Flat Earthers like to claim that the "light in the sky" we're calling the ISS is actually some other object that is flying significantly lower. It kind of makes sense - if the Earth is flat and space doesn't exist, satellites can't be a thing either, so the ISS must be something else, like a balloon or a drone. The possibility that people constructed something 400 km above the Earth's surface is in direct opposition to their worldview, for example because data from such an object directly proves that there is actually a vacuum there, and that the Earth is round.
So if there was a simple way of checking that the ISS is actually flying at an altitude of 400 km, it would be a hard blow to all kinds of flat Earth claims. Luckily for us, a fairly simple method exists. I suggested it a few years ago to a flat Earther, and eventually used it in practice in 2020. Here is how I did it.(more…)