The Phases of Venus

Enlarge Image Galileo used his telescope to show that Venus went through a complete set of phases, just like the Moon. This observation was among the most important in human history, for it provided the first conclusive observational proof that was consistent with the Copernican system but not the Ptolemaic system. The crucial point is the empirical fact that Venus is never very far from the Sun in our sky (see aspects & phases of the inferior planets). Thus, as the enlarged image indicates, in the Ptolemaic system Venus should always be in crescent phase as viewed from the Earth because as it moves around its epicycle it can never be far from the direction of the sun (which lies beyond it), but in the Copernican system Venus should exhibit a complete set of phases over time as viewed from the Earth because it is illuminated from the center of its orbit. It is important to note that this was the first empirical evidence (coming almost a century after Copernicus) that allowed a definitive test of the two models — by comparing the predictions of the models with observations. Until that point, both the Ptolemaic and Copernican models described the available data. The primary attraction of the Copernican system was that it described the data in a simpler fashion, but here finally was conclusive evidence that, not only was the Ptolemaic universe more complicated, it also was incorrect.

Excerpted and adapted from: Galileo: the Telescope & the Laws of Dynamics

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