Spherical right triangles and Napier's Rules [url=http://upload.wikimedia.org/wikipedia/commons/thumb/0/07/Spherical_trigonometry_Napier_right-angled.svg/443px-Spherical_trigonometry_Napier_right-angled.svg.png]http://upload.wikimedia.org/wikipedia/commons/thumb/0/07/Spherical_trigonometry_Napier_right-angled.svg/443px-Spherical_trigonometry_Napier_right-angled.svg.png[/url]