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.]