That's not a question about FET, you can read one of the hundreds of topics that already exist on that subject.
Actually it is, sorry. I should have made it more clear why I was asking. I have regular debates with this coworker of mine, and as a new member, I just wanted that one piece of evidence from a die-hard believer that would really deliver the "whammy" I'm looking for. Sorry to bother you, but anyone else can feel free to respond.
One easy one to observe is that when the moon and sun are in the sky together, the light of the moon does not point at the sun. It's called the Moon Terminator "Illusion".
RE'ers spam out perspective effects as an explanation, and link to close range examples where angles change at close range as bodies change distance radically, but they are unable actually show how such perspective changes are possible with Round Earth Geometry, where the earth and sun are far from the observer and equal distances away from the observer at all times.
It is possible to hold a pencil out horizontally and look at it from different angles close up to get the pencil to point upwards and downwards with close range perspective effects; but doing is harder to do so when the horizontal pencil is at a distance far from the observer. Those same motions would not cause the pencil to change. When the pencil is far from the observer it would require an equivalent change of ratio, which the Round Earth System cannot provide. In RET the sun and moon are far from the observer and equal distance from the observer at all times.
We are spammed with hallway examples where, when standing in the middle of a hallway, the angles of the ends of ceiling corners of the hallways are tilted upwards or downwards, but the concept of the sun and moon are "going down a hallway" and changing distance radically from the observer is actually the Flat Earth example, not a Round Earth example. Under RET the sun and moon are far away and maintain the same distance from the observer at all times. They do not change position in relation to the observer radically.
We are spammed with demands to hold a small ball up against the sky with the moon in the background and position yourself in a manner where the ball shows a similar scene to the moon. "Proof!" Yet, this is another dumb close range perspective effect, and is no different than the pencil explanation. It is possible to get a ball that is close to you into many orientations with slight movements, and, like the pencil, not so possible when the object is far away.
At some point astronomers did provided a mathematical equation to predict the changes, but unfortunately the distance to the Sun or Moon do not matter in the equations for the result; and so the predictive model is entirely frivolous as a Round Earth model.
It is a clear win for the Flat Earth model, and one that is easily observable to anyone. Look past the lies and excuses. The evidence is right there in the sky!