A geodesic crossing the equator with azimuth alpha_0 and passing through the point with latitude phi_1 and longitude lambda_1 and the point with latitude phi_2 and longitude lambda_2. Distance along geodesic denoted by 'sigma'.
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Derivative of <a class="external free" href="https://commons.wikimedia.org/wiki/File:Sphere_geodesic_4sigma.svg">https://commons.wikimedia.org/wiki/File:Sphere_geodesic_4sigma.svg</a> by Peter Mercator <a class="external free" href="https://en.wikipedia.org/wiki/User:Peter_Mercator/Draft_figures">https://en.wikipedia.org/wiki/User:Peter_Mercator/Draft_figures</a>