Before I kick off, I just want to announce that as semester is starting, I’ll be dropping back to fortnightly updates. They may even be a little more infrequent at times, during marking and such. So please consider using the field on the left of the page to subscribe and receive an update when I post.

Ok, so last week we left the world using the spirograph model of the universe. We can scoff as much as we want at this, but do remember that Ptolemy’s model was accurate enough not just for Columbus to get to the Americas (even if he was aiming for somewhere else entirely), but for Spain and Portugal to firmly establish their empires.

The first thinker to really upset this model of the universe was Copernicus. He was one of the first (after some Ancient Greeks who had been ignored for about 2,000 years) to place the sun at the centre of the solar system. Interestingly, this was primarily a philosophical shift, rather than any great revelation from new information. Although an avid astronomer, but Copernicus made few truly new observations. Instead, his great contribution was to show that placing the sun at the centre of the solar system explained many of the mathematical complexities of the planets’ orbits. It didn’t get rid of all the irregularities, however; and Copernicus still needed epicycles, albeit far fewer.

The orbit of Mars in particular offered problems for observers, but a German named Kepler was able to go a long way in explaining its strange orbit. He made several important contributions to the movement of the planets, but I’m going to concentrate on two. Firstly, up until Kepler no one had linked gravity on the earth to the motions of the planets; according to most thinkers of the age, they were still just sliding around crystalline spheres.

Instead, Kepler posited that there must be a universal force attracting everything to everything else in the solar system Unfortunately, he missed out on the invention of the term “gravity”, instead ascribing the force of attraction to magnetism.

Nevertheless, from this, Kepler was able to hypothesise that this force decreased exponentially with distance. Whilst this may sound more academic than interesting, it lead to his second key discovery—the orbits of the planets were ellipses, not circles, and the planets sped up and slowed down as they moved closer to and further from the Sun. This managed to account for the strange orbit of Mars (it has the most elliptical orbit of all the inner planets) without having to resort to any invisible epicycles.

Curiously, one of the opponents of elliptical orbits actually made an enormous contribution to the fields of astronomy and gravity—Galileo. The Italian’s contribution to the field of astronomy was largely to provide a way by which the planets continued to revolve around the sun without coming to rest. Our old friend Aristotle, upon whom I wrote last week, had decreed that an object’s natural inertia would inevitably lead to its deceleration and eventual rest. Consequently, Kepler hypothesised that in order to continue orbiting, the planets needed some sort of force to keep them moving. Through a series of careful experiments, primarily by rolling weights down a ramp, Galileo was able to determine that an object which met no resistance would continue forever. This meant that if the orbits of the planets were circular—if the momentum of the planets perfectly balanced the attractive force of the sun—they never stop. As for the problem that the planets did not appear to be moving circles, Galileo apparently proved to be a late disciple of Aristotle and simply ignored the issue away.

Nevertheless, by the middle of the 16th century, the problem largely seems to be that Aristotelian gravity was “all but dead” (p. 60), but no one seemed to know what to replace it with. Kepler’s elliptical orbits offered accurate predictions of where the planets would move next, but without the circular symmetry of Galileo, there was little understanding of how they could go round forever. The planets appeared to accelerate and decelerate as they went around the sun, but few could offer explanations for this apparently erratic behaviour.

It took the illustrious Newton to sort this all out and provide one unitary theory of gravitation. At the prompting of some of his friends,¹ Newton set to piecing all these disparate discoveries together. Starting with the Copernican assumption that the Sun was at the centre of the solar system, Newton made careful calculations as to what orbits would look like if everything in the universe were to have gravity that fell away with distance according to the inverse square law.²

Although Newton never attempted to set out the causes of gravity³ he did demonstrate that if gravity were to follow the inverse square law, the orbits of the planets would be elliptical—they would accelerate as they were pulled in close to the sun, and slow down as they shot away. Assuming that there was no wind resistance, Gallileo’s discovery around the conservation of momentum meant this would go on forever, without needing some mysterious force to slow down and speed up the planets, in spite of the utter lack of circles.⁴

The depth of this breakthrough is difficult to communicate. For many it seemed as if the laws of heaven had been discovered—the planets in their unchanging orbits could be mapped out with almost exacting precision. The key word in that sentence is, of course, “almost”. The orbit of Mercury did not quite fit the predicted route of Newtonian physics and would require Einstein’s general relativity to fix it.

Of course, everyone at the time used the time honoured Ancient Greek philosophical technique: they ignored it.

PS: Just a sly reminder—I’m going to be updating less regularly over the next few months, so you might consider subscribing to my blog (see right) to be notified of new posts.

Academic Sources

Ede, A & Cormack LB 2012, A History of Science in Society: From Philosophy to Utility, 2nd ed., University of Toronto Press, Toronto.

Holton, GJ 1988, Thematic Origins of Scientific Thought: Kepler to Einstein, Harvard University Press, Cambridge.

Horvitz, LA 2002, Eureka!: Scientific Breakthroughs that Changed the World, John Wiley & Sons, New Jersey.

Huff, TE 2003, The Rise of Early Modern Science: Islam, China and the West, Cambridge University Press Cambridge.