documentation of the use of a mariner's compass in the Middle ages is
found in the writings of an Englishman, Alexander Neckham of St. Albans.
Neckham wrote in his treatise, De Utensilibus, of a needle carried on
shipboard, which, when balanced on a pivot and allowed to come to rest,
showed mariners their direction even when the stars were obscured.
The box compass was invented by Petrus Peregrinus.
It involved a pivoting needle revolving around a graduated disc similar
to an astrolabe. Around 1302, the priniciples of Peregrinus' box compass
and the wind rose (rosa ventorum) were combined, positioning the magnetic
needle to rotate on the center of the wind rose, where it still rests
today. The wind rose, consisting of eight winds, with subdivisions for
half-winds and quarter-winds, was fitted to a metal cap and one or more
magnetic needles. This basic mechanical design has changed very little
in over 600 years of navigation.
With the use of a compass, a Navigator could
follow a rhumb line on a sailing chart.
Prior to the use of a compass, sailors could not safely navigate the
open ocean accurately, so many of the charts made during these times
were Portolan charts, or coastlines plotted
for sailing from port to port. If a navigator set out from a known port
and measured the direction of sail using a compass, he could accurately
project where he would land in relation to the origin point and to the
north pole (originally, many navigators thought that a compass point
was attracted to the north star). This is the basic instrument navigators
used to determine direction of travel.