You know how the moon looks HUGE when it is low on the horizon? That is just perspective, but you should write on the moon then. Your brain does this built in Zoom thing when it has something to scale against, and you could use that to your advantage.

Rather than smoothing the surface to reduce shadows, Why not write in a color other than black? I'm thinking a laser in Green would work well and would contrast against black and white quite well.

More authoritative, and fun http://what-if.xkcd.com/13/