Definition
Every great circle has two vertices, in opposite hemispheres and 180 degrees of longitude apart. At the vertex the track runs due east or due west, so the course is exactly 090 or 270 degrees true. Vertex latitude follows from Napier's rules as cos(latitude of vertex) equals cos(departure latitude) multiplied by sin(initial course). Checking the vertex before departure shows immediately whether ice or storm limits will be breached.
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