If the earth is flat we would not see it going from day to night and night to day the way we see it.
If the sun is always above the surface of the earth then there has to be a separation between light and dark because it is not always light and it is not always dark.
That means when it is light in our area we would see the darkness coming but it would still be light in our area and when the darkness passes through our area we would see the light moving away from us until it is gone.
And when it is dark in our area we would see the light coming but it would be dark in our area and the light would pass through our area and the darkness would move away from us until it is gone.
But on a globe when the sun goes down it is dark as far as our eye can see and when the sun comes up it is light as far as our eye can see.
We do not see the darkness chasing the light and the light chasing the dark.
They believe because the Bible says the 4 corners of the earth that the earth is a flat circle.
Psalms 113:3
From the rising of the sun unto the going down of the same the LORD'S name is to be praised.
If the sun is always above the surface of the earth on a flat earth how is it going up or down.