Slide 6 of 26
Notes:
In this slide S is the total power (in Watts) emitted from the sun - this is the output of the nuclear furnace deep in the interior. R is the distance between the sun and the Earth. As one moves out from the sun, the same total flow of energy, S, crosses the surface of each sphere of radius R. However, it is spread over a larger surface area . The INTENSITY thus falls off as 1/R2.
Question: What is the direct intensity at the distance of Mars from the sun (1.4 x R)? Direct intensity means on a surface directly facing the radiation.
Answer: Direct intensity = S/{4 ? (1.4 R)2 } = 1368 / (1.4)2 = 698 W/m2.