Abstract: A skylight cover for relatively blocking transmission of light through a lens opening of a skylight structure takes the form of a modular unit (10, 110), configured to fit between standard ceiling joists (16, 17). The unit includes a movable panel (30, 130), panel tracks (33, 133), and a panel drive mechanism (40, 45, 48; 161, 162,164), all housed in a common, readily installable and serviceable framework (12, 112). In one embodiment (10), the drive mechanism is a cable air cylinder (40) having end brackets (48) attached by a slide-in mounting assembly (58) to a frame member 39, and having a U-shaped cable bracket (48) deposited over posts (51) on the panel (30). In another embodiment (110), the drive mechanism is a motor (161) having an output gear (162) meshed with a gear track (164) on the panel (130).