# The Transformation of Astronomy in the Sixteenth Century: A Prelude to Contemporary Mechanics
The sixteenth century marked an essential shift in our comprehension of astronomy and cosmology. These developments—shaped by hundreds of years of Greek philosophical thought, culminating in the overthrow of Aristotle’s conception of the universe—set the stage for innovative theories of motion and mechanics that emerged in the seventeenth century. To truly understand the importance of these changes, one must explore the cosmological systems formulated by Aristotle and their evolution over the ages.
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## **Aristotle’s Cosmology: The Split Universe**
Aristotle (384–322 BCE) established one of the earliest systematic representations of the cosmos. His cosmology classified the universe into two separate realms, divided by the Moon’s orbit:
1. **The Sublunar Sphere**: This domain comprised the Earth and its immediate environment. It consisted of the four classical elements—earth, water, air, and fire—which were characterized by their inclination to seek their “natural place.” For instance, dense elements like earth and water naturally descended toward the Earth’s center, resulting in its spherical form. All other motions within this sphere were deemed “violent” (unnatural), requiring an external impetus.
2. **The Supralunar Sphere**: Beyond the Moon resided the celestial realm, an everlasting and immutable expanse made of a fifth element, **aether** (or quintessence). Unlike the terrestrial realm, motion in the heavens was natural, utterly uniform, and circular. These concepts traced back to earlier thinkers such as Plato and Empedocles.
Aristotle adhered to a **geocentric** cosmology, positing that the Earth was motionless at the universe’s center. All celestial bodies—Sun, Moon, planets, and stars—traveled in concentric crystalline spheres surrounding the Earth. These spheres were propelled by Aristotle’s notion of the **unmoved mover**, an eternal entity that instigated motion in the cosmos as an object of affection and longing, embodying the adage, “love makes the world go round.”
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## **The Puzzle of Retrograde Motion**
Aristotle’s framework encountered a significant observational dilemma: the retrograde motion of planets. To the untrained eye, planets sometimes seem to reverse direction in the celestial sphere, creating a quandary within the geocentric model. While contemporary astronomy attributes this to an illusion stemming from Earth’s swifter movement in a **heliocentric** setup, the ancient Greeks were constrained by their geocentric beliefs.
To resolve this, Greek mathematicians devised increasingly intricate geometric models:
– **Eudoxus (c. 390–c. 340 BCE)** proposed the concept of **homocentric spheres**, where various nested spheres rotated to mimic planetary movements. This model required 27 spheres to account for the motions of the seven recognized “planets” (including the Sun and Moon).
– **Callippus (c. 370–c. 300 BCE)** introduced additional spheres to enhance Eudoxus’ framework, bringing the total to 34.
– Aristotle later accepted this model, envisioning it not only as a mathematical construct but also as a physical reality where crystalline spheres consisted of aether and were mechanically linked. His version suggested a total sphere count between 47 and 55.
However, these models proved inadequate and could not comprehensively account for the observed planetary trajectories, leading to the emergence of more refined solutions.
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## **The Ptolemaic System: Deferents and Epicycles**
Acknowledging the shortcomings of homocentric spheres, **Apollonius (c. 240–190 BCE)** and subsequently **Ptolemaeus (Ptolemy) (fl. 150 CE)** developed the **deferent-epicycle** model. In this framework:
– Planets were believed to navigate in small circular orbits (epicycles) while these epicycles revolved around larger circular paths (deferents) centered on the Earth.
This model preserved a geocentric viewpoint but was mathematically proficient in predicting planetary locations and retrograde motion.
Although it contradicted Aristotle’s homocentric concept, Ptolemy’s model gained widespread approval due to its efficacy and was further integrated into **crystalline spheres** in Ptolemy’s *Planetary Hypotheses*. While it was mathematically sound, this model maintained an uneasy coexistence between the philosophical cosmology of Aristotle and the mathematical astronomy of Ptolemy.
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## **The Sixteenth Century: A Crisis in Cosmology**
The sixteenth century emerged as a watershed moment, characterized by observational challenges to the cosmologies of Aristotle and Ptolemy.
### **The Essence of Comets**
In Aristotle’s view, comets were **sublunar phenomena**, temporary disturbances of the atmosphere. However, observations from **Paolo dal Pozzo Toscanelli (1397–148