To measure the speed of light, Fizeau used a rotating disk with equally spaced holes during which light could pass. Light was directed at the rotating disk, passed during one of the holes, bounced from a distant mirror, and then the angular speed of the wheel was adjusted until the returning beam of light passed through the next hole in the disk. In one such experiment, the wheel is spun at 1000 rpm and the mirror is 18 km away. If the speed of light is 3.00 x 108 m/s, what is the needed angular spacing among the holes so the returning light will pass during the next hole?