The Flemish cartographer Gerardus Mercator published his famous world map in 1569, and for more than four centuries afterwards its design dominated the ways in which Europeans pictured their planet. Mercator's achievement was not to invent a new projection in isolation but to solve a specific practical problem. Sailors of his period wanted a map on which a line drawn at a constant compass bearing would appear as a straight line. Any mariner who could draw a straight line on such a chart could follow it with a compass and reach, eventually, the point at its end.

Earlier projections could not deliver this feature. On a globe, a course of constant compass bearing is not a straight line but a curve called a rhumb line. When the globe is flattened into a rectangle by any of the older methods, those rhumb lines become curves on paper as well, and a navigator drawing a straight line between two ports has no simple way of translating it into a sequence of compass headings. Mercator's trick was to stretch the vertical distance between lines of latitude in exactly the right amount, so that the east-west and north-south scales remained in step at every point.

The mathematical consequence was a map on which straight lines are rhumb lines, a property still indispensable in modern marine charts. The cost, however, was considerable distortion of area. Regions near the equator appear at roughly their correct relative size, but land masses near the poles are hugely exaggerated. On a Mercator map Greenland looks larger than South America, though it is in fact less than an eighth of its size, and Antarctica sprawls across the bottom of the page. Mercator himself acknowledged this trade-off in the map's Latin inscription, and urged readers not to use the map for comparing the areas of continents.

His warning was not always heeded. Successive editions of school atlases, printed cheaply and widely distributed, reproduced the projection without its original caveat, and generations of pupils in Europe and North America absorbed a subtly distorted mental image of the world. Educator Dr. Ines Cartwright, who has studied the images held by school-age children in several countries, argues that the Mercator projection taught a view in which temperate northern lands loomed larger than equatorial ones, an effect she calls a 'cartographic unconscious'.

Alternatives existed, but they were slow to gain traction. The Peters projection, promoted in the 1970s by the German historian Arno Peters, preserved area accurately and reduced the visual dominance of the north. Critics complained that its coastlines looked stretched and unnatural, and professional cartographers were divided about how much weight to give to honesty of area as against familiarity of shape. Other projections, such as the Winkel tripel used by National Geographic since 1998, attempt to compromise between shape, area and distance, accepting small errors in each rather than large errors in any.

For navigation, none of these alternatives has displaced Mercator's chart. GPS receivers at sea still display positions on a Mercator grid, because the straight-line property remains useful even when the original compass-based rationale has disappeared. For general geography, however, Mercator's map has been quietly retired in most modern atlases. Online mapping services, which need to display tiny city blocks as well as whole continents, use slightly modified versions of Mercator for their zoomed-in views and switch to other projections for global displays. Most users never notice the change.

Mercator's career was not confined to map-making. He taught mathematics, built globes, and spent time in prison on suspicion of heresy during a period of religious turbulence in the Low Countries. His 1569 world map, however, is the work for which he is still remembered, and it offers a minor case study in the power of a single design decision. By treating navigation as the priority and area as secondary, Mercator chose the problem his projection would solve and, in doing so, shaped how centuries of readers imagined the rest of the world.