Let C be the centre of the earth, A and B the stations of two observers, one in the northern, the other in the southern hemisphere, and M the moon. The lines C A Z and C B Z′ indicate the direction of the zenith at A and B respectively. Subtracting the measured angles at A and B each from 180° gives the inside angles at those points. The angle at C is equal to the sum of the latitudes of A and B since they are on opposite sides of the equator. With three angles known, the fourth, at M, is found by simply subtracting their sum from 360°. ]