Mercator and his Projection
Gerhard Mercator was a 16th century map-maker and business man who met a market niche by publishing a more navigator-friendly chart. The age of exploration was just beginning and Mercator's charts spurred it along.
Mercator's solution to the spherical-globe to flat-paper problem was to use mathematics such that the lines of latitude and longitude were projected onto an imaginary cylinder wrapped around it. It was as if a source of light was at the center of the earth and cast the shadows of the parallels and meridians onto the cylinder. The cylinder, when opened up, became the flat chart.
In the Mercator projection, the meridians of longitude are straight and parallel to each other. The lines of latitude, while still parallel are no longer a constant distance apart: that distance grows as the lines near the poles.
Mercator's projection is popular with navigators because the expansion of both lines is proportional and the relationships of the angles remain the same. Sailors and modern-day boaters can navigate a straight-line course - following an unchanging compass course - from place to place. Because Mercator charts are accurate and easy to use for most coastal navigation applications, they've been popular for hundreds of years.