| When
a sailor departs port and loses sight of land,
he must have some method of determining 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 view the Pole or North
Star. This star does not change
its position by the hour and it remains constant
in the north. The farther north the sailor
traveled, the higher the Pole Star appeared
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 were
accustomed to using different stars to determine
direction. |
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Determining
latitude, the distance from north to south
was measured by utilizing the Pole Star.
Measuring the altitude 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 assist in determining 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 indicate 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.
Determining longitude,
the distance from east to west, was problematic.
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 factor together
variables of 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.
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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
determined speed. This information combined with
the known direction of the compass would determine
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.
Portuguese
explorers | Portuguese
Ships | Navigation
Methods | 15th-century
maps & charts |
