In the exaggeration of popular scientific history, Newton’s *Philosophiæ Naturalis Principia Mathematica* (*The Mathematical Principles of Natural Philosophy*) is frequently designated as one of the most crucial — if not the most crucial — texts in the scientific narrative. If you inquire as to why it holds such a place, most individuals will likely respond that it is due to his discovery of the law of gravity. As I enjoy emphasizing, Newton didn’t discover the law of gravity; he validated it, which is an entirely different matter. While this played a fundamental role in what constitutes the true significance of Newton’s *Principia*, it is not the essence of it. What Newton accomplished with his monumental work was to illustrate that there is only one set of mechanical laws applicable to both terrestrial and celestial domains, thereby dismantling a division in natural philosophy that Aristotle had established two millennia prior.
Aristotle segmented the cosmos into two distinct zones: the sublunar, encompassing everything beneath the Moon’s orbit, and the supralunar, including everything above the Moon’s orbit. He posited that the two regions operated under different principles for motion, leading to separate celestial and terrestrial mechanics. The Stoics proposed a philosophical framework that transcended Aristotle’s dichotomy, yet Aristotle ultimately prevailed in the philosophical debate, maintaining his rigid separation from his own era through to the Early Modern period.
According to Aristotle, the sole natural motion in the sublunar realm was to fall directly to the earth, for which he developed a mechanical theory that was fundamentally flawed. He also crafted a theory of projectile motion that was even more incorrect. In the sixth century, John Philoponus began to dismantle Aristotle’s motion theories, experimentally demonstrating that his falling theory was erroneous and introducing what would later become the impetus theory for projectile motion. Philoponus’ ideas were adopted and elaborated upon by various Islamic scholars.
In the fourteenth century, the Oxford Calculatores and the Paris Physicists anticipated much of Galileo’s work regarding the laws of falling. The sixteenth century saw Tartaglia make further advancements in the theory of projectile motion. Throughout this century, additional contributions were made to the discussion surrounding falling laws, and Tartaglia’s former student, Giambattista Benedetti (1530–1590), articulated much of the falling body theory that would later be credited to Galileo, a full fifty years before Galileo’s own formulation.
As the seventeenth century approached, Guidobaldo dal Monte and Galileo revealed that a projectile’s trajectory is a parabola, while Isaac Beeckman introduced the accurate law of inertia. Galileo, naturally, experimentally confirmed that the Oxford Calculatores were correct, leading to the replacement of Aristotle’s falling and projectile motion theories with the laws recognized today.
Nonetheless, his theory regarding the division between terrestrial and celestial realms remained largely unchallenged well into the sixteenth century. Aristotle claimed that the only motion in the celestial sphere was perfect, uniform circular motion. It fell to astronomers to create mathematical models to reconcile the clear contradictions between this cosmological description and actual observed planetary movements. These mathematical frameworks were viewed as pragmatic fictions for evaluating planetary positions, yet were not deemed to represent reality, per the Aristotelian cosmic model.
While the terrestrial sphere comprised four elements—water, earth, fire, air—and was subject to change and decay, the celestial sphere contained the fifth element, aether, or quintessence, which was eternal and unchanging. The planets were believed to be transported in nested crystalline spheres propelled by a form of friction, with the outermost sphere moved by the unmoved mover, God, in the Christian interpretation of the Aristotelian cosmos.
Aristotle’s assertion that the celestial sphere must be eternal and unchanging caused him to classify comets as atmospheric events, and it was comets in the sixteenth century that ultimately led to the collapse of Aristotle’s cosmological framework. Already in the fifteenth century, Toscanelli (1397–1482) became the inaugural astronomer to regard comets as celestial objects and to monitor their trajectories. Concurrently, Regiomontanus sought to measure the parallax of a comet to determine whether it was sub- or supralunar. In the 1530s, a series of comets sparked renewed debate among prominent European astronomers regarding the nature of comets and whether they resided below or above the lunar sphere. This discussion coincided with a revived interest in Stoic philosophy. The Stoics, having rejected Aristotle’s sublunar-supralunar dichotomy, theorized that comets were celestial phenomena. This dialogue incited further intense interest in the matter.
In the 1570s, there was a supernova in 1572, followed by a significant comet in 1577. Both events were demonstrated to be supralunar.