is the angle measured upwards from the horizontal ( - ) plane through the centre of the sphere - the point's 'latitude', as it were.
is the 'longitude' of the point. This is the angle, measured anticlockwise around the 'equator', between two planes: the vertical plane containing the great circle* through the point and the - plane.
A neighbouring point is chosen, defining an area of the sphere's surface which, for small increases in and , is approximately rectangular.
Simple trigonometry will show that the radius of the 'circle of latitude' is . So an angle of at its centre is subtended by an arc of length on its circumference.
The angle is subtended at the centre of the great circle by an arc of length , since this circle has unit radius.
Hence the dimensions of the approximate rectangle are . Its area is therefore given byGrandad
* A great circle drawn on the surface of a sphere is one that has its centre at the centre of the sphere; its radius is equal to the radius of the sphere.