Abstract: A fan-like array of louvers around a hub in a half-circle shaped frame is adapted to cover a Palladian-style window. Two facing ring gear sectors are slidable in a channel in the hub to turn about an axis that is common to the half-circle, the channel bottom and the arc of the window. A series of pinions has pinion teeth between and engaged with the two gears, each pinion receiving a drive pin therein, each of which has a point directed away from the gear and embedded in a louver. A pivot pin has a point rotatably received in the opposite end of each louver and an outer end received and spring loaded in a blind hole in the outer, arched, frame member. A drive motor in the hub has a pinion with teeth between and engaged with the gears to drive the gears simultaneously but in opposite directions, thus turning the louvers in unison to open and shut them as and to the extent desired.