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| Frontispiece of Riccioli's 1651 New Almagest (Wikipedia) |
There were really three different classes of models:
- Geocentric - with the earth in the center and with roots in antiquity
- Geoheliocentric - with the earth in the center, and the sun orbiting the earth, but with most of the other planets orbiting the sun.
If you don't understand why this class of models gained prominence in the first half of the 17th century, you cannot really understand the science of the Galileo conflict. - Heliocentric - with the sun in the center, resembling our system today
- Geocentric with a stationary earth: the Ptolemaic system from antiquity
- Geocentric with a rotating earth
- Geohelicentric with the inner planets, Mercury and Venus, orbiting the sun. This system goes back to Martianus Capella in the 5th century and maybe even earlier
- Geoheliocentric with all planets except the earth in orbit around the sun. Proposed by Tycho Brahe in 1588, but before that by Nilakantha Somayaji in 1501 in Kerala in India. Stationary earth.
- Christen Sørensen Lomborg's (Longomontanus) model which is similar to Brahes, but with a rotating earth from around 1622
- Giovanni Battista Riccioli's model with Mercury, Venus and Mars in orbit around the sun and Jupiter and Saturn orbiting the earth as seen in the figure above.
- Riccioli later combined Brahe's model with Kepler's elliptic orbits
(C) Heliocentric
- The original Copernican system with circular orbits
- Kepler's system with elliptic planetary orbits
- Kepler's system modified by general relativity
The last system needs an explanation. The general theory of relativity was able to explain a problem that physicists had struggled with since at least 1859, concerning a small deviation in the orbit of Mercury, precession of the perihelion. During the solar eclipse in 1878, many astronomers had tried to find Vulcan, a hypothetical planet between the sun and Mercury, as it was one of the best candidates to explain this deviation. However, it was not possible to calculate Vulcan's orbit in a definitive way, and more importantly, Vulcan was never observed. Then Einstein came along and said that since Mercury is so close to the sun and moves so quickly, Newton's theory is not precise enough.
Strictly speaking, the systems above are valid only for a single planet orbiting a star. In particular the moon's orbit is perturbed due to the influence both of the earth and the sun.
In total this gives 10 different models for the movements of the earth, sun, and the planets, or 9 if I should follow the title and restrict myself to the 1600’s. In either case, it was definitely much more complex than Galileo portrayed it in his polemical book "Dialogue Concerning the Two Chief World Systems" in 1632. The two systems he compared were only A1 and C1 above, simplifying the question to absurdity.
Why do I say that the geoheliocentric system was so important? It is due to the lack of measurable parallax, something which the geocentric system requires. This is why the compromise system of geoheliocentricity was popular for a while. Parallax is a result of the earth's movement relative to the fixed stars, as the earth orbits the sun. It had been looked for since antiquity but never found. Due to the unexpected vastness of the universe, it is so small that it was first observed by Bessel in 1838. Before that the related effect of abberation was observed by Bradley around 1727.
(Text based on Norwegian text of Chapter 3 "Serious science in the Middle Ages and the narrow-minded Galileo" from my book The imagined conflict. On science and God, 2021 and inspired by Thony Christie's blog)
