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John Green reviews Halley's Comet, a celestial body visible from Earth once in a lifetime, and cholera, an infection caused by bacteria and people. Thanks to audible for sponsoring today's episode:

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Hello and welcome to The Anthropocene Reviewed, a podcast where we review different facets of the human-centered planet on a five-star scale. I’m John Green, and today I’ll be reviewing Halley’s Comet and also cholera.

[intro music]

But first, a quick thank-you to our sponsor, Audible. You can get a free audiobook right now by signing up for Audible at or by texting the word anthro to 500-500.

All right, let’s begin with Halley’s [Hay-lees] Comet, or possibly Halley’s [Hal-lees] Comet, or maybe even Halley's [Hall-lees] Comet? We don’t know for sure what to call it, because the astronomer for whom this comet is named spelled his own surname variously as Hailey, Halley, and Hawley. Like, we think language moves around a lot these days, with the emergence of emojis and the shifting meaning of words like literally, but at least we know how to spell our own last names. Anyway, I’m going to call it Halley’s [Hay-lees] Comet, because that’s what my parents called it when I was a child.

Halley’s Comet is the only periodic comet that can regularly be seen from Earth by the naked eye. It takes between 74 and 79 years for the Comet to chart its highly elliptical orbit around the sun, and so once in a good human lifetime the comet is visible in the night sky for several weeks. Or twice in a human lifetime, if you schedule things well. The American writer Mark Twain, for instance, was born as the Comet blazed above the Missouri sky. Seventy-three years later, he wrote, “I came in with Halley’s comet in 1835. It is coming again next year, and I expect to go out with it.” And so he did. Twain had a hell of a gift for narrative structure, especially when it came to memoir.

Seventy-six years later, Halley returned in the late winter of 1986. I was eight.

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The 1986 apparition of the comet was, to quote Wikipedia, “the least favorable on record,” with the comet much further from Earth than usual. That, combined with the tremendous growth of artificial light, rendered the comet invisible to many. I was living in Orlando, a town that throws a lot of light up at the night sky, but on Halley’s brightest weekend, my dad and I drove up to the Ocala National Forest, where our family owned a little cabin, and at the tail end of what I still consider to be one of the best days of my life, I saw the comet through my dad’s birding binoculars.

It was four Halleys earlier, in 1682, that Edmund Halley figured out that this comet had been seen before. Actually, we may have known that Halley was a repeating comet two thousand years ago—there is a reference in the Talmud to “a star that appears once in seventy years and makes the captains of ships err,” but back then it was common for humans to forget over time what they had already learned. Maybe not only back then, come to think of it, but regardless, the Englishman Edmond Halley noticed that the 1682 comet he observed appeared to have a very similar orbit to comets that had been reported in 1607 and 1531. Halley then predicted the comet would return in 1758. And it did. It's been named for him ever since.

Halley was a fascinating character—he developed an early magnetic compass; his writings on the hydrological cycle were tremendously influential; he funded the publication of Isaac Newton’s classic three-volume Principia; he also published an analysis of life expectancy that shaped the eventual development of actuarial tables and life insurance. On the other hand, Halley also he believed that there was a second planet inside of our Earth with its own atmosphere and possibly its own sentient inhabitants. 

By the time Halley showed up in 1986, even third-graders like me knew about the layers of the Earth. That day in the Ocala National Forest, my dad and I made a bench by nailing three two by fours to two sections of tree trunk.

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It wasn’t particularly challenging carpentry, but in my memory, at least, it took us most of the day, and then we started a fire, cooked some hot dogs, and waited for it to get properly dark—or as dark as Central Florida got in 1986.

I don’t know how to explain to you how important that bench was to me, how much it mattered that with my dad I’d made something that you could sit on, but that night, we sat next to each other on the bench, which just barely fit the two of us, and we passed the binoculars back and forth, looking at Halley’s comet, a white smudge in the blue-black sky. My parents sold the cabin fifteen years ago, but just before they did, I spent a weekend there with my girlfriend, who is now my wife. The bench was still there, its fat legs were termite-ridden, the 2 x 4s a little warped, but it still held my weight.

We are here, however, to review the comet, not my four-star childhood. Halley’s nucleus isn’t monolithic like I always imagined it to be, but instead it's a peanut-shaped rubble pile of many rocks that have coalesced together. In total its nucleus is nine miles long and five miles wide, but its tail of ionized gas and dust particles can extend more than sixty million miles through space. In 837 CE, the comet was much closer to Earth than usual, and its tail stretched across more than half of our sky. In 1910, Earth actually passed through the comet’s tail, and some people were terrified that the comet’s toxic gases would poison them—they bought gas masks and even anti-comet umbrellas, but passing through the tail proved a complete non-event. In fact Halley poses no kind of threat to us at all. It's approximately the same size as the object that struck Earth 66 million years ago leading to the extinction of dinosaurs and many other species, but it’s not on a collision course with us. 

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That said, Halley’s Comet will be more than five times closer to Earth in 2061 than it was in 1986. It’ll be brighter in the night sky than Jupiter, or any star. I’ll be 83—if I’m lucky.

When you measure time in Halleys rather than years, history starts to looks different. When Halley visited us in 1986, my dad had just brought home a personal computer—the first in our neighborhood. One Halley earlier, as Mark Twain lay dying, the first movie adaptation of Frankenstein was released. The Halley before that, Charles Darwin was aboard the HMS Beagle. The Halley before that, the United States wasn’t a country. The Halley before that, Louis XIV ruled France. Put another way: We are five human lifetimes removed from the building of the Taj Mahal, and two lifetimes removed from the abolition of slavery in the United States. History, like human life, is at once incredibly fast and agonizingly slow. This seems to me one of the central ways in which our lives are identical to Edmond Halley’s: In both time and space, we can zoom in or zoom out—we can look at life, and the universe, through both telescopes and microscopes. We can listen directly to the long-dead voices of the past through the comet charts they left behind, and predict futures we may not live to see.

Halley himself had been dead sixteen years when the comet came back as he had predicted it would. I find it so comforting that we know when Halley will return, and that it will return, whether we are here to see it or not. I give Halley’s comet five stars.


After the break, we’ll zoom in a little to examine a bacterium and the people who make it possible, but first a message from our sponsor, Audible.  

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Audible allows you to listen to books seamlessly—you can move from your phone to your tablet to your car without losing your place—and every month you get a credit good for any audiobook in Audible’s vast collection, including my novels Looking for Alaska and The Fault in Our Stars, and also Out of the Shadow of a Giant, a fascinating dual biography of Edmond Halley and Robert Hooke. You can get a free audiobook right now at or by texting anthro—that’s a-n-t-h-r-o to 500-500. Thanks again to Audible for sponsoring today’s episode. Now back to the show.


Okay, let’s turn our attention to cholera, which, spoiler alert, will be receiving our first ever lowest possible rating, one star. Cholera is a bacterial infection; its immediate cause is a comma-shaped bacterium called Vibrio cholerae, but in practice cholera is—like so many things in the Anthropocene—mostly caused by people.

The disease attacks the small intestine and leads to high-volume diarrhea and vomiting. It’s not uncommon for people with cholera to lose twenty pounds in a day, and although the illness usually only lasts for a few days, it can still kill you—in fact, certain strains can be fatal within hours of onset. For the past two centuries, it has been known as the “blue death,” because as dehydration becomes severe, human skin turns a bluish gray.

Cholera has infected humans for thousands of years—its name comes from the ancient Greek word for bile and its symptoms were described by Hippocrates. It probably first emerged on the Indian subcontinent, and although it’s been a part of the human story for a long time, outbreaks were limited by geography and relatively sparse populations until 1817, when the first global cholera pandemic began.

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It started between what is now Kolkata, India and what is now Dhaka, Bangladesh, and spread to Thailand, Indonesia, the Middle East, Turkey, and parts of East Africa. Over 100,000 people died on the Indonesian island of Java. In Iraq, 18,000 people died in less than a month. But by 1824, cholera had retreated as mysteriously as it had spread—that is, until five years later, when the second global cholera pandemic broke out.

This time, the disease traveled to Europe and the Americas. Five thousand people died in New Orleans, and 100,000 in France. Cholera riots broke out in Russia, where many believed the tsarist government was deliberately infecting people with the disease, and there were also riots in Liverpool, England, after rumors spread that hospitalized victims were being intentionally killed so that doctors could have corpses to dissect. Amid these riots in 1832, a British doctor named Thomas Latta developed the first intravenously delivered saline solution to rehydrate cholera victims. Latta died a year later, at the age of 37—not of cholera, but of tuberculosis.

Which is of note because even while cholera was terrifying--outbreaks seemed to come from nowhere and raged through communities like wildfires—the everyday assassin of tuberculosis was more common. TB, often known then as consumption, was also heavily romanticized in the 19th century: Thoreau wrote, “Decay and disease are often beautiful, like the pearly tear of the shellfish or the hectic glow of consumption.” Consumptive patients tended to have pale skin, waifish bodies, and pink cheeks—all of which became standards of beauty in 19th century Europe.

But there was no romanticizing the blue death, with its violent diarrhea and vomiting.

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In 1852, a third cholera pandemic appeared. It would be the deadliest yet. Over a million people died in Russia; 10,000 died in London alone. It was during this outbreak that humans began to make serious progress in understanding the disease—an Italian scientist, Filipo Pacini, identified the bacterium that causes cholera, and in London, the doctor John Snow famously proved that cholera was caused not by bad air, but instead by contaminated water. Snow noticed a high concentration of cholera cases in London among people using a particular well on Broad Street.

It was eventually established that cholera spreads via oral-fecal transmission. Human feces containing Vibrio cholerae contaminate water, where the bacteria survive until someone drinks the water, or eats food washed with it. So cholera is spread by people—specifically, our infected feces. And critically, not everyone who gets cholera gets sick—and so some people will travel after unknowingly contracting the disease, and poop in the new places they visit, introducing the disease to new communities. It’s hard to even fathom how fast and deadly cholera outbreaks could be—but to give one example, the tiny town of Boston, Indiana is located about 80 miles from where I’m recording this. On June 26th, 1849, Boston reported a single case of cholera. Five weeks later, half the town was dead.

The fourth cholera epidemic, which began in 1863, killed an estimated 30,000 people making the pilgrimage to Mecca; over a hundred thousand people died in Italy, and over 50,000 in the United States, where the disease spread up the Mississippi River and its tributaries as the sick and asymptomatically infected contaminated water supplies. The fifth pandemic, which began in 1881, was the first to reach South America; the sixth pandemic, which began in 1899, killed more than eight hundred thousand people in India.

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By then, we had the tools to fight cholera—we had a vaccine, although it still only provides protection for a few months; we had IV saline solution, and we understood what caused cholera and how to prevent it. But sanitation projects are expensive and logistically complicated, and by the beginning of the 20th century, cholera wasn’t much of a problem in rich nations. Poor communities, many of which were colonies of rich countries, couldn’t afford sanitation projects, and colonial authorities mostly invested in infrastructure projects that facilitated resource extraction, not healthcare delivery or expensive sewage systems. The seventh cholera pandemic began in 1961, and it's ongoing. Inexpensive oral re-hydration treatment and improved global health investments have made cholera less deadly than it was a generation ago, but still, there are millions of cases of cholera every year, and tens of thousands of deaths. The disease spread to Haiti via UN aid workers in 2010; a huge outbreak in Zimbabwe in 2008 caused thousands of deaths; and thousands more have died of cholera in Yemen after civil war there led to the breakdown of the sanitation system. In this pandemic, cholera has remained a global disease—but one that affects almost exclusively the poor.

As an illness, cholera probably predates the Anthropocene—but its global success highlights what seems to me a paradox of contemporary human life: cholera is a worldwide pandemic because of how profoundly interconnected we are. We're a global species—our bodies and our microbes intersect more and more all the time. But cholera is also only successful because the rich world doesn’t feel threatened by cholera. When an outbreak of the Ebola virus spread through Guinea, Sierra Leone, and Liberia in 2014, and began to affect a few residents of rich nations, the world responded with massive health aid, ensuring the end of the outbreak. 

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But we know cholera isn’t coming here—our drinking water is safe. And so the disease survives because of how deeply interconnected we are and because of how deeply interconnected we don't feel.

Human beings are both the cause of, and the solution to, cholera. In 160 years, we’ve gone from believing the disease was caused by dirty air to understanding how it spreads, how to treat it effectively, and how to prevent it entirely. Partly in response to cholera, most people in the world now have access to clean water. But hundreds of millions still don’t, and as the seventh cholera pandemic rages on, there is no mistaking who is responsible for the disease, and who is responsible for making the seventh pandemic the last: we are responsible.

I give cholera one star.


Thanks for listening to the Anthropocene Reviewed. This episode was written by me, and produced and edited by Rosianna Halse Rojas and Stan Muller. If you'd like to help our podcast, please consider writing a review of it on iTunes or wherever you rate and review podcasts. If you’d like to suggest a topic for review or just say hi, please do so. You can find us on Twitter or Facebook, or email us at

My favorite fact that didn’t make it in to today’s episode: It’s believed that one of the reasons most Americans say "Hailey’s" comet is because of the band Bill Haley and His Comets, whose song "Rock around the Clock" remains one of the bestselling singles of all time. Thanks again for listening. We’ll leave you today not with Bill Haley, but with the main theme from the underappreciated 1987 video game Halley’s Comet. 

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[video game theme]