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In Class With Bryan Penprase: "Archaeoastronomy"

In the classroom with astronomy and physics professor Bryan Penprase on "Archaeoastronomy"

Penprase: ... Hopefully you guys were able to see some sky this weekend. It was cloudy on Sunday, but Saturday was gorgeous, so I hope some of you went outside and looked up. Did anyone notice anything on Saturday?

Erin: I saw the moon in the sky during the day.

Penprase: What time?

Erin: About 11 a.m. to 1 p.m.

Penprase: Did anyone notice it later in the day?

David: Late afternoon. It was transiting.

Penprase: That’s when I was noticing it too. It was almost directly overhead and the sun was setting and you could see this perfect right angle going between sun, Earth and moon. I was playing softball with my daughter, and you could throw the ball overhead and see a half-illuminated ball going up in the sky next to the half-illuminated moon. I thought it was pretty dramatic. Of course, she was wondering why I didn’t throw the ball back at her. (Laughter.) I think she was impressed by it though. The more you can do this connecting with the sky and the celestial geometry, the more you’ll appreciate the class and, as you go on in life, the more you’ll appreciate the larger universe, which is always providing these insights. ...

Penprase: ... Aristotle included in his theory the five primary elements—earth, air, fire, water and ether or quintessence, and he added to these elements behaviors. Each element had a natural tendency. What would the earth element tend to do? It tends to sink to the center. If you take all the elements, the earthy stuff collapses and accumulates where we are, on the Earth. That’s why the Earth is naturally at the center because it’s composed of the stuff that seeks the center. What about air?

Adhana: Circulates upward.

Penprase: And smoke likewise, being a mixture of fire and air. It rises until it reaches the right height and turns itself into clouds by mixing with water. And then finally there is ether or quintessence. This is perfect, unchangeable stuff and its tendency is circular motion. That seems a little nonintuitive. Why would a perfect thing want to move around in circles? But when you think of the roles the circle played in Greek geometry and how one of the most amazing mathematical accomplishments of the Greek period was working on what pi was, the circle encompassed a lot of meaning. Pythagoras started this rolling with the circle being a perfect shape, and Plato spent a lot of time talking about the perfection of forms.

Aristotle picked up where Plato left off and began describing in concrete terms how everything worked. For him, ether was the element that existed out beyond Earth and was the so-called perfect material that made circular motion. So this is how we get these constructions. You have the Earth, you have the planets. We know now that the Earth is not in the center. But for them, for philosophical reasons, the Earth had to be in the middle. There were other practical reasons. What’s the problem with setting the Earth into orbit and spinning it? Remember we’re spinning 1000 miles per hour, orbiting at 65,000 miles per hour. ...

Thomas: ... No way to explain why we’re not blown off.

Penprase: They had no concept of gravity or inertia to work with. This seemed like a natural explanation. The problem is when you set up these systems with perfect circular motions, it doesn’t really match the planets. The stars worked great. The idea was the stars are the most perfect, the most distant and the most proper of these objects. But the planets are, of course, more complicated. We’ve seen some of them in action. Planets are doing their orbits and if we move time forward and lock on Saturn, for example, Saturn will begin moving around in its orbit. With time it will stop its motion and move backwards across the celestial sphere. We know that this is because the Earth is passing Saturn. It takes months to see, but it will slowly wobble back and forth. In Saturn you see the smallest and slowest of the retrograde loops. In order to account for this apparently noncircular behavior the Greeks invented this system of epicycles. ...

David: ... Would ancient people have any way to disprove this theory?

Penprase: Well, if it matches this well.

David: You couldn’t.

Penprase: That’s exactly the case. How do you disprove something like this? You’d have to accept some other construction that has the Earth moving and offset from the center of the solar system. For them, that’s absurd, it’s rejected out of hand. The logical argument would be that if you have to add more and more layers of complexity you eventually reach a breaking point. Sort of the Occam’s Razor argument—if it’s not the simplest explanation, it can’t be the right one. But for them, putting the Earth off to the side and moving it was an absurd proposition because Earth is the densest, most solid, stable place. It has to be in the middle. That’s why this idea survived for so long, for 1,500 years, because it works, and there is no easy way to disprove it without a telescope. ...

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