Six chief world systems were debated in Galileo’s world: • Ptolemaic: All planets revolve around the central Earth. Geocentric. • Platonic: Like the Ptolemaic, except switches the positions of Venus and Mercury. Geocentric. • Cappellan or Egyptian: Venus and Mercury revolve around the Sun. Otherwise geocentric. • Tychonic: All planets revolve around the Sun. The Sun in turn revolves around the Earth. The Earth and Moon are not planets. • Semi-Tychonic: Like the Tychonic, except Jupiter and Saturn revolve around the Earth. • Copernican: All planets revolve around the Sun. The Earth is a planet. The Moon is a satellite and not a planet. Heliocentric.