Michelson and Morley conducted their aether wind experiment and interposed it to ensure accuracy by rotating the apparatus 90 degrees. This allowed them to swap the paths of the light beams, helping to confirm that any differences in speed were not due to flaws in the setup. The goal was to detect an aether wind by comparing the travel times of two light rays, one parallel and one perpendicular to the supposed wind. The interposition was crucial for error correction, ensuring both paths were equal in length. The experiment ultimately contributed to discussions on length contraction and the validity of the aether theory.