My first year out of college, 22 years old and glimmering with hubris, I taught 9th-grade Earth Science. I’d last studied the subject myself in 8th grade, which made for a fun year, if “fun” can refer to wildly inaccurate lectures about glaciers.
When we arrived at the unit on astronomy—a topic I’d actually explored in college—I felt the exhausted, wild-eyed relief of a shipwrecked man crawling onto dry land. All right! Finally I could teach them something, not just spit back stuff I’d learned from their textbook like a mother bird offering regurgitated food. We began with the age-old debate of geocentrism vs. heliocentrism.
Geocentric models of the solar system put Earth at the center, with the sun and planets all orbiting around us. That’s how ancient Greeks saw the world.
The heliocentric model, meanwhile, places the sun at the center. At one point, the Catholic Church clung so tightly to the geocentric model that they put Galileo under house arrest for espousing heliocentrism. How dare he correctly propose that we’re not the center of the universe!
With my kids, I wanted to get past the historical scandals. I wanted to talk about data, predictions, and the nature of science.
Since antiquity, people have watched the planets. To the naked eye, they look just like stars, but with two differences. First, they don’t twinkle. And second, while the stars hold a fixed pattern from night to night, the planets drift. They wander across the sky, moving against the backdrop of stars. That’s what “planet” means, in fact—wanderer.
How did people explain this? Well, they imagined a universe with the Earth at the center, the stars in the distant background, and planets orbiting around us.
But this model had a problem. Each planet usually moves one direction, but sometimes it doubles back, reversing course for a few weeks, and then continuing on its original path. This is called retrograde motion, and it blew Greek minds. Why would planets do that?
Ptolemy had a solution. “The planets don’t orbit us in a simple fashion,” he said (though probably not in those precise words, and definitely not in English). “There are roller coaster loops in their orbits. That explains why they seem to move backwards.”
This theoretical invention helped, but the planets dance a very funny dance. To fully explain their motion, Ptolemy needed further tweaks. He proposed loops within loops. His model grew dauntingly complex. But it successfully predicted the motions of the planets, so for a thousand years, his book stood as the definitive text on planetary astronomy.
Enter Copernicus. He offered a much more elegant way to explain retrograde motion. It’s not that planets are orbiting us. It’s that we’re both orbiting the sun. Retrograde motion occurs because we’re orbiting at different speeds. Mars, for example, orbits the sun more slowly than we do. So when we’re at this point, Mars will seem to move one way.
But then, later in the year, we’re passing Mars, so it seems to move the other way.
And still later, as the orbits continue, Mars resumes its original direction in our sky.
Yes, Ptolemy’s geocentrism can predict the planets’ motion. But Copernicus’s heliocentrism has the same predictive power, and it’s far more elegant to boot. It’s what scientists call parsimonious.
I explained all this to my Earth Science class, pretty sure I was nailing it. “These kids will make great scientists, thanks to me!” I gloated mentally. Then Kenny—often the only one brave enough to speak up in that vacuum-quiet class—chimed in with a question.
“How did those old Greek people even know the planets existed?” (Kenny always had a way with words.) “They couldn’t see them, could they?”
“You… they… of course they could,” I said, blinking.
“Did those dudes have telescopes?” Kenny asked.
“No,” I said. “Planets are… you can just… see them. They look like stars, but they change position in the sky from night to night.”
“OHHHHHH!” the whole class exclaimed, and my heart sank. Here I was, contrasting theoretical frameworks to explain the movements of the planets, while my students didn’t even know that they’re visible from Earth. You don’t see much of a night sky growing up in Oakland.
In that awful moment, I realized I’d lost them. The whole class. I was like a general who’s been marching ahead with his nose in the air, and then looks up to realize that his army is nowhere in sight.
The problem wasn’t just that I relied too heavily on lecture, or that I launched into advanced material without checking my kids’ basic understandings. On a deeper level, I was applying the wrong model of the classroom. I was committing an error even graver than Ptolemy’s.
The class doesn’t revolve around me. Each student follows his own orbit around something that sheds a light far brighter than I do, and exerts a pull far greater: the content itself. As they trace their own elliptic paths around the material, their progress might appear to me like retrograde motion. But that doesn’t mean they’re caught in backwards loop-de-loops. It just means I’ve got to work to see the whole system through their eyes. A class isn’t teacher-centric. It’s truth-centric.
And for the first time in my teaching career, but certainly not the last, I realized that Galileo would be ashamed of me.