[and] also, if is any other point on the sphere,
We shall add here another theorem, which has appeared nowhere else, as far as we know, and which can often be used with advantage.
Let be four points on the sphere, and the angle which and make at their point of intersection. [Then we have]
The proof is easily obtained in the following way. Let
we have then
Therefore
Since each of the two great circles goes out from in two opposite directions, two supplementary angles are formed at this point. But it is seen from our analysis that those branches must be chosen, which go in the same sense from toward and from toward
Instead of the angle we can take also the distance of the pole of the great circle from the pole of the great circle However, since every great circle has two poles, we see that we must join those about which the great circles run in the same sense from toward and from toward respectively.
The development of the special case, where one or both of the arcs and are we leave to the reader.
6) Another useful theorem is obtained from the following analysis. Let be three points upon the sphere and put