Objects In Telescope Are Farther Than They Appear: How diffraction   tricked Galileo into mismeasuring the distances to the stars
  Galileo determined distances to stars based on the assumption that stars were suns, the apparent sizes of stars as seen through his telescope, and basic geometry. However, the apparent sizes that he measured were the result of diffraction and not related to the actual sizes of the stars. Galileo's methods and observations were good, but since he was unknowingly observing diffraction artifacts and not the physical bodies of stars he greatly underestimated the distances to the stars. 
Christopher M. Graney