I just read Galileo's Relativity Hypothesis from this website: http://physics.ucr.edu/~wudka/Physics7/Notes_www/node47.html
It states that - "any two observers moving at constant speed and direction with respect to one another will obtain the same results for all mechanical experiments"
Now imagine two people, one inside the hull of a ship, moving at a constant velocity relative to another stationary observer, on land. Lets say both of them are holding identical magnetic compasses. The person inside the ship can observe a movement of his compass needle, whereas the person on shore will not record any such observation. Moreover, using this, the person inside the ship can conclude that he is indeed moving!
Is this argument really inconsistent with Galileo's Relativity Hypothesis? Or is there a flaw in the argument? Or I havent understood the hypothesis correctly?
PS - I haven't read much about Einstein's theory. So I would love to know if this theory was later modified by him or someone else, which would be consistent with this argument (if it is indeed foolproof).