Mike, Your idea of a man sitting on the sun is certainly interesting, but I'm not sure I agree with your final analysis...
I'll accept your figure of 8 minutes for light to get here from the sun (any difference here won't affect the principle), but will turn it around to say that light from the earth takes 8 minutes to reach your observer on the sun. Light from the sun (albeit 8 minutes old) hits the earth and is reflected back towards your observer from the whole hemisphere facing the sun. Admittedly, the earth will have rotated through some small angle by the time your observer sees this light, 8 minutes later. However, in the meantime there has been a stream of light being reflected from the whole hemisphere facing the sun. This hemisphere will contain different geography, due to the earth's rotation, but will always be a complete hemisphere. Thus as far as I can tell, your observer will not see any dark limb. As soon as an unlit part of the earth moves into view, it is illuminated by light just reaching the earth having left the sun 8 minutes earlier (when that part of the earth was still in darkness). For the duration of that bit of earth passing across the hemisphere facing the sun, it will be lit and will reflect light back towards the sun. Hence the entire disc of the earth should be visible to your observer all the time. I hope this is a) correct ;-) b) reasonably understandable David Higgon London p.s. Given the right facilities, you could do a practical test bouncing sound waves echoing off a rotating body. Rotation speeds would not need to be unfeasably high since the speed of sound is more reasonable. If you were right, there would come a rotation speed where there was no echo from the body. Note that in light terms, if your theory were correct, the edge of the planet would not just be dark, it would literally be invisible. Objects that do not emit radiation are only visible because of the light they reflect. If your theory held and there was a limb of the body not reflecting light, you wouldn't be able to see it. In that sense my answer above is bound to be right in that you wouldn't see the dark limb even if it existed!
