Sundial

Museo della Civiltà Contadina del

Friuli Imperiale (Sundials courtyard)    

Photo n° 4

The intensity of a solar radiation that hits a unitary surface on the Earth, is called solar hemispheric irradiation. It is measured in W/m², and it is obtained by adding a direct component coming straight from the sun, a diffuse one coming from all directions, and the one reflected by the surrounding ground.

It depends on the distance between the Sun and the Earth, on the height of the sun above the horizon, on the conditions of the atmosphere, and on the sunbeams incidence angle in respect to the line perpendicular to the surface.