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The sea astrolabe (front view) was an adaptation of the astronomical type. It was much simplified, as the alidade and degree scale were really all that was required for the mariner's use. It was made much heavier to keep it vertical on the rolling platform of a ship, and cut-out to reduce disturbance in the wind. The sea astrolabe was introduced about 1460, but did not see general use until the beginning of the 16th century. Its use persisted until after 1670, particularly in the fleets of the Spanish and Portuguese, where it was in evidence early into the 18th century.
(Back view of the astrolabe showing the signs of the Zodiac)
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© 2008 USCG Lightship Sailors Association International Inc. Larry Ryan, President