Six chief world systems were debated in Galileo’s world, some geocentric and some heliocentric. The six systems, as they appeared side-by-side in a book by Athanasius Kircher in 1660, are as follows:
- Ptolemaic: Upper left. All planets revolve around the central Earth. Geocentric.
- Platonic: Upper right. Like the Ptolemaic, except the positions of Venus and Mercury are switched. Geocentric.
- Cappellan or Egyptian: Center left. Venus and Mercury revolve around the Sun. Otherwise geocentric.
- Tychonic: Center right. All planets revolve around the Sun. The Sun in turn revolves around the Earth. The Earth and Moon are not planets.
- Semi-Tychonic: Lower left. Like the Tychonic, except Jupiter and Saturn revolve around the Earth.
- Copernican: Lower right. All planets revolve around the Sun. The Earth is a planet. The Moon is a satellite and not a planet. Heliocentric.