Use of the Astrolabe in Ocean Navigation

Timeline

1. 1490–1494Lisbon · LisbonThe simplified mariner's astrolabe appearing in the later 15th century removed superfluous rings and weighted the plumb for steadier observation, better suited to measuring celestial altitudes on deck; in the early 1490s Portuguese instrument makers and navigators further improved its scales and sights for more reliable latitude readings.
2. 1492–1497North Atlantic · North AtlanticPortuguese fleets in the 1490s routinely carried astrolabes on voyages along the West African coast, with crews recording the sun's declination and their latitude; da Gama's 1497 route to India likewise relied on the astrolabe and solar observations for latitude, and though longitude still depended on estimation and landmarks, latitude control markedly improved ocean safety.
3. 1495–1499Iberian Peninsula · Iberian PeninsulaBy the late 1490s the astrolabe formed a matched set with the magnetic compass, portolan charts, and astronomical tables; Spain's Canary Islands and Portugal's Azores became relay points for calibrating latitude and resupplying, shifting 1490s navigation from coastal piloting to open-ocean positioning and paving the technical path for 16th-century global routes.