Hmm, this is an interesting question and is really one of scales. If the Earth was half way between the Moon and Sun (**which of course is not the case**) then the light at the Earth would be four times greater than at the Moon. That means the Moon only gets 1/4 the amount of light as the Earth. Also, if you assume that the shadow cast from a distant light source casts a shadow that is proportional to the distance between the obstacle (man at Earth) and the screen (moon) then the size of the shadow will be only twice the size of the man. So, trying to resolve the shadow about 10 feet long with only 1/4 of the light is very difficult. So, if we use a larger body (i.e. the Earth itself) the chances of being able to see the shadow is much greater. Alternatively, bringing the moon much closer will increase the intensity of the light that reaches the moon by a factor of 1/(r^2).