Great Circle

The arc of a great circle is the true course between two places, and the length of the track is the shortest distance between them. When a great circle track is drawn on a Mercator's chart it represents a curve, except on the equator and on north and south courses. A ship sailing on a great circle would be constantly changing the direction of her head, according to the curve represented on a Mercator's chart, while in reality she would sail in a direct line from place to place. This may be explained by stating that a Mercator's chart is distorted, and that which appears to be a straight line connecting two places is in reality a curve, and that the represented curve of a great circle is a straight line.