Universe systems through the ages: An Introduction

Before I start, here’s a link to a great talk by the eminent boss Terry Tao on how humankind’s ability to calculate cosmic distances has improved throughout the ages:

What is the structure of the cosmos and what is our place in it?  More specifically, what is the nature of Earth’s relations with the things we see in the sky?  At first glance, it would seem an obvious statement the Earth is stationary and objects like the Sun and Moon orbit around it (we’ll, after all, you don’t feel like you’re moving, do you?)  As far as how big the universe is, well there’s the Earth, the Moon, the Sun, those weird wandering planets, and the few thousand visible stars (whatever the hell those are).

The natural conclusion was what we now call a geocentric universe, literally meaning Earth-centered.  These ideas were most famously formalized by the Hellenistic astronomer Claudius Ptolemaeus in his book, the Amalgest in the 2nd century CE.  He placed the Earth at the center of the universe, and around it drew great spheres called deferents whose center is not aligned with the Earth but is halfway between the Earth and a point called the equant.  Why?  Because having just spheres around the Earth could give qualitative answers for things like the retrograde motion of the planets, but quantitatively broke down.  And since this was a pre-science age, if it didn’t work you could always just make shit up.  Even that wasn’t good enough, so the planets didn’t move around on the deferent, but rather on other sphers called epicycles attached to the deferents.  Here’s a picture:

If the math still didn’t work out, you could always just add more epicycles until it did work. Never mind that each planet needed its own set of arbitrary epicycles, and suddenly the system started to look pretty ugly.  I can only assume that they chose spheres to begin with for aesthetic mystical reasons, so this was a big problem.  This model was accepted as dogma for over a thousand years.

The Ancient Greeks weren’t completely clueless; Aristarchus of Samos postulated that it was the Sun, not the Earth, that lied at the center of the heavens and that the distance from the Sun to the stars was many times greater than was believed at the time.  The great polymath Archimedes dismissed this idea in his book The Sand Reckoner using some logic that I can’t follow (it almost seems to my humble self like a straw man argument, or something weird was going.  Aristarchus posited that the Earth revolved around the Sun and that the stars were vastly further away, as the surface of a sphere is to the center… wait, I don’t care.  Who cares about their argument, which ultimately ends up being who was less wrong).

Eventually, the house of cards had to come tumbling down, and it did with the 1543 publication of De revolutionibus orbium coelestium by Polish astronomer Nicholaus Copernicus who replaced the whole thing with a heliocentric model (that is, Sun-centered). We’ve already seen that this wasn’t the first heliocentric system to be proposed, and Copernicus did due diligence in an early manuscript by citing the contributions of Aristarchus and another Ancient Greek: Philolaus. These citations didn’t survive to the final publication though, so strike a point off of Copernicus’ intellectual integrity.  The Copernican system nicely simplified things by dumping all the deferents and epicycles and still solving the problem of retrograde motion.  Also, because he elected correctly to still have the Moon orbit the Earth, the phases of the Moon could now more simply be explained. While useful as a mathematical tool, few of his contemporaries were willing to abandon the geocentric model on theological grounds.

There was still a problem with lackluster and mismatching data. The Danish astronomer Tycho Brahe began a major research program for the time, cataloging astronomical data at a precision hitherto unseen, and this was before the invention of the telescope. Even Tycho wasn’t accurate enough to detect stellar parallax, and so he believed that the Earth was not in fact moving as Copernicus had postulated. Therefore, Tycho constructed a strange hybrid universe model which placed the Earth at the center, about which orbited the Sun, about which orbited all the other junk in the solar system. The system still used perfect circles, which would condemn it and all previous models to the trash bin of scientific history.

Tycho’s assistant and successor, Johannes Kepler, again went to work on the problem. Using Tycho’s superb data, Kepler found that the motions of the planets could not be made to work with circular orbits. After years of work, he worked out that the planets, including the Earth, must orbit the Sun in an ellipse with the Sun at a focus of the ellipse. That was his famous First Law. The Second Law related how quickly the planet moved about the ellipse and the Third Law allowed a computation of orbital period as a function of the planet’s distance from the Sun. He published these works in his books Astronomia nova (published in 1609) and Harmonices Mundi (published in 1619). Why the planets acted in a such a way was still a mystery.

It was Isaac Newton who formulated the laws of universal gravitation, from which he was able to derive Kepler’s laws from baser principles in his book Philosophiæ Naturalis Principia Mathematica published in 1687. Not only that, but it was now clear that it was not appropriate to place the Sun at the exact center of the universe, but rather at a common center of mass called the barycenter. Now, since the Sun holds most of the mass in the solar system, the barycenter is located within the Sun, biased from the center in whichever direction Jupiter currently is, essentially.

Over the next few centuries, evidence mounted that the Sun was not special but rather just one of many countless stars in the universe, and so the question of center began to become meaningless. Albert Einstein published the paper “On the Electrodynamics of Moving Bodies” in 1905 which argued convincingly against there being any preferred inertial reference frame and therefore any center of the universe proper. Nowadays we know that the Sun is just one of many perhaps 400 billion stars in our galaxy which is itself just a ho-hum member of perhaps 170 billion galaxies that comprise the visible universe (which is itself almost certainly not the whole of things).

With each passing age our grasp of the nature of the universe has become more clear, correctly predictive, and with firmer fundamentals as science has advanced. In lockstep, the importance of the human race and our place in the universe has diminished. We are but a shadow of a shadow on a speck of dust in an impossibly vast universe. But it is a knowable universe, and through science, logic, reason, and experiment we can come to unlock its secrets. Moreover, we are a part of the universe, and perhaps we can think of ourselves as a way that the universe is able to know itself. Ah, fuck it.

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