When the Moon passes in front of a star the star does not fade, it vanishes — instantly, because there is no air up there to blur the edge.

The disappearance takes a fraction of a second and is startling the first time you see it. Timing occultations was how the Moon's position was refined for two centuries, and how several stars were discovered to be double: they wink out in two steps. Watch the dark limb of a waxing moon and the star reappears from behind nothing at all.