Central idea of Michelson and Morley experiment was that if single speed of light predicted is speed of light relative to ether, then as the earth moves through the ether, light travelling along the direction of motion must travel at a different speed relative to the earth and light moving at right angles to the motion.
If I get the argument then something like saying say velocity of ball is fixed in air. Then someone watching the ball from a moving train will see the velocity of ball different in the direction of motion. But in the experiment, the measuring apparatus ( equivalent of person measuring speed of ball in train) and the what is being measured i.e. light are in the same frame (frame of earth). Why should the velocity come different in that experiment as they thought? The argument could have worked if someone to look at this experiment outside earth frame?
I know I am missing something just not able to pin point.