Galilean invariance or
Galilean relativity states that the laws of motion are the same in all
inertial frames.
Galileo Galilei first described this principle in 1632 in his
Dialogue Concerning the Two Chief World Systems using
the example of a ship travelling at constant velocity, without rocking, on a smooth sea; any observer doing experiments below the deck would not be able to tell whether the ship was moving or stationary. The fact that the Earth orbits around the sun at approximately 30 km/s offers a somewhat more dramatic example.