The distance R is therefore the radius of the sphere and the X, Y and Z coordinates may be defined in terms of the latitude 0 and longitude (b as follows: X Cos0 Cos Y Cos0 Sinqb Z Sin0 where, X -l-Y -l-Z =1 It may now be seen that, in order to algebraically express a straight line, that is, a great circle, the polar of the line may be used, where the polar is a point which is a quadrant away from