occulter

That which occults: specifically applied to the mechanism or device which, in lighthouses, periodically interrupts or occults the beam of light; also to the device used in heliographic signaling to interrupt or divert the beam of light reflected from the mirror of the heliostat, heliograph, or heliotrope.

Any object, natural or man-made, that blocks the light of an object from an observer, typically used in reference to astronomical events.