Why are earth-based parallax measurements limited to only the nearest star?
When astronomers average together many observations they cannot measure parallax with an uncertainty smaller than about 0.002 second of an arc. If you measure a parallax of 0.02 seconds of arc, the uncertainty is about 10 percent. That means that ground-based astronomers can’t measure accurate parallaxes smaller than about 0.02 second of arc, which corresponds to a distance of 50 pc. Consequently, ground-based parallax measurements are limited to only the closest stars.
