Why the Mercator map stretches the north
The Mercator projection turns a round Earth into a flat map by imagining the globe wrapped in a cylinder. To preserve local angles and shapes, it stretches the map more and more as it moves away from the equator. That stretching affects area: a patch of land near the top of the map occupies far more space on paper than an equally large patch near the equator.
The poles themselves cannot appear on a Mercator map. Reaching them would require stretching the map without limit, so the projection cuts off before then. The result is useful for navigation, but poor for comparing the size of regions at different latitudes.
The navigational tradeoff
The projection’s practical advantage is that a route following a constant compass bearing appears as a straight line. Such a route is called a rhumb line. A navigator could plot that line on a chart and steer a steady heading, an especially useful feature before modern navigation tools.
A straight line on a Mercator map is not necessarily the shortest route between two places. The shortest path over the globe usually curves on this kind of chart, so ships seeking to save distance may need to adjust their course rather than simply follow a plotted straight line.
Read the map with latitude in mind
Mercator maps remain handy for showing coastlines and local directions, but their visual areas can mislead. The farther north or south a place lies, the more its size is exaggerated. For comparing land area, an equal-area map projection is a better choice—even though it must distort some other feature, such as shape. A map is not a neutral window onto the world: its projection determines what becomes easiest to see.
