A long time ago, I thought about writing a series of books called How Do We Know? I still think it’s a good idea. However, I think it might work better as a series of books for kids rather than for a general audience.
That’s today’s insight. I’m probably better off writing science books for kids than for adults. At least for now.
Here’s an example of what I have in mind for the Astronomy book.
How do we know Earth is round?
When you look at pictures of Earth taken from space, it’s easy to see that Earth is round.
While it’s not a perfect sphere, satellites can go all the way around Earth in as little as about 90 minutes or so.
The real question is: How did we first realize that Earth is round?
Eratosthenes, a Greek scholar and librarian, is credited with being the first person to calculate the circumference of Earth. He did this before the year 200 BC, which means at least some humans knew Earth was round before then.
How did they know?
They probably watched ships sailing out into the ocean. As the ships got further away, they looked smaller.
More to the point, the top of the ship was still visible above the horizon after the bottom of the ship could no longer be seen.
If Earth was flat, the ship would just get smaller and smaller until you could no longer see the whole thing.
Instead, people could see that the bottom of a ship drops below the horizon and out of view before the top of the ship.
The simplest and correct explanation for this behavior is that Earth is round.
How Do We Know?
If I’m not mistaken, I first learned about this while teaching Conceptual Physics, using Paul Hewitt’s wonderful book.
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