In Italian, the term “meridiana” has come to assume, with the passage of time, the generic meaning of "sundial", whether it be flat or solid, vertical or horizontal, concave or convex. Deriving from the Latin meridianus (the celestial circle passing through the cardinal points of north and south), and from meridies (midday), the term “meridian” properly indicates the line of intersection between the local meridian and the horizon plane. In horizontal sundials, it is thus the line that extends in the north-south direction, along which the gnomon's shadow falls at midday, throughout the year. By measuring the length of this shadow, day after day, in relation to the height of the gnomon, some fundamental astronomical parameters can be determined, such as the dates of the spring and autumn equinoxes and the summer and winter solstices. Over the course of the centuries, sundials of this type took on notable dimensions and a primary religious function, being used to correctly determine the dates of the spring equinox and the Christian feast days, first among them Easter.

Drawing a sundial

The dial of a sundial is no other than the gnomonic projection of the celestial sphere onto a flat, concave or convex surface. The term "gnomonic projection", refers to the projection of the celestial circles generated by a projection point located at the center of the sphere. The design of the lines differs in relation to the geometry of the projection surface and its orientation.