Let's go through all of the proofs for a spherical earth.
1. The Sky:
On earth, we see the sky rotating. On the north pole, we see the sky rotating counterclockwise, and on the south pole, we see the sky rotating clockwise. Also, on the north pole, we see a completely different set of stars than the south pole. Also, when you are on the equator, you see all of the stars moving in a straight line across the horizon, rather than apart of a rotating plane. Also, over thousands of years, we can have new north stars.
How does one explain all of these phenomena? Well, it's simple: the earth is a rotating sphere, orbiting the sun, and is moving through the galaxy over time. When you are standing on the north pole, you are rotating along with the earth, and so from your perspective, instead of you rotating, it's the stars that appear to be rotating. And a person who is standing on the south pole is upside down relative to the person standing on the north pole, so they are going to see the sky rotating in opposite directions, because they themselves are rotating in opposite directions, but for both of them, they are standing upright. We see two different sets of stars on the two poles because one set of stars can only be viewed on one half of the earth, and the other set of stars can only be seen on the other half of the earth. And finally, we get things like a different north star every six thousand years because the earth is traveling through space, along with its orbit around the sun, and the sun is traveling through the galaxy.
Explaining all of these phenomena with a spherical earth is really easy. Not for flat earth. How could you explain all of these phenomena using a flat earth? And make sure to explain all of them simultaneously, because of course, all of these phenomena are happening simultaneously.
2. Day and Night:
On earth, there is a period of time in which the sun is lighting up the area everywhere, which we call day, and there is a period of time in which the sun is not lighting up the area everywhere, and everything is much darker, which we call night. But day and night are not as rigid as just twelve hours of day and twelve hours of night. If you were to go to the north or south pole, it is daytime for six months, and nighttime for six months. And areas in between that have in between quantities of day and night. How does this happen? Well, let's go back to our spherical earth model. The earth is spinning, once per day, and is orbiting the sun. If the earth is a sphere, then the intense light from the sun is only lighting up exactly half of the earth at any given moment. The half of the earth that is facing away from the sun is nighttime, because the sunlight is not capable of reaching that side of the earth. And the reason the poles experience day for six months and night for six months is because the angle of the earth's rotation is bringing them a significantly shorter distance across itself than if you were on the equator. Here, your movement going along with the earth traces a much smaller circle than if you were on the equator. Such a circle could potentially be so small that it never actually reaches the other side of the earth, and thus, it never has the chance to escape the sun's luminescence. But since the earth is tilted, this means that this is only true for six months. Once it reaches the other side of the sun, now the opposite is true, where your circle of movement that moves along with the rotation of the earth never reaches the other side.
That was difficult to describe in words, but rest assured, we know exactly why this is happening. How do you explain this on a flat earth? How do you get exactly half of the earth to experience day, and exactly half of the earth to experience night at any given point in time? And while you do that, have the poles experience six month days and six month nights? I'd like to see you try.
For three months, the weather is pretty nice. Then for the next three months, the weather gets significantly hotter. Then for the next three months, the weather gets significantly cooler, and the leaves on the trees turn bright red and orange colors, and then they fall off of the trees. And t hen for the final three months, it is significantly colder still, so cold that the water vapor floating in the air (clouds) freezes, and clumps of water particles crystalize into a snow flake. This causes billions of snow flakes to fall down to the ground, and these snow flakes clump together to create what we know as snow. How does this happen? Well, let's go back to our spherical earth model. Why is the earth tilted? The very specific tilt of the earth serves a very specific purpose. In areas where the path of the sun's heat is directly on, it is much hotter, because the heat is not spread out at all. But near the edges where the sun's heat nearly misses the earth entirely, the heat is now traveling horizontally relative to the pole instead of vertically. This causes the distribution of sun heat to be much greater, meaning the amount of heat per square mile is significantly less. This means that it will be hotter closer to the equator, and cooler closer to the poles. But such an earth would not be very efficient, as it would mean that there are entire chunks of the earth where they are too cold to be habitable all year 'round. So what did God do? He tilted the earth by 23.5 degrees. This means that for six months, a particular area on the equator was pointing at an upward angle during the day, but then when the earth looped around to the other side of the sun, that part of the earth was pointing at a downward angle during the day. This alternates the distribution of heat, which means that if an area is uninhabitably cold for a time, it can be circulated so that it's less cold. This is the phenomenon we know of as seasons.
How do you explain seasons on a flat earth? I'd like to see you try.
4. Eye Witness:
I wanted to save this for last because it is the most commonly sighted thing when people look for arguments for round earth, but of course, technically all we have is the people's word. But there are a select few number of people on this planet who have actually seen the earth with their own two eyes. We have even taken pictures of the earth and what it looks like from space. These people would have a thing or two to say about the flat earth idea, because they have seen for themselves the fact that the earth is a sphere. This is why many flat earthers resort to conspiracy, as otherwise, such evidence would be far too much to handle.