Physical Astronomy for the Mechanistic Universe
Aristotelian cosmology was still present in 17th century understanding of the cosmos. This section briefly explores the contributions of Rene Descartes and Isaac Newton to the development of a new mechanical model for describing the relationship between heavenly bodies. In continental Europe, Rene Descartes theory of vorticies served as a powerful conceptual tool for theorizing the nature of the heavens. In England, Isaac Newton developed a universal theory of gravitation that would provide an underlying mechanism for describing a wide range of celestial and terrestrial motions.
The overlapping circles in Tycho Brahe's geocentric model of the cosmos created a significant problem for the Aristotelian notions of the heavenly spheres. If Brahe was right and the orbits of the planets crossed each other each other then they couldn't be a set of solid.
Rene Descartes offered a solution to this problem in his 1644 Principia Philosophiae. In Descartes system, like Aristotle's, the universe was full of matter, there was no such thing as empty space. To explain motion Descartes introduced the concept of vortices. The system consisted of different kinds of mater or elements rubbing up against each other. His model included three different kinds of elements: luminous, transparent, and opaque. Luminous was the smallest and was what the stars were made of. Earth and the planets were made up of the denser opaque. The space between the planets and the stars was made up of transparent He stated that Lumnious would settle at the center of these vortices and the transparent and opaque elements would keep shifting around each other. This shifting created the movement of objects in the heavens.
Popularity of Cartesian Vortices
Descartes theory of vortices solved a series of existing problems for astronomers and philosophers. It was becoming increasingly difficult to keep the Aristotelian notions of solid crystalline spheres intact. Descartes vortices offered a new mechanism for explaining the movement of the heavens and filled a need to establish an underlying theory to support the new astronomy. As a result, Descartes' ideas about vortices were widely adopted as a way of thinking about the cosmos.
This way of describing the heavens worked as a theoretical philosophy, but in practice was not particularly good at explaining phenomena. In hindsight, Descartes' vortices served an important cultural role as an underlying philosophy for a Copernican sun centred theory, but very little of the ideas in this work continue on. The model of vortices played an important role in advancing the idea that the stars themselves are suns, and that there may be a plurality of planets orbiting those suns.
Newton's Principia and the Genesis of Universal Gravitation
Isaac Newton's 1687 Principia Mathematica Philosophiae Naturalis (Mathematical Principles of Natural Philosophy) generally referred to as the Principia, is often cited as one of the most important books in the history of the physical sciences. Where Descartes had offered an explanation of how a sun centred heavens could work with his theory of vortices, Newton offered a mechanical model of the cosmos anchored in a set of mathematical laws.
In the 1660s Newton had explored Kepler's laws of planetary motion. By working with the orbit of the moon he had roughly confirmed the idea that force acting on objects diminished inversely by the square of the distance between the objects. That is, the closer objects are to each other the more they pull on each other. The data however wasn't particularly strong to confirm his ideas, and he did not publish on this issue until twenty years after he made his discovery.
In 1684 Newton asked astronomer John Flamsteed for data on the orbits of Jupiter's moons. One of the things he wanted to know is if there was any evidence to suggest that Jupiter might be affecting the orbit of Saturn. The question pointed to Newton's developing ideas about an underlying universal theory of gravitation. He wanted to know if he could see if one planet was pulling on another.
Uniting the Celestial and the Terrestrial
Newton's universal law of gravitation bridged the terrestrial and celestial realms in a single set of laws. By positing that an object's gravity pulled on other objects Newton simultaneously explained the movement of the planets, the comets, the moon, the earth, and the tides in the oceans. Principia provided a logic that explained the behavior that Kepler had documented in his descriptive work on the movement of the planets. At its time of publication, the book was controversial. In particular, the idea that objects could act on each other across a distance through empty space was unsatisfying to many. This action at a distance just violated conventional thinking about how forces work in the world. Descartes' model of the cosmos as vortices filled with matter was continued to be broadly popular in Europe and also had adherents in England.
In hindsight, the Principia is often cited as a defining moment in the development of modern science. The mention of "natural philosophy" in its full title suggests its important connections to the past as well. Our notion of "science" is very different and much more modern than Newton's. Aside from his work on gravitation and optics, he had a range of research interests that we would not associate with science today. For example, Newton spent time conducting research in alchemy and working on biblical chronology. In the context of his time science and religion were intertwined and alchemy, biblical chronology, astronomy and mathematics were all legitimate ways to understand the natural world.