In order to calculate the material value of a shadow we have to engage in some physics:
Scientific calculations indicate that the sun emanates radiation that amounts to approximately 61,000 kW per square meter of its surface area. Just to give a comparison: 61,000 kW equals the output of a large generator in a power plant. Converted into the old unit of horsepower (multiplied by a factor of 1.36), this would be 83,000 PS – and this is per square meter of the sun’s surface area!
At a distance of about 150 million kilometres from the sun, our enormous source of energy, travels the tiny earth that receives this energy. At the periphery of its atmosphere, the earth absorbs 1.36 kW/m². With this amount of energy a small electric heater could be run, and an electric company would charge us several cents per hour for this. Astronomers call this value the “solar constant”.
Now the idea of an electric heater on each square meter of heaven above us might be alarming, but fortunately two-thirds of this energy are absorbed by the atmosphere. Only 420 W arrive on each square meter of the earth’s surface area. And we also have to take into account that in most regions of the earth the sun does not shine down vertically or does so for short periods of time only; most of the time its rays are coming in at an angle.
So let’s imagine a beautiful morning in ancient Greece with a cloudless sky. The sun is standing about 15° above the horizon. For our calculations, we take an area of one square meter that is situated perpendicular to the sun. So the sun’s energy of 420 W is coming down on this area. This corresponds to the power consumption of four bright light bulbs. In one hour, they consume about 420 Wh. Since 1 kWh is supplied by the electric company for a few cents, the shadow generated by this energy would also be worth a few cents only – provided we could transform the sun’s energy into a usable form of energy.