 From September 1610 , Galileo observed that Venus exhibited a full set of phases similar to that of the Moon . The heliocentric model of the solar system developed by Nicolaus Copernicus predicted that all phases would be visible since the orbit of Venus around the Sun would cause its illuminated hemisphere to face the Earth when it was on the opposite side of the Sun and to face away from the Earth when it was on the Earth @-@ side of the Sun . On the other hand , in Ptolemy 's geocentric model it was impossible for any of the planets ' orbits to intersect the spherical shell carrying the Sun . Traditionally the orbit of Venus was placed entirely on the near side of the Sun , where it could exhibit only crescent and new phases . It was , however , also possible to place it entirely on the far side of the Sun , where it could exhibit only gibbous and full phases . After Galileo 's telescopic observations of the crescent , gibbous and full phases of Venus , therefore , this Ptolemaic model became untenable . Thus in the early 17th century as a result of his discovery the great majority of astronomers converted to one of the various geo @-@ heliocentric planetary models , such as the Tychonic , Capellan and Extended Capellan models , each either with or without a daily rotating Earth . These all had the virtue of explaining the phases of Venus without the vice of the ' refutation ' of full heliocentrism 's prediction of stellar parallax . Galileo 's discovery of the phases of Venus was thus arguably his most empirically practically influential contribution to the two @-@ stage transition from full geocentrism to full heliocentrism via geo @-@ heliocentrism . 
