solar constant
A theoretical numerical value resulting from calculating the amount of solar radiation that, per unit of time, strikes a surface of one square meter perpendicular to the radiant flux and located at the outer limit of the atmosphere, with the Earth at its average distance from the sun (149.6 million kilometers). Satellite measurements confirm that solar irradiance varies slightly over time, so the aforementioned "constant" is in fact an average value, equal to 1367 W/m2. In most Meteorology texts it is identified with the letter S.