I have read that a star is chosen from the left hand side of pluto and as pluto moves it blocks the starlight and when the star appears on the right hand side again, meaning it reappears we get the diameter of pluto. Can somebody explain this to me in layman's terms?
We time it. We ideally want a bright star. We time when the light from the star goes out and when it reappears. We know how fast Pluto is moving in its orbit, the star is treated as fixed in position, so that is one way we measure it, but we will only get a true diameter if the equator of Pluto passes across the star.
I think I have the answer. Distance is speed into time. So when we know plutos speed and time it takes for pluto to move which in the above example is 20 seconds. The answer to the above hence will be 5x20=100 (COrrect me if I am wrong and also give me the units). This distance will be the diameter of pluto.