"Cosmos" host and astrophysicist Neil deGrasse Tyson recently shot down the ideas of anyone who still thinks the Earth is flat. In a new YouTube video on the scientist's StarTalk channel, he used examples ranging from space observations to ancient Greek experiments in a conversation with stand-up comedian Chuck Nice.

"So, tell me, Neil, is the Earth flat?" Nice said to open the conversation.

"We have video from space of the rotating, spherical Earth. The Earth is round," was Tyson's laconic response. [Even a Kid Can Prove the Earth Is Round: Here's How]

Satisfied, Nice joked, "Thank you for joining us on this episode," and pretended to wrap things up. But the co-hosts continued their conversation in a 9-minute video that is based on a chapter in Tyson's new book, "Astrophysics for People in a Hurry" (W.W. Norton, 2017).

The science argument

Astrophysicist Neil deGrasse Tyson wants everyone to know that the world is round. (Image credit: National Geographic Channel)

"What's odd," Tyson continued, "is there are people who think that Earth is flat but recognize that the moon is round. Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and the sun are all spheres. But Earth is flat ... something doesn't square here."

Tyson explained that because of the laws of physics and the way energy works, the universe "favors the sphere" when forming planets and other bodies. Sometimes, a sphere might be distorted because it's rotating very fast. But almost everything in the universe, he added, is spherical or almost spherical.

Tyson did not address asteroids, which are small bodies of ice and rock and are irregularly shaped. It is widely recognized, however, that these asteroids have a gravity too low to pull their mass into a sphere. Worlds orbiting the sun that do have spherical bodies are sometimes called planets, but only if they meet certain criteria set by the International Astronomical Union.

In the example of Pluto — once considered a planet — the IAU ruled in 2006 that the body is not big enough to clear debris from its path and therefore should be reclassified as a dwarf planet. That's a controversial definition, one that Pluto's New Horizons spacecraft team does not agree with. But for his part, Tyson (as shown in many other sources, such as this 2017 clip from "The Late Show with Stephen Colbert ") really, really doesn't believe Pluto fits the definition of planethood.

Tyson was similarly outspoken in the new StarTalk video. He said there are people in the U.S. who believe in a flat Earth for two reasons: the country protects free speech, and its educational system doesn't teach students to think critically about the evidence.

The geometry argument

There's even more evidence that the Earth is round, Tyson said. During lunar eclipses, the moon passes into the Earth's shadow and lies directly opposite to the sun. The shape of the Earth's shadow is always round in these eclipses. If Earth were a flat planet, at times, you would see a flat shadow on the moon. But that's never happened, Tyson added. The Earth must be a sphere, because that's the only thing that casts a perfect circle every time.

Or think about a ship sailing toward the horizon, Tyson said — it gradually disappears, because the Earth is curved. Or, he said, if he were to send co-host Nice on a journey around the Earth, Tyson could turn his chair 180 degrees and eventually see Nice arriving back where the comedian had started. (Nice quipped that would take him 150 years to accomplish the trip, because "I'm no Forrest Gump. It would take me that long to run the Earth.")

Tyson further described an ancient experiment by the Greek Eratosthenes, who lived in Alexandria around 250 B.C. Eratosthenes noted that in an Egyptian city called Syene (near modern-day Aswan), you could see right to the bottom of a well at noon during the summer solstice. On the same day, you couldn't see to the bottom of a well in Alexandria.

There are two possible explanations for that observation, Tyson added. The first is that the Earth is flat and has a small sun, close to the planet. The second is that the Earth is curved, with a sun further from the planet. But if you were to extend the argument to three wells, there's no way a flat Earth's geometry would fit the experiment's results, he added.

So, what if people still believe the Earth is flat?

"That's OK," Tyson quipped, "as long as you don't run NASA."

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