The solar constant Ics is the average energy radiated by the Sun per time unit on a unitary surface situated outside the Earth’s atmosphere and perpendicular to the Sun’s rays. It measures 1367 W/m2 . Considering the atmospheric phenomenon of absorption and diffusion and the Sun’s inclination above the horizon, on the Earth’s soil the solar constant reaches a maximum of 1000 W/m2 (radiation on land, at midday, during a clear sky day) [1, 4].

The total power radiated by the Sun can be calculated as follows:

P = 4kRJIcs = 4n(150 • 109)21367 = 3.8 • 1026 W (1)

where Rm is the average distance between the Earth and the Sun [2].

The Earth intercepts only 1.73 ■ 1017 W of that power. Owing to nuclear reac­tions, a mass of 4.27 ■ 109 kg can be destroyed in a second; thus, nearly 0.0067% of the solar mass will be lost in a billion years [1].

Once we know the power emitted by the Sun, it is easy to calculate the heat produced internally per unit of solar volume:

q = P/(4/ 3)nR3 = 3.8 • 1026 /(4/ 3)n(7.25 • 108 )3 = 0.24W/ m3 (2)

where R (= 7.25 ■ 108 m) is the solar ray.

The quantity calculated above is a particularly low value considering that, for example, the human body’s heat production per unit volume is roughly 1400 W/m3 [2].