Philosophers and poets in times past tried to figure out why the stainless moon "smoothly polished, like a diamond" in Dante's words, had stains. The agreed solution was that, like a mirror, it reflected the imperfect Earth. Today we smile, but it was a clever way to understand the Moon in a manner that was consistent with the beliefs of their age. The Moon is no longer the "in" thing. We see it as often as the Sun and give it little thought -- we've...