In 1569 the Flemish cartographer and mathematician Gerardus Mercator published a new world map under the title “New and more complete representation of the terrestrial globe properly adapted for use in navigation.” The title of the map points to Mercator’s main claim for its usefulness, which he expounded upon in the map’s legends. Mercator presented his map as not only an accurate representation of the known world, but also as a particularly useful map for the purposes of navigation. As described in the third legend, Mercator aimed to maintain conformity to the shape of land masses even towards the poles and to have straight lines on the map accurately represent directionality. To achieve his goals Mercator used a projection in which lines of longitude and latitude were made perpendicular at all values by increasing the distance between degrees of latitude as they reach the pole.1 Mercator’s projection had the benefit that straight lines drawn on the map are rhumb lines, lines of constant bearing that pass every degree of longitude at the same angle. Theoretically this simplified oceanic navigation; a ship captain could draw a straight line from one port to another, calculate the bearing, and maintain that bearing along the voyage. However, 16th-century navigators used magnetic courses and not longitude and latitude values as Mercator’s map assumed.2 An accurate means to measure longitude at sea was only discovered in the second half of the 18th century with the development of the sextant and later the marine chronometer.3
World Map by Gerardus Mercator, 1569
The Mercator projection was designed with certain uses in mind. Mercator’s emphasis on perpendicular lines of longitude and latitude and the equivalence of straight lines and rhumb lines were meant to simplify navigation and have recently proved useful for online mapping services. However, the stretching of latitudes towards the poles distorts the size of land masses, making those closer to the poles appear larger than those near the equator. The stress on rhumb lines in Mercator’s map also highlights the difference between lines of constant bearing (rhumb or loxodrome lines) and the shortest distance between two points (great circles). Due to Earth’s ellipsoidal nature, the shortest distance between two points is not necessarily a straight line. For instance, to fly from Los Angeles to Amsterdam, one would not want to fly in a straight line of constant bearing at 78 degrees. Instead, you would want to make an arc to the north to take advantage of the ellipsoidal shape of the Earth. By flying along the great circle from Los Angeles to Amsterdam one would travel 1120 kilometers less than flying along the rhumb line.
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