We can see an object farther over Earth’s curvature if we and the object are higher. Mountain peaks are the highest points on Earth. As a result, from a mountain peak, it is possible to observe another peak very far away, even over hundreds of kilometers. The website Beyond Horizons discusses these records of observation.
Flat-Earthers use some of the world records of the farthest peak to peak observation as “proof” of flat Earth. In reality, these observations are possible because the peaks are very high, and atmospheric refraction helps the observation. These observations agree with the expectation if Earth is a sphere.
The website Beyond Horizons discusses the world records of the farthest peak to peak observation. When this post is published, the current record is 443 km, from the peak of Finestrelles in Spain, observing Pic Gaspard in France. Beyond Horizons is aware that Earth’s curvature and atmospheric refraction affect the observation outcome, and they have entire pages to explain the fact.
These world records of the farthest observation are consistent with the expectation of what will happen if Earth is a sphere and cannot be regarded as “proof” of flat Earth. On the other hand, these observations are impossible if the observers did not do it from a mountain peak, and this fact shows that Earth is a sphere.