gnomonic chart

gnomonic chart, a chart which is very useful in great circle sailing based on the gnomonic projection. This is a perspective projection in which part of a spherical surface is projected from the centre of the sphere onto a plane surface tangential to the sphere's surface. The principal property of this projection is that great circle arcs are projected as straight lines.

In order to draw a great circle on a Mercator chart—the projection being a relatively complex curve always concave to the equator—the route is first drawn on a gnomonic chart by connecting the plotted positions of the places of departure and destination with a straight line. Positions of a series of points on this line are taken from the gnomonic chart and marked on the Mercator chart. A fair curve is then drawn through these points, which is the required projection of the great circle route on the Mercator chart.

The gnomonic chart became popular with the publication by Hugh Godfray in 1858 of two polar gnomonic charts covering the greater part of the world, one for the northern and the other for the southern hemisphere. Although it was generally believed that Godfray was the original inventor of this method of great circle sailing, it is interesting to note that a complete explanation of the construction of a polar gnomonic chart, with a detailed example of a great circle route from the Lizard to the Bermudas, appeared in Samuel Sturmey's Mariners' Mirror, of 1669.