Galileo's military compass. 17th-century illustrations of the design of military compass invented by Italian mathematician, engineer, physicist and astronomer Galileo Galilei (1564-1642). Also known as a proportional compass or a sector, this instrument is used for trigonometry, surveying, navigation and aiming artillery such as cannon. It consists of two rulers of equal length joined by a hinge. When opened, a curved quadrant (upper right) enables it to be used as a quadrant. This page of illustrations is from Galileo's 'Le operazioni del compasso geometrico, et militare di Galileo Galilei' (Padua, 1606). The publishing location and name of the printer are at upper left.