Mercator's 1569 projection made straight-line navigation and won the sea chart

In 1569 Gerardus Mercator published an 18-sheet world map titled 'A new and augmented description of Earth corrected for the use of sailors'. Its innovation was to spread the parallels ever wider toward the poles so that lines of constant compass bearing — rhumb lines — appear as straight lines on the paper.

The benefit for navigation was immediate in principle: sailors could draw a straight line between two ports and read the course instead of repeatedly correcting bearings. Mercator's contemporaries did not immediately recognize the value, and the academic review of the map notes it was Edward Wright (1599) and Henry Bond (1645) who supplied the mathematics that made the projection reproducible and standard for sea charts.

The design choice is why it endured: it preserves angles, so a bearing measured on the chart matches the compass. Even after the projection drew criticism for inflating high-latitude landmasses, it remained the standard for nautical charts, and the review notes electronic charts still rely on the projection Mercator invented.

Straight lines on the map = constant compass course

Straight rhumb lines let navigators plot a course with a ruler

It preserves angles, so chart bearings match the compass

Wright and Bond supplied the math that made it reproducible

Sea charts — and later electronic charts — inherited the projection

A design that removes one painful step — plotting a course — can conquer a whole profession, even when the inventor cannot explain how it works.

Mercator became the standard sea-chart projection from the 18th century on; its size distortion later pushed general world maps toward other projections, but marine charts never left it, and Web Mercator revived it as the basis of online maps.