Carl Sagan - How We Know The Earth Is Round

[Swede]

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[Blue Owl]

1. K&R

[Deuxcents]

2. I miss Carl Sagan

I taped every one of his “Cosmos” series on PBS years ago. He has a video called The Blue Dot and it’s got Pink Floyd’s “On The Turning Away” to make his point. It’s on YouTube but I’m not techie to transfer it here. Carl Sagan left us too soon but was good to see him here. Thanks

[mitch96]

8. Is it this one? Just copy/paste the URL and it shows up automagically...

[Wounded Bear]

3. Which of course leads us to the Columbus question...

Simple minds think that there was an argument about whether the earth was flat or a sphere, but in fact 'everybody' pretty much knew the Earth was round, the argument was about how far around it was, and how far away China would be on that globe going west. Columbus was actually wrong by about 10k miles or more, and of course he ran into a continent that most folks didn't know was there.

[msongs]

4. a good experiment for both science classes and flat earthers nt

[mitch96]

9. Definitely an education for flat earther's.. nt

[abqtommy]

5. "More Than 2,300 Years Ago, A Greek Geographer Proved The Earth Was Round"

https://www.ancient-code.com/2300-years-ago-greek-geographer-proved-earth-round/

Yes, most people in the know back in the days of Columbus knew the Earth was round.
But legends and misinformation always persist, even in our modern times.

[William Seger]

10. That article makes the same mistake the Cosmos segment does

The experiment does not "prove" the Earth is round unless you accept the premise that the sun is so far away that the rays are parallel. An alternate explanation for the difference between the shadow's lengths is that the sun is close, i.e. if it were about 65,000 km away from a flat Earth, that would explain the 7-degree difference.

[abqtommy]

11. And yet at the end of the day we have sufficient proof that the earth is round. It interests

me that you appear to dispute this.

[William Seger]

12. Well, what I actually said was, *that* experiment doesn't prove the Earth is round

... unless you accept the premise of a very distant sun. The critical thinking that Sagan admirably championed requires examining the argument, doesn't it?

[muriel_volestrangler]

13. But it does mention, before Eratosthenes, circumpolar stars and ones never visible in the north

which is good evidence for a round Earth. With the angle of the pole star above the horizon differing by the same amount as the difference in shadow angle at the 2 locations, you do have a good idea that it really is a sphere, and the distance between the locations then gives its size.

[William Seger]

14. Yes, I know

The round Earth theory was accepted because it was the most logical explanation for ALL the available observations, and there were several more, such as ships going over the horizon and the shadow of the Earth on the moon during an eclipse. The only point I was making was that Eratosthenes' analysis doesn't "prove" the theory -- it's not that simple -- but I guess I didn't make the point very well.

As for Eratosthenes' calculation of the circumference of the Earth, that still depends on the premise that the sun is far enough away that the rays are essentially parallel. If the sun was, say, only 100,000 kilometers away, only part of the angle Eratosthenes measured would be due to the curvature of the Earth, which would mean a much larger Earth. You can't really use the shadow observations to calculate the size of the Earth unless you know the distance to the sun, but Eratosthenes' calculation turned out to be accurate because the premise turned out to be true. It's sort of like the case that some of Galileo's arguments for a heliocentric solar system depended on premises he couldn't really prove, but which turned out to be correct.

[BootinUp]

6. One of my favorite Sagan segments. Nt

[William Seger]

7. Actually, that segment has always bugged me for being over-simplified

When he says that the only explanation for the shadows is that the Earth's surface is curved, that's only true if what he says later is true: that the sun is so far away that the rays are parallel. He doesn't explain how Eratosthenes knew he was calculating the circumference of a round Earth instead of assuming he was calculating the distance from a flat Earth to a fairly close sun.