The Copernican Principle
Published on Feb 16, 2006 at 11:26 am.
1 Comment.
Filed under cosmology.
Back when I was an undergraduate, I took a course entitled “The Philosophy of Science.â€Â As a physics major, this seemed an interesting elective. I wasn’t alone in that feeling, as a bit over half of the class was composed of science and engineering students. There were some interesting things that we talked about in that class. I came to realize that some of the philosophers that we covered knew less science about how science worked than they did philosophy. However, others seemed to really be on to something. I think that some of the things we talked about were oversimplifications of how science works, but then what do you expect of such courses? In our introductory science courses, we talk about “the scientific method,†as if there were only one method used by scientists. In reality, science uses all sorts of methods to understand the universe — some methods being better than others. However, that is a bit complicated to get across to freshmen who are looking for a cookbook way of doing their laboratory exercises. We get them used to the traditional way, and then we move on in later classes to the variations used by scientists in the real world.
At any rate, during the philosophy of science class, I remember being introduced to the term Copernican principle. Having an interest in astronomy at the time, it stuck. Later, I ran into the term in some introductory astronomy and cosmology textbooks. I think that the philosophers were the first to come up with this idea, and then it worked its way into astronomy and cosmology. Certainly it was not something that Copernicus came up with. So, what is the Copernican principle?Â
To answer this, it helps to refresh ourselves with Nicholas Copernicus. He’s an interesting person in his own right, and well worth reading up on. Copernicus was born February 19, 1473 (Hey, he’s sort of got a birthday coming up in a couple of days, sort of, but not really, taking into account the calendar changes between then and now, …), and he was a lay worker in the Church. It was in this capacity, in an attempt to understand the problems with the calendar (Easter was not being calculated correctly), he realized that you could much more easily calculate the positions of the planets if you assumed that they as well as the Earth all went around the Sun. This was the heliocentric model of the Solar System. At the time, the prevailing idea was Geocentric, or that the Sun, Moon, Planets, and stars all went around the Earth. But, Earth going around the Sun meant that Earth is not the center of all. That is the heart of what we call the Copernican principle: we do not have a special location in the cosmos.
As I said, though, the true significance of this did not come for a number of years. It was Geordano Bruno who most vocally pointed out the significance of both Earth and planets going around the Sun. It meant that Earth was a planet. He went on to say that that meant that the planets were Earths, and that they might be inhabited. He was ordered to quit saying such things; however, he continued to popularize his arguments. For this he was burned at the stake. Then, of course, he quit saying such things.
However, old ideas die hard. With the realization that the stars were bodies like our sun, only much farther away, came the notion of the galaxy. Astronomers tried to map the galaxy by estimating stellar distances. All maps looked similar: the galaxy appeared as a sort of disk-like structure with the Sun at its center. Even as early as the beginning of the 20th Century, most astronomers thought that the Milky Way galaxy was the entire universe, and that it was only a few ten thousand lightyears across, with the Sun almost exactly at its center. In 1920, though, Harlow Shapley showed that globular clusters formed a spherical halo around the galaxy, and that the center of that halo was NOT the Sun, but rather a point tens of thousands of lightyears away, in the general direction of Sagittarius, beyond where the edge of the universe was thought to be! Not too many years after this, in 1924, Edwin Hubble presented conclusive evidence that some of the “spiral nebulae†that astronomers had puzzled over for nearly a century were in fact entire galaxies located millions of lightyears away. We have now found galaxies many billions of lighyears away.Â
So, the Earth orbits the Sun as its third planet, the Sun orbits the center of our galaxy about ¾ of the way out from the center, our galaxy and the Andromeda Galaxy are dancing about each other in a local group of galaxies (actually, the Milky Way and the Andromeda galaxy are in a death spiral and will crash into each other in a few billion years), the local group of galaxies are moving around the Virgo cluster of galaxies, which is the center of a supercluster of galaxies, which forms a great wall of galaxies running through the universe. There is nothing particularly unique about any of that. So, we are not really the center of everything. Hence the Copernican Principle.
 So, all I have to do is convince my students that they are NOT the center of the universe.
-Astroprof
PS:Oh, and just to confuse the issue, it turns out that since the universe is finite in age, and isotropic, and thus you can see the same distance in all direction, so in that sense, we are the center of the observable universe, or at least I am! However, that could be said about every point in the universe, so again our vantage point is nothing special.
Bob Walberg on November 26, 2008 at 9:40 am: 1
i thought copernicus had somewhere in his writings another postulate that the sun’s orbit was in a spiral and of course the earth was following the sun. do you know if there has been any work done on connecting the analemma with this possibility?
Bob