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The Sidereal Compass

 

Basic to the entire navigational system is the "star structure," as the navigators call it. Observed near the equator, the stars appear to rotate around the earth on a north-south axis. Some rise and set farther to the north and some farther to the south, and they do so in succession at different times.

The "star structure" divides the great circle of the horizon into 32 points where the stars (other than Polaris) for which the points are named are observed to rise and set. These 32 points form a sidereal (star) compass that provides the system of reference for organizing all directional information about winds, currents, ocean swells, and the relative positions of islands, shoals, reefs, and other seamarks. The diametrically opposite points of this compass are seen as connecting in straight lines through a central point. A navigator thinks of himself or of any place from which he is determining directions as at this central point. Thus, whatever compass point he faces, there is a reciprocal point at his back.