The nature of the Great Circle route

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Great circle routes are used by ships and aircraft where currents and winds are not a significant factor. For aircraft traveling westerly between continents in the northern hemisphere these paths will extend northward near or into the Arctic region, while easterly flights will often fly a more southerly track to take advantage of the jet stream.

A great circle is a circle on the surface of a sphere that has the same circumference as the sphere, dividing the sphere into two equal hemispheres, says Wikipedia.

A great circle is the largest circle that can be drawn on a given sphere. The diameter of a great circle is same as the diameter of the sphere.

When long distance aviation or nautical routes are drawn on a flat map (for instance, the Mercator projection), they often look curved. This is because they lie on great circles. A route that would look like a straight line on the map would actually be longer.

On the Earth, the meridians are on great circles, and the equator is a great circle. Other lines of latitude are not great circles, because they are smaller than the equator; their centers are not at the center of the Earth -- they are small circles instead.