There are soooo many easy ways to know the earth is round. I will give you 1, that is centuries old and UNDENIABLE!
A flat disc can cast a round shadow only if the light hits it straight on.
But lunar eclipses happen at many different angles as Earth, Sun, and Moon move — yet the shadow is always perfectly circular.
The only 3‑D shape that produces a circle from all orientations is a sphere.
This has been observed for thousands of years, from Aristotle to modern astronomy, and is still visible today.