- A person on a moving ship drops a rock. They see it fall straight down to land at their feet.
- Someone on shore sees the rock continue to move horizontally as it falls, and says the trajectory is parabolic.
- Another observer on another ship sees it move horizontally with a different speed and sees a different parabola.
- They all see it land at the same time. Galileo concluded that the horizontal motion cannot influence the vertical motion, and that what takes place on the ship is independent of the ship’s motion.
- Galileo argued this applies to all physical and biological processes.
- The same principle applies to what takes place on a moving Earth.
Galileo thought about the question of a moving Earth. If the Earth moves, why do we not notice this motion, as Aristotle claimed we would. Here is the thought experiment he carried out in answer to this puzzle:
A person on a ship drops a rock. To that person its trajectory is a straight line. An observer on shore sees the trajectory as a parabola. Someone on another ship sees yet a different parabola. All of them are making valid observations. They all see it land at the same time.
Galileo concluded that the vertical motion of the rock must in principle be independent of its horizontal motion. Furthermore he concluded that everything that happens on board the ship must be independent of its motion. He argued that whatever you take with you on the ship, insects, fish, physics experiments, etc will be unable to detect the horizontal motion of the ship. He was assuming, of course, that this motion is perfectly uniform and smooth.
This is called the Galilean principle of relativity and explains immediately why we do not sense the motion of the Earth about the Sun.