When people look at a map, they usually have no idea of which projection cartographers used to construct that map. However, almost everyone has heard of or seen the most frequently used projection-the Mercator-developed by Gerardus Mercator in 1569. It was the map projection used by European navigators in their voyages of exploration and empire building in the years that followed. In his projection Mercator drew the lines of latitude and longitude as straight lines that crossed each other at right angles so that navigators could draw straight lines to plot their courses.

This meant that the distance between parallels, or lines of latitude, increased as latitude increased. If Mercator had drawn the parallels with even spacing, as they are shown on a globe, straight lines plotted on them would not represent true compass bearings. The actual mathematical computations that Mercator used to develop his projection were complex, but the idea behind his projection was quite simple.

Picture a spherical balloon inside a hollow cylinder. Then, imagine that the balloon has lines of latitude and longitude marked on it and that the line marking the equator just touches the wall of the cylinder. Now imagine blowing up the balloon. As the balloon stretches, the lines of longitude lie as straight lines along the walls of the cylinder and the lines of latitude are stretched apart in proportion.

As a result, the farther away from the equator and towards the poles you look, the greater the distortion in scale. Also, although land shapes appear correct, the areas of the landmasses are greatly distorted. Nevertheless, for every point on the map, the angles shown are uniform in every direction. This allows a navigator to plot a straight-line course, because a line connecting any two points follows a single compass direction.

The Mercator projection is a valuable navigational tool. It is convenient to use because the shapes of the continents are correct and are easily recognized. But as a result of frequently seeing the Mercator projection, many people have no idea of the proper size of the continents in relation to one another.