I’ve been on a history binge recently, and I was reading Herodotus’s ‘Histories’, and there’s a particular story he tells about the Phoenician circumnavigation of Africa (which he admittedly didn’t believe) where he describes a report from one of the sailors where they “on a westerly course round the southern end of Africa, had the sun to their right — to the northward of them.” I saw a lecture recently where Tom Holland (the historian) claimed that this story was true, because Earth is a sphere. If this is true, it’s an incredible story, and I’d love to have it in my back pocket as a story that shows the amazing things we can learn by looking a step or two deeper into the stories and/or data that we’re provided. But I just don’t quite understand what he meant, and how it shows what it shows. Can any of you history folks help a math nerd who happens to love historical anecdotes put this story into my repertoire?
Herodotus, or the Phoenecian circumnavigation of Africa, do not prove that the Earth is round. The observation that the sun was to their north during the southern part of their journey is entirely compatible with a round earth, but it does not prove it.
Consider a flat Earth, with the sun 1000km above the equator. Travel to 1000km south of the equator, and the sun will be to the north, at an elevation of 45 degrees. The observed position of the sun during the Phoenician circumnavigation of Africa can be explained in terms of such a model, and does not require a round Earth. (It does require a nearby sun.)
There are other cosmologies that can also explain these observations, such as cosmologies where light doesn't travel in straight lines. AFAICT, the most interesting of these mainly result in the actual distance of the nearby sun being different from the "straight line" distance. The round Earth and flat Earth cosmologies can be differentiated by precision measurements of the altitude of the sun vs distance travelled south or north of the equator, but Herodotus in this account does not provide such precise measurements. Noting that flat Earth cosmologies require a nearby sun (and stars), this can provide a means of rejecting them. Of course, the observation that the shadow of the Earth on the Moon during a lunar eclipse shows that the Earth is spherical is also available.
Noting the very long time I took to answer this question, I'll also link the recently-asked identi-question: https://www.reddit.com/r/AskHistorians/comments/brj7bw/did_herodotus_accidentally_prove_that_the_world/
For some fun reading on ancient flat Earth comsmologies, see Dirk L. Couprie, When the Earth Was Flat: Studies in Ancient Greek and Chinese Cosmology, Springer 2018.