Spherical trigonometry has fallen somewhat out of fashion and so it was a salutary experience to be consulted recently by a would-be circumnavigator of the world.

A great circle route can be analysed graphically by first using a gnomonic chart. Such a chart is obtained by projection from the centre of the earth on to any plane. The distance of the plane from the centre determines the scale of the chart. On a gnomonic chart every great circle appears as a straight line and every straight line on the chart represents a great circle. Of course chart can only represent one hemisphere and even then the whole hemisphere would need an infinite chart.