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.
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
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.