# Do Great Circles Wiggle?

October 28, 2012 pm31 9:35 pm

Cool applet takes two cities and draws the great circle between them. You plug in airport codes, the app does the rest. Here’s JFK – Narita (Tokyo)

But look what happens when we go from Buenos Aires to Istanbul:

Help me out folks. Is that a little bit of an “S” curve? Why? And is there a city pair that gives a more obvious “S”?

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The map projection stretches the high latitudes more than low latitudes. Try Christchurch (CHC) to Helsinki (HEL). The sheer size of Russia (and Antarctica!) is an obvious indication of the distortion.

Right, it’s distortion from fitting a round surface onto a plane. The point of inflection should be right on the equator.

Thanks and thanks. I was guessing more like JBL, but something about your comment, Hao. Go on, try Johannesburg to Anchorage… (JOH-ANC)

Hah! Due north and over the pole, and it doesn’t know how to cope :)

Sail manufacturers make sails with depth and curvature by cutting panels out of cloth and curving the seams. Deconstructing a globe is the reverse process. The illustration does not do this, so the line gets curved. A Mercator projection would reveal a broken, but straight line ( I suppose ).

No, this almost certainly is a Mercator projection. Mercator represents lines of constant course (aka rhumb lines) as straight, but great circles (= geodesics, = shortest paths) are not rhumb lines and so look curved on a Mercator projection, as seen here.