The easiest nail in the coffin of the Flat Earth
An object that is far away looks smaller than an object near to you. This perspective effect is universal and the required math is simple. And it works identically with any object, big or small.
One reader of my blogs recently sent me a link to a video that “proved” that the Sun is actually much closer than the claimed 150 million kilometers. Unsurprisingly, the video’s approach to demonstrating “size” was flawed, and the truth can be revealed by pretty much any camera these days.