He thought that while this observation was incompatible with the Ptolemaic system , it was a natural consequence of the heliocentric system .
In December 1610 , Galileo Galilei used his telescope to observe that Venus showed all phases , just like the Moon .
Galileo Galilei thought that while this observation was incompatible with the Ptolemaic system , it was a natural consequence of the heliocentric system .
In December 1610 , Galileo Galilei used his telescope to observe that Venus showed all phases , just like the Moon .