Eyelids

The eyelids on the star projectors on the spheres at either end of the Zeiss Mk. IV Planetarium Projector occlude the projections that would otherwise land on the audience (and the hemispherical view that would be obscured by the earth, given the position of the projection).These ingenious devices work simply through gravity.

This is the same projector (from the Griffith Observatory) of which I posted some pictures a couple of weeks or so ago.