Wikipedia: Mercator projection

[Gerardus Mercator]? developed the Mercator projection in 1569 as a navigation tool. Like all map projections attempting to fit a curved surface onto a flat sheet, there are distortions - the Mercator projection stretches East-West distances by an increasing amount as the distance from the equator increases. The end case is the poles, where the two geographical points have become lines at the top and bottom of the map. Though the map distorts distances and areas, with the distortions increasing towards the poles, it has the useful property to navigators that it preserves directions. A straight line on the map between two points actually does represent the direction you would need to sail between those points, although, because of the length distortions, it is not very useful for determining the distance between two points. At the time the projection was invented, there were no accurate ways of determining the distance sailed anyway, so this was not a problem.