A method of navigating a ship along the shortest navigable distance between the point of departure and the point of arrival. The shortest distance between any two points on a sphere is the circumference of the circle which joins them and whose centre is at the centre of the sphere. In terms of the earth, this is a great circle, and if it were possible for a ship to sail along the great circle connecting its point of departure with its point of arrival, it would sail the shortest distance between the two. But unless both these points lie on the equator, which is of course a great circle, and along which it can steer a steady course due east or west, the ship cannot do this unless it sails a continuing curve, permanently altering course to keep itself on the great circle.
The theory of great circle sailing has long been known and understood and is described in many early books on navigation. But it was of little use to a ship dependent on the wind to get from one place to another since it was impossible for such a vessel to stick to a predetermined course. However, when steam propulsion was introduced a ship could steer a course irrespective of the wind, so the economics of great circle sailing in terms of fuel consumption and voyage time were quickly appreciated.
One of the properties of a gnomonic chart is that a great circle appears on it as a straight line. To plot a great circle track on a Mercator chart the navigator joins his point of departure and his point of arrival by a straight line drawn on a gnomonic chart, and then transfers a series of positions on this straight line—read off in latitude and longitude—to his Mercator chart. These positions will then lie on a curve, which he can sketch in although he will not be able to steer along it. He approximates to the curve by joining up successive points on the curve with straight courses which are in effect chords of the great circle and which appear as straight lines on the Mercator chart. He will thus keep his ship as close as possible to the great circle, altering course as each position on the curve is reached. In this way he will be sailing the shortest reasonable approximation to the great circle, thus saving fuel and time. For illus. see mercator projection.
Subjects: Maritime History.