system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did ancient people know that the earth was round?
gpt
The two major arguments known to the ancients were these:

* Observing that if you travel northwards or southwards by any specific distance, the angle of the North Star above the horizon will also change by a specific predictable amount, implying that the Earth’s surface has a consistent curvature, like a sphere.

* Observing that the shadow the Earth casts on the Moon during a lunar eclipse is always a circle, and realizing that the only shape which always casts a circular shadow is a sphere.