Flat-earthers use flawed math and doctored images and videos to infect the gullible. They claim it gets dark at night because the sun is simply too far away and gone past the vanishing point. But when out on a big body of water with a true level horizon, the sun and moon are clearly not reduced in size as they clearly dip below or rise above the horizon. If anything, the sun and moon appear larger because of the extra thicknesd and curvature of the atmosphere acting as a magnifying lens / prism.

Then the days in the southern hemisphere would be shorter in their summers than ours and the sun would have to travel through the sky faster, just like if you put a dot near the edge of a record and a dot near the center. The dot near the outside edge aleays travel faster. That is not the experience in the southern hemisphere.