They lived a long time ago, therefore they had to be stupid. I find this offensive. An ancient Greek philosopher calculated the circumfrence of the Earth with pretty decent accuracy given the tools available to him at the time. The ancient Egyptians were not dumbasses unable to make observations and record them over time.

https://en.wikipedia.org/wiki/Eratosthenes

https://www.khanacademy.org/humanities/big-history-project/solar-system-and-earth/knowing-solar-system-earth/a/eratosthenes-of-cyrene