 In the Starry Messenger , Galileo reported that stars appeared as mere blazes of light , essentially unaltered in appearance by the telescope , and contrasted them to planets , which the telescope revealed to be discs . But shortly thereafter , in his letters on sunspots , he reported that the telescope revealed the shapes of both stars and planets to be " quite round " . From that point forward , he continued to report that telescopes showed the roundness of stars , and that stars seen through the telescope measured a few seconds of arc in diameter . He also devised a method for measuring the apparent size of a star without a telescope . As described in his Dialogue Concerning the two Chief World Systems , his method was to hang a thin rope in his line of sight to the star and measure the maximum distance from which it would wholly obscure the star . From his measurements of this distance and of the width of the rope , he could calculate the angle subtended by the star at his viewing point . In his Dialogue , he reported that he had found the apparent diameter of a star of first magnitude to be no more than 5 arcseconds , and that of one of sixth magnitude to be about 5 / 6 arcseconds . Like most astronomers of his day , Galileo did not recognise that the apparent sizes of stars that he measured were spurious , caused by diffraction and atmospheric distortion ( see seeing disk or Airy disk ) , and did not represent the true sizes of stars . However , Galileo 's values were much smaller than previous estimates of the apparent sizes of the brightest stars , such as those made by Tycho Brahe ( see Magnitude ) and enabled Galileo to counter anti @-@ Copernican arguments such as those made by Tycho that these stars would have to be absurdly large for their annual parallaxes to be undetectable . Other astronomers such as Simon Marius , Giovanni Battista Riccioli , and Martinus Hortensius made similar measurements of stars , and Marius and Riccioli concluded the smaller sizes were not small enough to answer Tycho 's argument . 
