Competing Cosmological Models
In 1543, Copernicus suggested the sun was at the center of the cosmos. However, it was centuries before a sun-centered model became widely accepted. The history of science is often thought of as a procession of discoveries and advances. This obscures the complex stories of how theories and models can compete and coexist over long periods of time. When the Copernican model eventually won out it had been so extensively refined that the name "Copernican" doesn't tell the whole story.
As evidence mounted in favor of a sun centered model, it remained one of many competing models for describing and explaining the heavens. By looking at how Tycho Brahe's model, the Copernican model and the Ptolemaic model were represented in the 17th and 18th centuries, we will see the long period of competition between these models.
Weighing Cosmological Models in 1651
When Jesuit astronomer, Giovanni Battista Riccioli published his Almagestrum Novum or "New Almagest" the title alone suggested the boldness of the project. This was to be a new and updated take on Ptolemy's Almagest. The book offered new insight into the state of thought about the cosmos in 17th century Europe.
The frontispiece to Almagestrum Novum illustrates Riccioli's evaluation of three models of the universe. Discarded at the bottom left of the image is the Ptolemaic model. In the center Urania, the muse of astronomy, weighs a variant of Tycho Brahe's revised Earth-centered model against Copernicus sun-centered model. Brahe's model, in which the planets orbit the sun and the sun orbits the Earth, beats out Copernicus model in this evaluation. From Riccioli's evaluation, the Earth-centered model of the cosmos was still the best choice.
In the book, Riccioli presents problems with the Ptolemaic, Copernican and Tychonic models and then offers a variant of Tycho Brahe's model which he believes slightly more correct. For him, it made the most sense to have Mercury, Venus and Mars orbit the sun but still have Jupiter and Saturn orbit the Earth.
One of Riccioli's arguments in favor of the Tychonic model was that since everything was created for humanity it simply made more sense that Earth would be at the center of creation. The argument about our place in the cosmos was a major philosophical issue that scientists, philosophers and theologians debated for centuries.
Beyond the argument about the centrality of humanity and Earth in the universe Riccioli leans on the authority of a number of contemporary and historical thinkers. He lists 38 different astronomers and thinkers, such as Aristotle, Ptolemy and others who believe the Earth to be the center of the universe. He compares them to the 16 astronomers, including Copernicus, Kepler, and Descartes, who favor a sun centered model. The frontispiece and text of Almagestrum Novum show that in 1651 there was still a vibrant debate between the merits of competing models for the cosmos, and suggests that the Earth centered model was still favored.
Ongoing Model Competition and Refinement
Illustrations juxtaposing the Copernican, Tychonic and Ptolemaic models of the universe are found on maps and atlases well into the 18th century. René Descartes ideas about the universe as a series of vortices and Isaac Newton's theory of universal gravitation had provided a robust framework in support of a sun centered solar system. However, as the illustrations in a range of sources show, competing models of the structure of the heavens were still being presented as viable explanations into the 18th century.
Reviewing illustrations of the cosmos over time reveals the gradual acceptance and refinement of celestial models. Many of these gradual shifts are evident in Johann Doppelmayr's Systema Solare et Planetarium. Printed as part of an atlas in 1742, this presentation of a sun-centered system includes a wealth of information on understanding the cosmos at the time. For example, it includes 4 of Jupiter's moons and 5 of Saturn's.
Some of the most interesting details in this illustration are tucked away in the corners. In the upper right corner, among the clouds, are small representations of additional solar systems. Beyond the central diagram, the mapmaker shows the concept of the plurality of worlds. Each of these little sets of circles represents its own solar system with a star and planets. This image directly draws on the literary author, de Fontenelle, who building on the ideas of Newton and Descartes’, explored the significance of living in a universe with a plurality of worlds each orbiting their own stars. For comparison, see the frontispiece from de Fontenelle's 1686 book Conversations on the Plurality of Worlds for similar visual representations.
In the bottom right corner (shown larger here) are the Ptolemaic, Tychonic and Copernican models of the cosmos. The models are arranged from left to right and upward, communicating progress. This illustration is in direct contrast to the frontispiece from Riccioli’s Almagestrum Novum, which showed the Tychonic model winning out over the Copernican when weighed on the scale.
The story of models of the solar system in the 17th and 18th centuries shows how competing explanations and theories can persist over considerable periods of time. The sun-centered model was gradually accepted and promoted, but only after a range of evidence and theory converged to support and substantially refine it.