How it began
The mariner's astrolabe was a simplified version of the astrolabe that Arab astronomers had developed for measuring the height of heavenly bodies. According to Royal Museums Greenwich it came into use in navigation by about 1470. Sources give several dates for the first examples, and one old claim credits Martin Behaim, but the creation and perfecting of the instrument is attributed to Portuguese navigators at the start of the Portuguese discoveries.
Before it existed
Portuguese seamen returning from trading posts in West Africa were forced by winds and currents to sail far out into the open ocean, out of sight of land. They needed a way to know their latitude. The planispheric astrolabe used on land was light and complex and was poorly suited to a rolling deck in wind. Navigators also used the quadrant and the cross staff.
How it works
The instrument is a graduated circle with a pivoting rule, the alidade, that carries two vanes with pinholes. It is hung from a ring so that it stays vertical. At night the navigator sights the Pole Star through the pinholes. By day he holds it low and turns the rule until sunlight passes through the upper pinhole onto the lower one, then reads the altitude in degrees on the rim. The mariner's version was heavier, and parts of the disc were cut away to reduce wind resistance.
How it spread
Vasco da Gama took a small brass astrolabe and a larger wooden one on his voyage around Africa to India in 1497 to 1499. Christopher Columbus carried an astrolabe in 1492, though he had trouble using it on a pitching ship. Spanish navigators used the instrument too, and Martín Cortés de Albacar published the earliest known description of how to make and use one in 1551.
What lasted
The mariner's astrolabe was in use until the middle or end of the 17th century, when more accurate instruments such as the Davis quadrant replaced it. In its time it replaced or complemented the cross staff and the quadrant as the navigator's instrument, and it played a key part in the voyages of the Age of Discovery.
Where it stands today
Mariner's astrolabes are rare. Only 108 were known in 2017, and the largest collection is in museums in Portugal. The oldest known, dated between 1496 and 1501, was found in 2014 on the wreck of the Portuguese ship Esmeralda, which sank off Oman in 1503.
