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Sand glass

     Telling time at sea was an involved and inaccurate procedure. First, local noon had to be determined by the use of a quadrant, astrolabe, or cross staff. Then a sand glass was used to keep track of the passing time. As the ship traveled east or west the local time changed, so the sand glass could only tell the time relative to the last noon sighting.

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     The nocturnal was first described in 1272 as an instrument for determining the latitude and for telling time at night. Another name for the  nocturnal was nocturlabe.

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     It was not until the 18th century that clocks were designed to keep time accurately at sea for long periods of time. When they were time also became a means for determining the longitude.