If you want to see the exact formulae for the calculations, see mapfunc.js. Look for the functions "calcdist", "fuel", "calchm", and "context".
What is a "Great circle?"
This is the shortest route between two points on the surface of the earth, and lies on a plane passing through the earth's centre.
Flights usually lie along great circle routes, although national airspace restrictions, jet streams, etc., may also influence the route.
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If you like this interactive tool, perhaps you would like to try the
interactive Java Climate Model
linking individual emissions to global temperature rise.