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Greenwich (Valentia) Astrolabe

A ring shaped brass instrument with four cut out sections and alidade.

Greenwich (Valentia) Astrolabe, © National Maritime Museum, Greenwich, London

  • Intro
    Museum's Description
    Teacher notes
    • If you were a captain in charge of a ship on a long sea voyage, wouldn’t it be useful to know where in the ocean you were at any one time? This mariner’s (a sailor's) astrolabe used the Pole Star or the sun to pinpoint the ship’s position. The astrolabe was held below the waist and the Alidade (the middle piece) was adjusted so that a beam of sunlight passed through the top pinhole onto the bottom one.

      Astrolabes can be used in timekeeping, surveying, geography, and astronomy all from the comfort of land but the mariner's version like this one had to be heavier.  It also had holes in it to make it easier for the wind to pass through. 

      In 1588 Philip II of Spain sailed to England with a fleet of warships called an ‘armada’ to remove Queen Elizabeth I from the throne. He didn’t succeed and many of his ships were lost to storms. This astrolabe was found in 1845 in Valentia on the Irish coast, close to where three of Philip’s ships were wrecked.

      Things to think about:
      • Why do you think mariner astrolabes were this shape with holes in and heavier than ones used on land?
      • Why do you think it was so important that mariners had an accurate way of knowing their location?
      • Have you ever found anything from another time or place washed up on a beach or the river’s edge?
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