I am trying to understand an example in my Modern Physics textbook (Example 3.1, page 5 in thishttp://phy240.ahepl.org/Chp3-QT-of-Light-Serway.pdf" [Broken] or pg 69 using the book numbering) I don't understand why the average earth-sun distance is being used in the conservation of energy equation instead of the radius of the earth. Isn't the idea that whatever total power is emitted from the sun must equal the total power received at the earth? [I think e_total can be power received, too, right? It just depends on context?] So, to make sure the power at each end of the journey is equal, we multiply the power per unit area (the values we have) by the surface area of each body. But in that case we would want 4pi*(Earth Radius) not 4pi*(Earth-Sun Distance) I am also a bit worried that no energy is "lost" on the way to the Earth. The book doesn't really talk about that. How do we *know* that energy is conserved in this way? Thanks!