To observe parallax on a small scale hold your hands out in front of your face with the left elbow bent at a right angle and the right elbow straight, but try to keep both hands at eye-level. Make certain there is a gap between your hands. Now, move your head to the left and watch the gap close and your right hand disappear. Voila, parallax! (Click pic.)
Parallax is the apparent shift in background (right hand) as the point of observation (head) relative to the observed object (left hand) moves.
Copernicus resurrected the heliocentric (sun-centered) model of our solar system, which was set to replace the geocentric (earth-centered) model. This, however, was difficult to prove given the unnoticeable parallax phenomenon. If the earth were moving, then how come the other planets' positions did not change relative to the stars? It should be mentioned that the stars were considered to be lined up on a single plane of their own, beyond the farthest known planet, Saturn.
Parallax wasn't proven until 300 years after Copernicus's model.
Another popular argument against the movement of planet Earth was the tower argument. Simply put, when a stone is dropped from the top of a tower, why doesn't it land many feet away from the base of the tower? After all, if the earth is moving, the ground beneath the falling rock would shift accordingly, causing it's impact to be a hypotenuse away from the tower's base...
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