the now-discontinued blog of the OU History of Science Collections. Neither this post nor any of its content should be taken as an official communication of the University of Oklahoma.
Today’s 
Astronomy Picture of the Day
features a photo of the night sky at Alamut, in the Alborz Mountains of Iran, northeast of Tehran. Alamut Castle was for a while the home of the Persian mathematician and astronomer Nasir al-Din al-Tusi (1201-1274). Click the link and take a look at the sky as al-Tusi might have known it.
The History of Science Collections hold a copy of the version of Euclid’s Geometry associated with the circle of al-Tusi (right). This is a printed edition, not a manuscript, although through a technological feat it displays ligatures and other features of Arabic writing. However, this edition was not printed in Baghdad or Cairo, but in Rome in 1594. The Medici set up a printing press to make important Arabic works like this one available for European scholars who were willing to learn Arabic in order to make further advances in their fields.
Al-Tusi worked on problems raised by Ptolemy’s Planetary Hypotheses, a work concerned with describing possible physical structures of the universe that would correspond with the geometrical models of the Almagest. Al-Tusi is best known for developing a geometrical device called the “Tusi couple” that acts like a crank mechanism, sliding a planet directly toward or away from the center of the deferent circle. The Tusi couple resolved a problem with Ptolemy’s lunar models, which accurately predicted the position of the Moon but required the Moon to appear with a greatly varying diameter (which is not observed). Therefore the Tusi couple could move the Moon farther out or closer in as needed to maintain a more constant apparent diameter of the Moon. In the 16th century, Copernicus used a Tusi couple in his lunar theory, directly appropriating this technique from Islamic astronomers. [See former OU professor Jamil F. Ragep, “Tusi and Copernicus,” Science in Context, 2001, 14:145-163.]
