Christopher: In 1632 London, official records listed more people dying from 'the planet' than from cancer. Lucas: Hold on, from 'the planet'? What does that even mean? Like, they tripped over a globe? Christopher: No, it wasn't a metaphor. They believed the planets—Mars, Jupiter, their alignment in the sky—could literally reach down and kill you. It was an official cause of death. Lucas: Wow. That is an absolutely wild piece of trivia. It’s hard to even imagine a world that works like that. Christopher: And that’s the journey we’re on today. We're exploring the book that charts our species' epic voyage from that kind of magical thinking to landing robots on other worlds. And that journey is masterfully told in Cosmos by Carl Sagan. Lucas: Right, the book that accompanied that iconic TV series with the turtleneck and the spaceship of the imagination! What's amazing is that Sagan, who was a top-tier NASA astronomer working on their biggest missions, was driven to write it after seeing how little the media cared about the first real images coming back from Mars. He wanted to share that raw wonder directly with everyone. Christopher: Exactly. He felt the public was being short-changed, that they were far more intelligent and curious than they were given credit for. And he starts this journey way, way back, showing just how deeply we've always tried to connect our tiny, everyday lives to the grand cosmos… sometimes in very, very strange ways.

Lucas: Okay, you can't leave me hanging. How strange are we talking? Stranger than blaming Jupiter for your untimely demise? Christopher: Oh, much stranger, and much more specific. Sagan tells this incredible story from ancient Assyria, around 1000 B.C. Imagine you have a terrible toothache. What’s the cause? Lucas: Uh, a cavity? I didn't eat my vegetables? I need to floss more? Christopher: Wrong. The cause is a cosmic drama that began at the dawn of time. According to the Assyrian belief, after the great gods created the universe—the sky, the earth, the rivers, the canals—the canals created the mud, and the mud created… a worm. Lucas: A worm. Of course. The final act of creation. Christopher: This little worm goes before the gods, weeping, and asks, "What will be my food? What will be my drink?" The gods offer him ripe figs and apricots, but the worm refuses. He has a different idea. He begs them, "Let me live between the teeth! Let me dwell in the gums! That I may drink the blood of the tooth and gnaw upon the roots of the gum!" Lucas: That is horrifyingly specific. So my dentist should just be reciting an ancient spell to exorcise a greedy, cosmic worm from my mouth? Christopher: Precisely. The cure was a potion of beer and oil, and you had to recite this entire creation story three times while applying it. The point Sagan makes is that for them, a toothache wasn't a biological problem; it was a failure in the cosmic order. Everything was connected, from the creation of the heavens to the throbbing in your jaw. Lucas: I see the logic, in a terrifying way. It gives you a sense of control, an explanation for the random pains of life. But how did we ever break out of that kind of thinking? How do you get from a cosmic tooth worm to, say, modern dentistry or… astronomy? Christopher: With a stick. And the sun. And one of the greatest "aha!" moments in human history. Sagan introduces us to a man named Eratosthenes, who lived in the 3rd century B.C. He was the head of the legendary Library of Alexandria, the single greatest repository of knowledge in the ancient world. Lucas: Okay, so this guy was no slouch. He’s surrounded by all the world’s information. Christopher: Right. And one day he reads a report in a papyrus scroll from a town in southern Egypt called Syene. The report said that on the longest day of the year, June 21st, the midday sun shone directly down a deep well, illuminating the very bottom. On that day, at that time, a vertical stick in Syene would cast no shadow. Lucas: Huh. That’s a neat observation. But it doesn't sound world-changing. Christopher: For most people, it wasn't. But Eratosthenes was a scientist. He was curious. He asked a simple question: "Is the same thing true here in Alexandria?" So, on the next June 21st, he planted a stick in the ground and waited for noon. And sure enough, it cast a shadow. Lucas: Okay, so the sun hits the two cities differently. Why? Christopher: This is the genius leap. Eratosthenes reasoned that if the Earth were flat, the sun's rays would be parallel and both sticks should have no shadow. The only way a stick in Alexandria could cast a shadow while one in Syene didn't was if the surface of the Earth was curved. Lucas: Wow. Okay, I see it now. The shadow is there because the stick in Alexandria is at a different angle to the sun. Christopher: Exactly. And he didn't stop there. He measured the angle of that shadow and found it was about 7 degrees. Now, a full circle is 360 degrees, and 7 degrees is about 1/50th of that circle. So he reasoned that the distance from Alexandria to Syene must be 1/50th of the total circumference of the Earth. Lucas: That’s brilliant. But how did he know the distance? It’s not like they had Google Maps. Christopher: He hired a man to walk it. A professional surveyor. The distance was measured to be about 800 kilometers. So, Eratosthenes did the simple math: 800 kilometers times 50. He got an answer of 40,000 kilometers for the circumference of the Earth. Lucas: Get out of here. What’s the real number? Christopher: The actual circumference of the Earth is 40,075 kilometers. Lucas: No way. He did that with a stick, a well, and his brain? That is a mic drop moment in human history. He measured the entire planet without ever leaving Egypt. Christopher: It's the perfect contrast to the tooth worm. One is a story you tell yourself to feel better. The other is a method you use to find out what's actually true, even if it’s a truth as staggering as the size of your own world. It’s the birth of science. Lucas: It really makes you wonder what "obvious" myths we believe today. What are the things we assume are true that future generations are just going to look back on and laugh at, the same way we do about the cosmic worm? Christopher: That is the question that echoes through all of Cosmos. And that leap by Eratosthenes was huge, but for nearly 2,000 years after him, our picture of the heavens was still fundamentally wrong. We had the size of our planet right, but we still thought everything—the sun, the planets, the stars—revolved around us. And the man who finally shattered that, Johannes Kepler, started from a place of pure mystical beauty.

Lucas: Wait, the man who corrected our view of the solar system started with mysticism? That sounds like a step backward from Eratosthenes and his hard data. Christopher: It does, but it’s what makes his story so powerful. Kepler, working in the late 1500s, was a profoundly religious man. He wasn't just a mathematician; he believed he was reading the mind of God. He was obsessed with a question: why are there six planets, and why are they spaced out the way they are? He wanted to find the divine architecture. Lucas: A noble goal. How did he try to solve it? Christopher: He had this breathtakingly elegant idea. He knew that in all of geometry, there are only five "perfect" 3D shapes, the Platonic solids—the cube, tetrahedron, and so on. And he noticed there were six planets with five gaps between them. Lucas: Oh, I see where this is going. Five perfect solids, five gaps between planets. It’s too neat to be a coincidence. Christopher: That's exactly what he thought! He proposed a model where the orbit of each planet was held in place by one of these perfect solids, nested one inside the other like a set of Russian dolls. It was beautiful, it was mathematical, it felt like uncovering God's secret blueprint for the universe. He called it his "Cosmic Mystery." Lucas: That is an incredibly cool idea. I want it to be true. Was it? Christopher: Not even close. It was a beautiful theory slain by an ugly fact. Or, more accurately, by thousands of them. Kepler knew his model was only as good as the data, and the best data in the world belonged to one man: a flamboyant, hard-partying, Danish nobleman named Tycho Brahe. Lucas: Tycho Brahe. He’s the guy with the metal nose, right? Lost his real one in a duel over a math equation. Christopher: The very same. Tycho was the greatest observational astronomer of his age. He had an island observatory and spent decades meticulously charting the positions of the planets with unprecedented accuracy. Kepler, a poor schoolteacher at the time, knew he needed Tycho's numbers to prove his theory. So he went to work for him. Lucas: So you have the master theorist and the master observer under one roof. That sounds like a dream team. Christopher: It was more like a nightmare. They had a tense, difficult relationship. Tycho was secretive and didn't want to share his life's work. But then, after a banquet, Tycho suddenly died. And on his deathbed, he bequeathed all his data to Kepler. Kepler now had the keys to the kingdom. Lucas: So he plugs Tycho's super-accurate numbers into his beautiful Platonic solids model and… Christopher: And it fails. Miserably. Especially for the planet Mars. Its orbit just wouldn't fit a perfect circle. The discrepancy wasn't huge—it was a tiny error of just eight minutes of arc. To put that in perspective, that's about the width of a single coin seen from across a football field. Lucas: That’s tiny. Most people would just fudge the numbers, right? Call it a rounding error and publish their beautiful theory. Christopher: But Kepler couldn't. He wrote, "If I had believed that we could ignore these eight minutes, I would have patched up my hypothesis accordingly. But since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy." His intellectual honesty, his devotion to the data, was stronger than his love for his own beautiful theory. Lucas: So his own data proved his beautiful theory was wrong? That must have been crushing. He had to kill his own brilliant idea. Christopher: It was his finest moment. He threw out centuries of dogma about perfect circular orbits and started from scratch, trying to find the true shape of Mars's path. After years of grueling calculations, he finally found it. It wasn't a perfect circle. It was a squashed circle. An ellipse. Lucas: An ellipse. It's not as pretty as a perfect circle, is it? Christopher: It’s not. But it was the truth. And this is where Sagan really contrasts the method of science with the religious dogma of the time. Kepler himself was deeply religious, but his scientific honesty forced him to accept a universe that was less perfect, less elegant than he had imagined. And he did this while facing incredible personal hardship—his mother was accused of witchcraft, and he had to rush home to act as her defense lawyer, using his scientific reasoning to dismantle the superstitious claims against her. Lucas: That's just an unbelievable story. He's revolutionizing our entire conception of the cosmos while simultaneously fighting medieval superstition in his own family. It’s a heartbreaking and inspiring conflict. Christopher: It is. He gives us the real laws of planetary motion, the true rules of the road for the solar system. And once we had those rules, it was only a matter of time. Fast forward a few centuries, and we're not just looking at the planets anymore. We're trying to go there, especially to our most intriguing neighbor, Mars.

Lucas: Ah, Mars. The red planet. The source of so much of our science fiction, our hopes, and our fears. Christopher: And for a long time, our scientific speculation was just as wild as the fiction. Sagan dives into the story of Percival Lowell, a wealthy Bostonian astronomer who, in the late 1800s, became convinced he was seeing canals on Mars. Lucas: The famous canals of Mars! I've heard of this. He thought Martians built them, right? Christopher: He did. He built a state-of-the-art observatory in Arizona and spent years drawing these incredibly intricate maps of a planet-wide network of straight lines. He didn't just see a few lines; he saw hundreds. He constructed this grand, romantic narrative of an old, dying Martian civilization, a planet drying out, whose brilliant engineers had built a colossal irrigation system to carry water from the polar ice caps to their thirsty cities. Lucas: That is an amazing story. It’s like a sci-fi novel written in an observatory. Christopher: It captivated the world. But other astronomers looked through their own telescopes and saw nothing. Lowell's canals, it turned out, were most likely an optical illusion, perhaps the brain's tendency to connect random dots into straight lines, maybe even influenced by the pattern of blood vessels in his own eye pressed against the eyepiece. Sagan has this great line: "Lowell always said that the regularity of the canals was an unmistakable sign of their intelligent origin. He was right. The only question was which side of the telescope the intelligence was on." Lucas: Ouch. A brilliant burn. So, how did we finally get the real picture? Christopher: We sent robots. Sagan was a central figure in the Viking missions in 1976, which landed two spacecraft on the surface of Mars. This was humanity's first real attempt to search for life on another world. And the reality they found was so much more complex and ambiguous than Lowell's grand vision. Lucas: What did they find? Did they find life? Christopher: That's the billion-dollar question. The Viking landers had a robotic arm that scooped up Martian soil and put it into a miniature onboard laboratory. They ran three different experiments designed to detect microbial life. And the results were baffling. One experiment came back positive—it showed a burst of gas, just as you'd expect if microorganisms were metabolizing the nutrients they were given. Lucas: So they found it! Christopher: Well, the other two experiments came back negative. And a different instrument on the lander found no organic molecules in the soil, the building blocks of life as we know it. The scientists were left with a puzzle. Was it biology, or was it just some weird, unexpected Martian chemistry that was mimicking the signs of life? The consensus eventually landed on chemistry, but the ambiguity was profound. Lucas: It's the same pattern again, isn't it? We want to see something, so we see it. Lowell wanted to see canals. The Viking scientists, and the whole world, wanted a clear 'yes' or 'no' on life, and instead they got a 'maybe?' Christopher: Exactly. Even the first color picture from the surface was a source of this. The sky came back a kind of salmon pink. And the press at the Jet Propulsion Laboratory actually booed. They wanted a familiar, Earth-like blue sky. They were disappointed that Mars was so… alien. Lucas: We just can't help ourselves. We keep trying to project our own image onto the cosmos. So what does Sagan think this all means for us? This constant, fumbling, hopeful search?

Christopher: I think for Sagan, the entire story of Cosmos—from that ancient toothache worm to the ambiguous data from the Viking robots—is a lesson in humility. It’s a story about the scientific method as a tool for decentering ourselves. Lucas: What do you mean by 'decentering'? Christopher: We started out as the protagonists of the universe. The gods created a worm just for our toothache. The sun and stars dutifully revolved around our planet. We were the center of everything. Science, at its best, has been a slow, painful process of dethroning ourselves. Lucas: I can see that. Copernicus and Kepler kicked us out of the center of the solar system. Then we learned our sun is just one of a hundred billion stars in a galaxy that's one of hundreds of billions of galaxies. Christopher: Precisely. And the search for life on Mars is part of that. Lowell's story was another attempt to put intelligence, something like us, at the center of the Martian story. The real science of Viking gave us a much more humbling answer: a complex, beautiful, and possibly sterile world that operates on its own terms, not ours. Sagan's point is that the greatest gift of science isn't just the knowledge it gives us, but the character it builds. It forces us to accept the evidence, even when it's not the story we wanted—like Kepler's 'ugly' ellipses or a Mars that stubbornly refuses to give us a straight answer. Lucas: And it leaves you with such a powerful question: What are the 'canals' we're seeing today? The patterns we're imposing on reality—in our politics, in our personal lives, on the internet—because we so desperately want them to be true? Christopher: That's the perfect question to end on. The book is more than forty years old, but that challenge feels more relevant now than ever. It's a deep thought, and we'd love to hear your take. What modern 'myths' or 'canals' do you think science, or just honest observation, will eventually overturn? Let us know your thoughts on our community channels. Lucas: It’s a call to be a little more like Kepler, and a little less like Lowell. This is Aibrary, signing off.