It is one of the most beautiful and recognizable abbeys in the world: Rising above the medieval walls of a tidal island on France’s northwestern coast, the monastery of Mont Saint-Michel seems to float in the glassy waters of the surrounding bay. At least, that’s how it appears from the mainland, less than half a mile away. Seen from space during the late autumn and winter months, when the sun is lower in the Northern Hemisphere sky, the monastery’s walls and spires cast a perfect shadow onto the bay. The…