Galileo's ship

Galileo's Dialogue Concerning the Two Chief World Systems considers all the common arguments against the idea that the Earth moves. One of these is that if the Earth is spinning on its axis, then we are all moving to the east at hundreds of miles an hour; hence, a ball dropped from a tower, moving straight down, will fall to the west of the tower, which has moved some distance east in the interim. Similarly, a cannon ball fired to the east will land closer to the cannon than one fired to the west, as the cannon will be moving east and partly catching up with it.

Galileo's advocate Salviati responds with an illustration of the classical principle of relativity. According to this principle as formulated by later physicists, there is no internal observation (without, as it were, looking out the window) by which one can distinguish between a system that is moving in a straight line at constant speed and one that is at rest. Hence, any two systems moving without acceleration are equivalent, and unaccelerated motion is relative. The difficulties in reconciling this with the behavior of light led Einstein to the Special theory of relativity.

Salviati's experiment goes as follows:

Shut yourself up with some friend in the main cabin below decks on some large ship, and have with you there some flies, butterflies, and other small flying animals. Have a large bowl of water with some fish in it; hang up a bottle that empties drop by drop into a wide vessel beneath it. With the ship standing still, observe carefully how the little animals fly with equal speed to all sides of the cabin. The fish swim indifferently in all directions; the drops fall into the vessel beneath; and, in throwing something to your friend, you need throw it no more strongly in one direction than another, the distances being equal; jumping with your feet together, you pass equal spaces in every direction. When you have observed all these things carefully (though there is no doubt that when the ship is standing still everything must happen in this way), have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still. In jumping, you will pass on the floor the same spaces as before, nor will you make larger jumps toward the stern than toward the prow even though the ship is moving quite rapidly, despite the fact that during the time that you are in the air the floor under you will be going in a direction opposite to your jump. In throwing something to your companion, you will need no more force to get it to him whether he is in the direction of the bow or the stern, with yourself situated opposite. The droplets will fall as before into the vessel beneath without dropping toward the stern, although while the drops are in the air the ship runs many spans. The fish in their water will swim toward the front of their bowl with no more effort than toward the back, and will go with equal ease to bait placed anywhere around the edges of the bowl. Finally the butterfles and flies will continue their flights indifferently toward every side, nor will it ever happen that they are concentrated toward the stern, as if tired out from keeping up with the course of the ship, from which they will have been separated during long intervals by keeping themselves in the air. And if smoke is made by burning some incense, it will be seen going up in the form of a little cloud, remaining still and moving no more toward one side than the other. The cause of all these correspondences of effects is the fact that the ship's motion is common to all the things contained in it, and to the air also. That is why I said you should be below decks; for if this took place above in the open air, which would not follow the course of the ship, more or less noticeable differences would be seen in some of the effects noted.
Dialogue Concerning the Two Chief World Systems, translated by Stillman Drake, University of California Press, 1953, pp. 186 - 187 (Second Day).
The above text, describing the thought experiment on the moving ship, is released under the GNU Free Documentation License (GFDL) by the estate of Stillman Drake.