Cosmologies are systems that model or demonstrate the order of the universe. They can be used to explain the movement of the planets and other astronomical phenomena, or for the creation of predictive calendars. Cosmologies attempt to understand how and why astronomical bodies move as they do, and to present an explanation for these movements that matches what is known of them.

The Ptolemaic System

The geocentric (Ptolemaic) model saw the earth as the center of the solar system with the planets and stars in fixed spheres that rotate around the central Earth. The model was based on observations, dating back to at least the 2nd century, made without any magnification. Viewing the planets and stars from Earth with the naked eye gives the perspective that everything else revolves around us at regular intervals. Ptolemy used an armillary astrolabe, a complex nest of seven concentric rings, which took the positions of planetary bodies but did not magnify them. Because the planets and stars moved in the same patterns, Ptolemy’s model could consistently predict their motion and generally explained observations made with the naked eye.

The Copernican System

When Copernicus published his heliocentric, or sun-centered, model in 1543, astronomers were still viewing planetary bodies through instruments lacking any magnification. Copernicus’s instruments were the armillary sphere and quadrant that had been used over a thousand years before by Ptolemy. Copernicus’s model was not the result of new observations, but a philosophical attempt to create a new cosmological model and to better explain issues such as retrograde motion, in which planets appear to move backwards in their orbits. The goal of his cosmology was to imagine a system for the order of the planets’ rotations, and it was built on mathematics rather than new observations.

Tycho Brahe's large mural quadrant at Uraniborg. Engraving from the book: Tycho Brahe (1598), Astronomiae instauratae mechanica, Wandsbeck.

The Tychonian System

Tycho Brahe was an astronomer in the generation between Copernicus and Galileo whose astronomical observations were considered the best of his time. His observations were made with quadrants mounted on walls to obtain greater positional accuracy. Tycho disagreed with the Copernican model because it did not adhere to his observational data and Aristotelian physics. Therefore, Tycho developed a hybrid system in which the moon and sun revolved around the earth but other planets revolved around the sun. Where Copernicus had made logical leaps in order to explain his system, Tycho adhered strictly to the available observational evidence.

Galileo and Cosmologies

Galileo’s enhancements to the telescope allowed him to see more of the cosmos than any astronomer before him could do. While his discoveries clashed with Aristotelian physics and added evidence on the side of the Copernican model, they failed to definitively reveal a new order for the cosmos. Discovering Jupiter’s moons proved that planetary bodies could revolve around bodies other than the Earth, but Earth could still be seen as the center of the universe, as it was in the Tychonian system. Similarly, while Venus should not have appeared fully or partly illuminated in the order that it does if the Ptolemaic order of planets was correct, it did not disprove the Tychonian system.