Sun setting over the beach

The curve of the Earth can be observed by experimenting with the horizon over varying distances. However, we have additional proof of Earth's roundness in the rising and setting of the sun. If the Earth was flat and the sun was always above the plane of the Earth, then why is it's light not always visible and why can we not see it?

Speed of Light Testing Apparatus (Fizeau)

At this time (2019) flat Earth theorists have devised two wide-spread theories about what causes "gravity", or at least what causes the effect that we call gravity. The first theory, which is fading among the older flat-earthers, is that the flat earth is accelerating upwards which causes a downward force on the surface of the earth. The second is density theory. We will address universal acceleration in this post.