After learning the compass points, a student of navigation is taught all their reciprocals. The reciprocal of the rising of Vega in the northeast, for example, is the setting of Antares in the southwest. For every reciprocal pair a student must then learn what other reciprocal pair lies at a right angle to it. A compass star on the beam can thus serve as a guide when the star on which one's course is set is not visible.
A feel for the angular distances from one to another of
all the compass points enables a navigator to maintain his
course at the appropriate angle to any visible compass star
or any other visible star known to rise and set at about the
same place as a compass star. To be able to use the stars
this way is essential, the navigator shifting from one to
another as they rise and set in the course of a night. When
no stars are visible, as in daylight or in overcast at
night, a navigator still orients himself with reference to
the star compass. Knowing the compass direction of wind and
ocean swell, he can keep track of where he is headed.
World Cultures: Ancient and Modern