Why do stellar parallax measurements work only with relatively nearby stars?
Parallax measurements work by measuring the relative change in position of stars as the earth goes around the sun. Think of it this way: hold your right index finger a foot from your face, and your left at about 2 feet. Now close one eye and shift your head without moving your hands. Notice how your fingers appear to get closer or further apart. Now pick two objects across the room. Notice the effect is the same (as you move your head the objects shift in relative position), but the effect is much smaller. The same thing happens as the earth moves around the sun; nearby stars appear to move relative to far-away stars. However the effect gets smaller as the stars get further away. It took until the 19th century before we developed powerful enough telescopes to detect any stellar parallax. Telescopes today can still only detect parallax of nearby stars; the parallax of far away stars is so small that our telescopes can not see it.
