Mercator Projection

In the year 1512, Gerardus Mercator was born. He would eventually have a great and lasting impact on navigation. Young Mercator grew up to become a geographer of worldwide fame.

And why does the name "Mercator" have a familiar ring to most of us mariners? If you look in the title block of your navigation charts, you'll find that with very few exceptions, the name "Mercator" will be printed there.

Gerardus Mercator produced maps and globes. He was particularly interested in the differences between spherical (globes) and flat (maps) representations of the same geography. Did you have ever cut a globe in half and try to flatten it out for use as a map? If so, you have something in common with Mercator.

Just as it is impossible to flatten an orange peel out without tearing or distorting it, the "globe to map" transition was also difficult. In the 1500's, Mercator endeavored to produce a "flat map" of the globe, but without the usual distortion. 

Meridians of longitude (vertical lines), parallels of latitude (horizontal lines), and land masses, all wrap around a spherically shaped globe. How does one flatten those things out and end up with an accurate map? 

Mercator pondered this problem for over half of his life before finding a solution. In 1569, he decided to abandon the theories of all other chart makers. It was in that year that he published his eighteen page world map, which employed his revolutionary theory.

What he ultimately developed is what we refer to today as the "Mercator Projection." This new method of converting the spherical graphics of the globe to the flat graphics of a chart was revolutionary.

On the globe, the meridians of longitude are spaced furthest apart at the equator and all terminate together at the poles. At no point on the globe is there a right angle between a meridian of longitude and a parallel of latitude. 

Imagine starting with a globe that has the meridians of longitude, parallels of latitude and all land masses drawn on it. Next, roll a sheet of paper into a cylinder equal to the diameter of the globe. Put a light source into the center of the globe and slip the globe into the paper cylinder. What was drawn on the globe will be "projected" onto the paper. 

Now you know how Mercator was able to "project" a spherical image onto a flat surface. The result is that the meridians of longitude and the parallels of latitude end up intersecting each other at right angles. While there is still a minimal amount of distortion, it is negligible.

On a Mercator projection, a "rhumb line" course appears as a straight line. This has been a great advantage to mariners over the "spherical" alternative. 

Given the "right angle" aspect of the latitude and longitude lines, the North and South Poles cannot be shown by Mercator projection. The poles are the two points at which the longitude lines terminate. In a Mercator projection, the longitude lines never converge. 

While Mercator's achievement was monumental, it would be almost seventy years before it would be in widespread use. In the middle of it all, Mercator narrowly escaped religious prosecution while being tried for heresy in 1544. Who ever said that scientific study was easy?

Until next time, I wish you clear skies, fair winds, and calm seas!