Models of the Universe

Understanding our place in the universe has always been complex. Complicated theories and the mathematical equations that back them up are often hard to understand. For over two millennia, astronomers have turned to cartography to create familiar diagrams to accompany their explanations. These visual representations often focused on explaining the way the universe worked according to different theories. Celestial cartographers like Cellarius and Doppelmayr committed to exploring different models of the universe in their celestial atlases: the geocentric model that put the Earth at the center of the universe, the heliocentric model that insisted that the Earth revolved around the Sun, and other models that combined aspects of the two.

The Geocentric Model

For thousands of years, humans observed the skies from Earth and believed that the Earth was the center of the Universe. There were two primary reasons for this belief. First, from our perspective, the stars, planets, Sun, and Moon appear to revolve in predictable patterns around us about once per day. Second, the Earth seems to be stationary - if you didn't know it was moving, would you be able to tell?

Thinkers like Aristotle helped to advance the theory of geocentrism, but Claudius Ptolemy, a second-century CE astronomer, standardized the system. His Almagest, published sometime around 150 CE, combined centuries of astronomical work done by Greek, Roman, and Babylonian astronomers. Ptolemy's system placed Earth at the center of the Universe with each planet occupying a celestial sphere surrounding Earth. Ptolemy's work became the foundation for modern astronomy - the Almagest is one of the most influential scientific works in history!

The Heliocentric Model

While the geocentric model dominated astronomy for well over a thousand years, by the 1500s some problems had become apparent. The predictive equations Ptolemy laid out in the Almagest were discovered to be slightly inaccurate, which complicated the model. Nicolaus Copernicus, a Polish astronomer, theorized that the Earth was not the center of the Universe, but that is was just one of the planets that orbited around the Sun. His theory helped explain why the planets did not maintain consistent paths across the Earth's sky (also called "retrograde motion"), and created the idea that the Earth rotated daily, and revolved around the Sun on a yearly basis - thus explaining the seasons.

Because of these complications, the fact that his system did not improve the accuracy of predictions over Ptolemy's model, and the lack of proof that the Earth is in motion, Copernicus's model did not catch on immediately. Later astronomers would help prove the heliocentric theory: Galileo Galilei's discovery that Jupiter had four moons definitively proved that the Earth was not the center that everything revolved around, Johannes Kepler's argument for elliptical orbits simplified the model, and Isaac Newton's law of universal gravitation provided an explanation for why the planets would be in motion. These scientists and their discoveries led to the widespread adoption of heliocentric theory by the end of the 1600s.

Models of the Universe

After Copernicus published On the Revolutions of the Heavenly Spheres, astronomers began to explore alternatives to the Ptolemaic model. Copernicus's system, as originally proposed, relied on some of the more complex parts of Ptolemy's, while also requiring the reader to accept an unprovable (at the time) concept: that the Earth moved.

Many astronomers tried to explain the universe with models of their own, model that updated Ptolemy, but did not rely on unobservable truths. The primary opponent of the heliocentric model after Copernicus's death was Tycho Brahe - a Danish astronomer who served as the court astronomer and astrologer to kings. Brahe admired the Copernican system but felt it had problems with physics, observations, and religion. Although he acknowledged that the daily motion of the heavens could be explained by Copernicus's theory, his primary reason for opposing heliocentrism was the Earth's "hulking, lazy body, unfit for motion".

As a response to his doubts about the Copernican system, Brahe created his own model - often called the Tychonic model. In the Tychonic system, the Earth remains the center of the Universe, with the Moon, Sun, and Stars revolving around it. Meanwhile, the planets each revolved around the Sun.