Venus’s Phases: Evidence Supporting Heliocentrism

The affirmation of the Copernican system marks a milestone in the history of science. However, in educational settings, theoretical and philosophical aspects are often prioritized, with less attention paid to the empirical and quantitative data supporting heliocentrism. The discovery of Venus phases by Galileo Galilei through his early telescope represents a compelling piece of evidence corroborating the Sun-centered model. A renowned drawing published in Galilei's treatise ‘Il Saggiatore’ (1623) illustrates this phenomenon and the anti-correlation between phases and apparent diameter. The analysis of the illustration reveals precise quantitative information comparable in accuracy to that which can be extracted from contemporary astrophotography. Inspired by this result, the galileian discovery has been revisited and tailored into activities suitable for both high school and college teaching levels. These activities invite students to construct their own empirical evidence for heliocentrism by analyzing digital images and using interactive digital tools for image manipulation. The topic has also been proposed at the university level through more advanced analysis techniques. The common aim at each level is to engage students in a dynamic ‘learning by doing’ experience, replicating pivotal moments from the history of science and transforming a topic traditionally confined to the history of ideas into a living, interactive subject.