Mercator cut Earth into slices

To help marine navigators, Mercator decided to make a map, in which compass directions are always straight lines. The idea is simple, but brilliant.

The challenge that Mercator gave himself was to invent a new map projection, by which both parallels of latitude and longitude are straight, i.e. vertical and horizontal lines respectively.

His revolutionizing world map was completed in 1569, but seafarers were not ready to use it. Not until navigators got an accurate clock in the 1700s, that could help them determine the degree of longitude, and they learned the difference between geographical and magnetic north, they realized the quality of Mercator's maps.

The weakness of his projection is that the distances do not match. Close to the poles, there is much distortion, and at 70+ degrees S/N, the map cannot be used in practice. However, only a few % of the world’s area are located there, and shipping traffic in the polar regions is minimal, so in spite of the problem, his projection is often used for nautical charts even today.