1. Eratosthenes then measured the angle of a shadow cast by a stick at noon on the summer solstice in Alexandria, and found it made an angle of about 7.2 degrees, or about 1/50 of a complete circle. He realized that if he knew the distance from Alexandria to Syene, he could easily calculate the circumference of Earth.

  2. For those asking:

    A detail left out is that he did this on the summer solstice. It was known that at noon on the solstice in Syene such an obelisk would cast no shadow. This was not the case in Alexandria. So all he needed was the length of the shadow at noon on the solstice in Alexandria to construe the angle between the two obelisks and the distance between the two cities.

  3. *Demon Haunted World* might be my favorite. He writes like you’re sitting next to him. To be a genius like him would be grand, but to also have the emotional intelligence he had to not only educate, but to be so empathetic in combatting pseudoscience…we need another Sagan and I know that’s just not in the cards. What a gift to humanity.

  4. I’ve always been fascinated by how ancient philosophers & mathematicians developed complex scientific models or technology that we would assume belongs in a later century.

    If I said that photovoltaic panels were invented in the late 19th century, and the first solar panel was installed in 1884 New York, people would be rushing to Google to confirm my claim. [Here’s a nice write-up.](https://www.smithsonianmag.com/sponsored/brief-history-solar-panels-180972006/)

  5. Flatearthers will argue it’s flat because they have eyes and brains as well.

    So, it’s not that obvious. At least to some ‘special’ people.

  6. Can somebody explain to me how they were able to accurately determine that they were measuring the shadows at the same time? I’ve always loved this story, but my brain can never get past this one detail.

  7. This doesn’t make sense because how would the Greek guy know the different lengths of the shadows at the extract same time of day, did they have an accurate way to tell time, so he could stand at each obelisk and measure the shadow?

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