The Sidereal Compass

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.