Mercator projection

Sailors needed straight lines that stayed straight. On a globe, the shortest path between two points curves—a great circle arc. But navigating by constantly changing compass bearings was impractical. Sailors wanted to set a course and hold it. In 1569, Gerardus Mercator created a map where such constant-bearing routes appeared as straight lines.

The mathematical trick was counterintuitive: stretch the map vertically more and more as latitude increases from the equator. This preserves angles at the cost of distorting areas. Greenland appears larger than Africa on a Mercator map, though Africa is actually 14 times bigger. But a navigator drawing a straight line between two ports could read the constant compass bearing directly from the map.

Mercator titled his 18-sheet world map "Nova et Aucta Orbis Terrae Descriptio ad Usum Navigantium Emendata"—A new and augmented description of Earth corrected for the use of sailors. The title announces the purpose: this was not a geographic reference but a navigational tool. The projection solved the problem that had plagued Portuguese and Spanish explorers: a course of constant compass bearing (called a rhumb line or loxodrome) did not correspond to a straight line on existing charts.

The mathematics required understanding that emerged from portolan charts—Mediterranean sailing maps that used compass bearings—and the globe projections pioneered by Mercator himself in 1541. The conceptual leap was realizing that preserving angles mattered more than preserving areas for navigation.

Adoption was slow at first. Edward Wright published the mathematical tables needed to construct the projection in 1599, making it reproducible. By the 18th century, Mercator projection had become the standard for nautical charts, a position it still holds.

The projection's success locked it in for purposes beyond navigation. Its familiarity made it the default for world maps generally, embedding its area distortions into popular geographic understanding. The very effectiveness of Mercator's navigational solution created path dependence that persists in classrooms and news graphics four centuries later.