Having some time ago been requested to design a sundial for the Hydrographic Institute of Genoa (Fig. 1) I went through various books and found that the methods indicated for drawing sundials were all based on graphical constructions. Although quite interesting these methods have the inconvenience of all graphical solutions, that they require constructions which, besides being complicated, turn out to be rather inaccurate, especially when lines cross at small angles, as inevitably happens outside a limited central area of the drawing. To avoid this I worked out a numerical system, related to the familiar methods of astro-navigation, which, by means of simple formulae, gives the coordinates of the points necessary to design a sundial. As the bibliography I examined is, of course, limited, I am not in a position to state whether this method of calculation is new or not; I describe it here hoping it may interest people who have sometimes wondered how sundials are made.