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Mariner's Astrolabe
© Museum of the History of Science, Oxford
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Normally, astrolabes are sophisticated, beautiful and valuable objects, and were made for very rich clients, including princes and sultans. They would usually be coated with gold and often decorated with silver. Owners of astrolabes would not have understood everything they could do because they were so complicated. Instead, astrolabes were treasured as works of art, kept under lock and key in display cases and brought out only on special occasions.

This astrolabe however is very different from most other astrolabes. It is a special type of astrolabe called a 'mariner's astrolabe' because it was used at sea by sailors. It is very plain compared to other astrolabes: there is no gold, no complex scales, and no fine decoration. It is also very strong because it had to withstand the difficult conditions on board a ship.

Unlike other astrolabes, a mariner's astrolabe can only be used to do one thing: to measure the height of the sun or of a star above the horizon. Mariners needed to measure the height of the sun or stars in the sky because it helped them to work out their position. Knowing the height of the sun at noon or of the pole star at night told them their latitude. This was their position north or south on the earth. Unfortunately, it only half helped them determine their position exactly, because they also needed to know their longitude. This was their position east or west on the earth. Longitude was much harder to work out than latitude, because sailors needed very accurate clocks that could keep good time even when shaken around on board a ship. In fact, it was impossible for sailors to work out their longitude accurately until around 1750, when John Harrison invented his famous sea clocks.

This mariner's astrolabe is made of a metal circle with a scale around its edge and a rule with sights (vanes with tiny holes in) at each end. To measure the height of the sun or the stars a mariner would line up the rule with the sun or the stars, either by looking thorough the sights directly, in the case of the stars, or by making sunlight passing through one sight and fall on the other, in the case of the sun. The position of the rule on the scale could then be read off and converted to a latitude reading.

There are two more special design features on this astrolabe that show it was used at sea. It has windows cut out of its body and it is weighted at the bottom with more metal. Both these things would have helped mariners when they were using it to make observations. The cut out portions meant that it would not blow around in the wind as much as a solid astrolabe. The weight at the bottom made it more likely to keep hanging vertically even when the sea was quite rough.

Mariner's astrolabes were used up until the 17th century when they were replaced by sextants and octants. Mariner's astrolabes are actually quite valuable, because they are so rare. This is both because they were always used by their owners (unlike other astrolabes) so they would have had a good chance of being broken and thrown away and because many of them went to the bottom of the sea when the ships that they were being used on sank. Sometimes, after big storms, mariner's astrolabes are washed up on beaches near shipwrecks. This particular mariner's astrolabe was brought up from the bottom of the sea at Veracruz in the Gulf of Mexico and probably comes from a 16th century Spanish warship.

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