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When a sailor leaves port and loses sight of land, he must have some method of working out his direction. Early captains relied on nature to provide the answers. We all know the sun rises in the east and sets in the west. A rising sun on the left-hand side of the ship, for example, meant it was sailing south.
At night, the pilot could look for the Pole or North Star (as long as he is in the Northern Hemisphere). This star does not change its position by the hour and it remains in the same place in the north of the sky. The farther north the sailor traveled, the higher the Pole Star appeared to be in the sky. The farther south he sailed, the lower the star appeared in the sky. When mariners reached the equator, the star disappeared. Navigators in the southern hemisphere had to use different stars to determine direction, like the Southern Cross. (back to top)
Working out
latitude, the distance from north to south, was done by
using the Pole Star. Measuring the height of the star from
the horizon and reading it in degrees was the same as the
degrees of latitude above the equator. The quadrant,
a quarter circle measuring 0 to 90 degrees marked around its
curved edge, was a common instrument to use in working out
latitude. Its straight edges had tiny holes or sights on
each end. A plumb line hung from the top. The navigator
lined up the sights on the Pole Star and the plumb line
would hang straight down over the curved area at a
particular point. This would show the height of the star in
degrees latitude.
Another way of determining latitude was with the use of the
astrolabe.
This was a simple wooden or brass disk with degrees marked around its
edge. It had a rotating arm with small holes at either end. The disk
would be hung vertically from a ring. The user could move the arm
until the sunlight shone through the hole at one end and fell on the
hole on the other end. The arm then would indicate the altitude by
the degrees marked around the edge of the disk.
The drawback for both the quadrant and
the astrolabe was the movement of the ship, which made it
difficult to make an accurate measurement. The
cross-staff,
invented in the sixteenth century, solved this
problem.
Working out longitude, the distance from east
to west, was more difficult. It is impossible to measure it without
an accurate timepiece. (The chronometer
was not invented until the eighteenth century.) For fifteenth-century
sailors, the only way to measure it was to put together information
about compass direction, speed, or dead
reckoning. The compass
was well known to Europeans in the fifteenth century. It had been
used in China and Arabia centuries before. Compasses of the fifteenth
century were made with an iron needle magnetized by a
lodestone
on a small piece of wood floating in a container of water. This was
eventually replaced by a brass canister where a magnetic needle swung
around an upright pin. The compass was not always accurate because
magnetic north is not the same as true north. (back
to top)
Another method of navigating open sea was the complicated process of dead reckoning. The pilot had to estimate the ship's speed with a chip log, which had a weighted wooden float attached to a line with knots in it. This line would be thrown from the stern. Time was measured with one-minute glasses. The number of knots pulled off the reel by the drifting log told you the speed. This information combined with the known direction of the compass would show the ship's progress along longitudinal lines. Time, distance, and direction were measured each time the ship changed tack due to wind direction. This zigzag plotting was calculated with a traverse board. Dead reckoning also included observations of the surroundings. Cloud formations, wave patterns and directions, birds, and floating debris were all taken into account.