We can also calculate the amount of energy arrives at the upper atmosphere using the inverse square law, if we know the distance to the sun and its diameter. Doing the math for the one spot directly below the sun, and thus receiving the sun’s rays directly perpendicular, we will begin with RE assumptions. In this model, the energy arriving at earth varies with the distance from earth to sun as we orbit. The earth is closest to the sun at perihelion in January, when the earth is 147 million kilometers out. That works out to 211.1 solar radii, which results in an inverse square result of 1414 watts per square meter at the spot directly facing the sun. The outermost point in the orbit, aphelion, happens in July. We are at a distance of 152 million kilometers, or 218.3 solar radii. The inverse square result that day is therefore somewhat less, at 1322 watts. The quoted average on the internet varies; some sites use 1360 watts per square meter, some use 1370, while Wikipedia settles on the satellite-measured value of 1361. Next, let’s move away from the subsolar point and consider Portland Oregon, just above the 45th parallel. On a round earth, Portland will be slightly further from the sun than the equator will be, as curving a little bit around the planet adds a little bit of distance. That difference is only 1866 km, a vanishingly small fraction of the 5 million kilometer difference between aphelion to perihelion. Therefore we need only consider the effects caused by the difference in the incident angle of the radiation (which in effect takes a square meter of radiation measured perpendicular to the sun, and spreads it out over a larger surface area due to the curvature of the earth) and a bit more atmospheric attenuation (due to having a longer path through the atmosphere before reaching the ground). The angle of incidence reduces the energy to 70% of the initial value. And to my surprise, my searches indicate that the difference between equatorial atmospheric attenuation and higher latitudes appears to be negligible (one source called it “a second-order effect”).