- Steve Phelps
In Euclidean geometry, the ORTHOCENTER is the intersection of the three altitudes of a triangle. In other words, the altitudes are concurrent at the orthocenter. In the sketch below, the RED great circles are the altitudes of the BLUE triangle, and the RED point is the point of concurrency of the altitudes; the spherical orthocenter. Drag the blue points around, but do so slowly! Things change quickly on a sphere!