Galileo designed his geometric and military compass (“compasso geometrico e militare”) in 1597. Known in English as the sector, the compass was widely used until the invention of logarithmic tables. In Galileo’s model, an arc could be inserted between the two arms of the compass to make the instrument into a quadrant. A plumb bob could be attached.
The Compasso was Galileo’s first published book and was printed in Padua, where he had taught mathematics at the University since 1592. Written in the vernacular, the book was directed to a wide audience. It was dedicated to Cosimo II de’ Medici, the Grand Duke of Tuscany, who had been one of Galileo’s students and who would hire him as his philosopher and mathematician a few years later.