  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.
