posted by rohit .
The parallax of the red giant Betelguese is just barely measurable and has a value of about 0.005 arc seconds. What is its distance? Suppose the measurement is in error by + or -0.003 arc seconds. What limits can you set on its distance?
1/0.005 = 200 parsecs. Convert that to light years, if you wish. Try using 0.002 or 0.008 instead of 0.005 to get an idea of the limits of uncertainty.