Parallax works by shooting an angle to a star from two positions which form the bottom two angles on a very long triangle. We use trigonometry to figure out how far away the other vertex of the triangle is. Our ability to do this is limited over long distances. Here's an example. Suppose you could see and point at some point in California from your position in Maryland. Now you shoot an angle to it. Now you move over a half an inch and shoot another angle. You can see how close these two lines are to being parallel. And it is very difficult to tell where they intersect, or how far off of 90 degrees they are to the 1/2-inch baseline. It is actually about these same proportions that we are trying to measure stars with. The earth's orbital diameter would be the half inch. Mind boggling that we can do even a fraction of this.