I have always imagined how an early astronomer like Tycho Brahe (1546-1601) would address the present-day flat-earth advocates.

I believe he might say something like this:

“Good people, I did not rely on speculation—I measured the heavens night after night with instruments of great precision, long before telescopes existed.

First, consider the stars. As one travels north or south on Earth, the positions of stars change in the sky. New stars rise into view while others disappear below the horizon. This only makes sense if you are standing on a curved surface. On a flat plane, all observers would see the same stars in the same positions.

Second, observe the elevation of the Pole Star. As you journey north, it climbs higher in the sky; as you go south, it sinks. I measured such changes carefully. This is exactly what one expects on a sphere, where your angle relative to the heavens changes with position.

Third, ships at sea vanish hull-first over the horizon. I did not need a telescope to understand this—geometry alone explains it. A curved surface hides the lower parts first.

Finally, during lunar eclipses, the Earth’s shadow cast upon the Moon is always round. Only a sphere casts a consistently circular shadow from every angle.

These are not matters of opinion. They are measurements—repeated, consistent, and visible to anyone willing to observe.

If the Earth were flat, the heavens would behave very differently. But they do not.”

My imaginary Tycho Brahe statement above, is also the entire premise and assertion of r/flateartbs.

Enjoy this subreddit and above all have fun and be kind in whatever your view of Earth.