1. The problem statement, all variables and given/known data I am try to understand how the value of 1370 watts per metre squared for the intensity of sunlight hitting the earth has been derived. Is there a way to see this mathematically? 2. Relevant equations I understand about how the intensity is related to Poyntings vector, by taking it's average over the wavelength (ie multiplying it by 0.5) but I don't see how this has anything to do with the radius of the orbit. 3. The attempt at a solution The equation for I is 0.5*c*ε_0 * E^2_0 where E_0 is the amplitude of the Electric part of the EM wave, and ε_0 is the permitivity of free space 8.85*10^-12. How do I calculate the E_0 value? And does that vary with radius? I can't imagine that it does, so I'm back to square 1! I'm trying to calculate the radiation pressure on a surface at differing distances from the sun, so knowing how and why the intensity changes would be greatly helpful Any help would be greatly appreciated.