Are Directions Preserved?
Whenever one plans moving from a point on Earth to another far enough to be out of sight, or on the sea or above the clouds where no paths are marked, some very related questions arise: • what’s the initial heading, i.e., direction to turn, for establishing a path with a “straight” course? • if no course corrections are taken, which points will be visited along the way? Directions on a surface are stated as a bearing, i.e., the angle from a reference line. On Earth, this line is the meridian through the current location, and the bearing is usually measured in degrees, from 0° northwards and increasing clockwise to 360° northwards again. The direction of the current meridian can be obtained from a magnetic compass, whose needle is always aligned with the North-South direction, but one must account for magnetic declination, the deviation of magnetic North from true geographic North. Magnetic declination is not uniform throughout the world and slowly changes with time (in millions of year
