Visible Stars as Apparent Observational Evidence in Favor of the Copernican Principle in the Early 17th Century

The Copernican Principle (which says the Earth and sun are not unique) should have observational consequences and thus be testable. Galileo Galilei thought he could measure the true angular diameters of stars with his telescope; according to him, stars visible to the naked eye range in diameter from a fraction of a second to several seconds of arc. He used this and the Copernican Principle assumption that stars are suns as a method of determining stellar distances. The expected numbers of naked eye stars brighter than a given magnitude can be calculated via Galileo’s methods; the results are consistent with data obtained from counting naked eye stars. Thus the total number of stars visible to the naked eye as a function of magnitude would appear to Galileo to be data supporting the Copernican Principle.