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Galileo Galilei
In 1632, Galileo Galilei described the concept of relativity in his "Dialogue Concerning the Two Chief World Systems". In it he used an example of a ship travelling at constant speed on a smooth sea. An observer doing experiments below the deck would not be able to tell whether the ship was moving or stationary.
Today one can make a similar observation while travelling in an aeroplane cruising at a set altitude. There is no discernable difference between being at rest and moving at a constant velocity. For example, we are oblivious to the fact that the Earth travels at approximately thirty kilometers per second in its orbit around the Sun. This effect is known as Galilean Invariance or Newtonian Relativity.
We may sumarise this concept as "all motion is relative". However, in addition to being relative, any effects of the motion are symmetrical in this version of relativity. We will discuss the relevance of that symmetry later.
Further information on Galilean Invariance may be found here.
Further information on Galileo's many achievements may be found here.
