Mercator's projection

Gerardus Mercator published a world map on which a line of constant compass direction is a straight line, which helped sailors. It exaggerates the size of lands near the poles.

Context

Gerardus Mercator, a Flemish cartographer, issued his large world map in 1569 with the title in Latin that translates roughly as a new and improved description of the world for use in navigation. On it, lines of longitude are parallel and spacing between lines of latitude increases toward the poles in a way that keeps angles correct.

This property is called conformality. A ship following a constant compass bearing, known as a rhumb line, appears as a straight line, so navigators could set a course with a ruler and protractor. Mercator did not publish the mathematical formula; the English mathematician Edward Wright later explained it in 1599.

The cost is that scale grows with distance from the equator, so Greenland looks about as large as Africa when Africa is roughly 14 times bigger. The projection cannot show the poles at all. Modern maps for navigation still use variants, including the Web Mercator used by many online mapping services, while atlases often prefer other projections to show areas fairly.

Mercator also popularized the word atlas for a bound collection of maps, with a volume published after his death.