Coriolis explicitly analyzed rotating systems such as a waterwheel, but his conclusions apply to the atmosphere and the rotation of the Earth. Galileo’s principle of the relativity of motion depends upon an analogy between the Earth sailing through space and a ship at sea. A moving object on a ship will follow a path, according to an on-board witness, identical to how it appears over solid ground. But in one respect the Earth is not a ship: the analogy breaks down for motion northward or southward, when an object is moving inward or outward toward or away from the Earth’s axis. If a cannon fires a ball northward on a rotating Earth, will the ball veer to one side as it moves toward the pole? If a cannon at the pole fires a ball southward on a rotating Earth, will the ball veer to one side as it moves toward the equator, away from the center? Might this possible effect of the Earth’s rotation be detectable?