In this session, we discuss the historical journey from the geocentric model to the realization that the Earth orbits the Sun. For almost 2000 years, the geocentric model, with the Earth at the center, dominated astronomical thought. This model, proposed by Ptolemy, involved complex systems of deferents and epicycles to explain planetary motions. The Renaissance in the 16th century marked a period of significant change. Nicolaus Copernicus reexamined Ptolemy’s model and proposed a heliocentric model, simplifying the mathematics by placing the Sun at the center. However, Copernicus’s model still relied on perfect circles and deferents, making it not much simpler than Ptolemy’s. Key to the eventual acceptance of the heliocentric model was the work of Tycho Brahe and Johannes Kepler. Tycho Brahe’s precise observations of Mars’ position over 20 years were instrumental. After Brahe’s death, Kepler gained access to his data and made groundbreaking discoveries. Kepler abandoned the idea of perfect circular orbits, instead proposing that planets move in ellipses. He formulated three laws of planetary motion: 1. Planets orbit the Sun in ellipses with the Sun at one focus. 2. 3. The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit (harmonic law). Kepler’s work was published in 1609 and 1619, laying the foundation of modern astrophysics by basing his laws on empirical data. This session highlights the transition from the geocentric to the heliocentric model, illustrating how scientific thought evolved through observation, improved accuracy, and the abandonment of long-held beliefs in perfect celestial circles. Next time, we’ll discuss Galileo’s contributions and the further development of Newtonian mechanics, which cemented the heliocentric view of our solar system.