Great Circle

Have you ever heard of a "great circle?" How about a "big circle" or a "big loop." What is the difference between the three terms? All three terms are used in navigation to refer to a "route" for a long-range cruise. A "route" is a series of "courses," usually between a departure point and a destination point.

A "great circle" is the shortest distance between two points on a sphere, such as the planet Earth. A "great circle" is defined in several ways. One definition calls it the largest circle that can be placed around a sphere. In the case of the Earth, the Equator and the meridians of longitude are the "primary" great circles.

Another definition of a "great circle" is that it divides a sphere into two "equal" parts. A different way of expressing that is to say that a "great circle," except for the Equator, intersects the Equator at two points that are 180 degrees apart. Lastly, you can define a "great circle" as a circle around a sphere that is situated such that it is half north of the Equator and half south of the Equator.

To put this concept into simpler terms, lets try an experiment. Mark two points on a beach ball, just as you would on the globe to show your departure and destination points. Wrap a large rubber band around the beach ball. Position the rubber band so that passes through the two points and so that it also divides the beach ball into two equal halves. That's a "great circle."

Now that your rubber band is in the right position to represent a "great circle," go to a place between your two points and slide the rubber band over a few inches. Did the distance along the rubber band between the points get a little longer? Of course it did.

Lets look at an example of why a "great circle" route is used. Imagine a departure point on the U.S. east coast and a destination point on Europe's west coast, both at the same latitude. Your first inclination might be to simply follow the latitude line eastward. That approach would yield a True course of ninety degrees.

However, if you stretched a string between those two points on the globe, the entire course line between the points would be above the latitude line, except at the two points. The string represents part of a "great circle" while the latitude line does not. Unlike the string (course line), the latitude line is the same distance form the North Pole throughout its length.

Why do I bring this up? Usually, when I hear the term "great circle" being used in conversation, it does not conform what we now know is a "great circle." In most cases, "great circle" is confused with the terms "big circle" or "big loop," which is a boat trip.

The boat trip includes the U.S. east coast, Florida's west coast, the northern Gulf coast, the rivers north to the Great Lakes and back to the east coast via the St. Lawrence Seaway or the Erie Canal. In short, the boat trip is not a great circle, it's a "big circle" or a "big loop!"

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