Why are parallax measurements of stars limited to the closest stars?
In astronomy, parallax (sometimes called “annual parallax”) means—specifically—the apparent angular displacement of a star against the background of much more distant stars, as seen by an observer on Earth moving in orbit around the Sun. The trigonometric parallax is defined as half the annual parallax. Parallax of a star is measured in arc seconds (1 arc second = 1″ =1/3600 degree). Error in the calculation of distance from parallax increases with diminishing parallax, and different authors accept different limits on distances over which parallax is useful in calculating distance with accuracy. The resolution of current ground-based telescopes limits determinations of trigonometric parallax to around 0.01″. Atmospheric distortion of images makes measurement of smaller angles too unreliable to be useful. This limits distance measurement by parallax to approximately 100 pc. From the ground, the distances to only about 100 stars can be calculated within 5% accuracy. The resolution of spa
