Four Instruments in Martín Cortés' Breve Compendio de la Sphera and a Rule for Finding the Age of the Moon

This paper investigates and discusses four instruments found in a sixteenth century Spanish book of navigation. The night clock is a rather standard application of similar instruments known since the fourteenth century in Europe. Cortés’ explanation of the causes of the deviation of the magnetic and geographical north is quite original. His instrument to correct for that the Polaris deviates from the North Celestial Pole is an original way of presenting the practical tradition of Spanish mariners. The general sundial is essentially a copy of the same instrument appearing in in Peter Apian’s books Cosmographia LIber and Ein kunstlich Instrument but with an addition for finding the azimuth of sunrise and sunset. The paper also investigates a simple rule for finding the age of the Moon for any date. Observations of the Moon and its phases were an important part of navigational art for predicting with the tides.