Cartography Word of the Day: Great Circle

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Cartography Word of the Day: Great Circle

This term goes back to navigation and ways to find one’s way on a sphere–the globe of our world.

Recall from the discussion of loxodromes or rhumb lines that they are lines of constant direction. On Mercator’s projection these manifest as clearly followable straight lines, but it will be seen that, if plotted on the 3D solid surface of the globe, that these are curves. They are the shortest lines on the flat plot but no so on the globe.

Now, take a plane and slice the globe in two, making sure that the plane also goes through the center of the sphere. The path described by the intersection of the plane and the sphere connects any two points on that path by the actual physical shortest distance. This path is a great circle. An obvious example of a great circle on a globe would be the Equator or any opposite pair of longitude lines (such as 0 degrees-180 degrees).

The difference can be made evident with any globe. Taking any two points plot the rhumb between them. On the globe the rhumb will form a curve. Now, take a string, and affix each end to the end of the plotted rhumb. The difference between the two should become immediately apparent; the string will be a straight course whilst the rhumb will “bow” outward from it.

Conversely, plotted on the Mercator projection, the rhumbs will appear as straight lines whilst the great circles will appear to “bow” away from the rhumbs.