A chart for sailors

In 1569 Gerardus Mercator published a world map "corrected for the use of sailors". Its key idea: a line of constant compass bearing is a straight line, so a navigator can rule a course and follow it. Keeping every angle true forces the map to stretch the world more and more toward the poles.

The stretch is the price of the property, not a flaw. Scale grows with the secant of the latitude and area with its square: at 60°, twice the stretch and four times the area. Greenland, near 72° north, is drawn about ten times too large; DR Congo, on the equator, is barely touched. Here they look alike. In truth DR Congo is the larger.

For three centuries the chart stayed on ships. Once chronometers solved longitude in the 1800s it moved to classroom walls and atlases, a job it was never designed for.