I watched Behind the Curve (2018), a documentary about the Flat Earth movement. In the beginning, Mark Sargent says (I paraphrase), “I know the Earth is not round because I can see Seattle from here [Whidbey Island].”

If I knew the distance from the Space Needle to Sargent’s house, the altitude of that house and the altitude of the lowest part of the Space Needle visible from there, I could put an upper bound on the curvature.

I recently read the Long Earth saga by Terry Pratchett and Stephen Baxter. The story begins in the near future when an eccentric engineer anonymously publishes plans for a “stepper box” which takes the user to a parallel world, adjacent in a chain of millions.

It soon emerges that a few humans have the talent of Stepping without a box. In one episode, two of these (including an ancestor of a main character) help the Underground Railroad, provoking in me a question they did not ask: Rather than sneaking the escapees to Canada, what if we leave them in a side-world? They’d have to learn to live Paleolithic-style (metallic iron cannot be transported), but that life evidently was not so bad.