I would love to hear someone's answer to this.
1: There is a live webcam on the I.S.S. which allows you to see a view of the earth 24/7. So either NASA is somehow able to draw up photorealistic 30 frame per second video of the earth from a consistent viewpoint, constantly changing and showing the actual events on the ground and in the weather as they are happening, or there is something wrong with the 'space flight doesn't exist because it doesn't support my opinions' conspiracy.
2: You can see satellites from the ground. And the I.S.S. You can find dozens of amateur astronomy websites which will tell you when the I.S.S. will pass over any part of the world next. And you can go and watch it. I've done it myself several times. I was even lucky enough to have it pass over while a space shuttle was approaching to dock, so that I could see the two arcing over the sky, seemingly with the space shuttle chasing the station down as it closed to dock. I've even had the pleasure of seeing the work of an amateur photographer who's hobby is taking pictures of the I.S.S. from the ground. Yes, real pictures taken with a normal nongovernment funded camera from right here on Earth. My favorite was his picture of the I.S.S. passing in front of the moon, a rather impressive feat of precision.
3: You can see the moon landings. I've also had the pleasure of seeing through a friend's telescope the dropped Saturn V booster which impacted the moon to provide data for geological sensors left behind by a previous Apollo mission. Undoubtedly, FE supporters will claim with no proof that any images of things such as the moon buggy or the bottom half of the lander are hoaxes because they disagree with their opinions. Nonetheless I will leave these rather cutting pieces of evidence to the side at the insistence of FE supporters that any photo evidence that contradicts their opinion is fake.
4: Commercial and international spaceflight. It's one thing to say that the Apollo program, and the literal millions of people involved with it all had 100% loyalty to the government, and that of all the countless scores of people who've been involved with the space program remained faithful and never gave up any secrets despite the fact that they were holding the truth away from the whole world for reasons that didn't benefit them in any way. But it's a whole other thing to say that every country involved in the space industry has never had anyone ever become disillusioned and gave away their secrets? What about the U.S.S.R.? They aren't even a country anymore, and most of their former population now resents or even hates what they did to them. So out of these all now resentful people, not one of them decided they would spill the beans that they had been lying for their country the whole time? What utter foolishness.
5: Seeing the curvature of the earth. I have seen the curvature of the earth. I am a hiker, and my favorite hikes are those that bring me to those glorious peaks, thousands of feet above the ground below, where the air is thin and the sky clear for hundreds of miles. And I have seen the curvature of the earth. No illusion, I have legitimately seen the world drop away as it gets farther away, so that far ridges and mountains look like they are half buried beneath the horizon. Not to mention the fact that through amateur rocketry I've also recorded video of the earth miles above where I can stand and look down from. And the curvature only becomes more apparent. Someone once on this forum said that this can be accredited to distortion of the camera. So, just for fun, I set up a straight grid of paper facing directly at the camera and took a picture. As you might expect, the grid was not distorted at all. The lines were still completely straight, not a pixel off.
6: Communications satellites. Communications satellites such as GPS, satphone, TV, etc. Are for the most part commercially funded and launched. And their affects are measurable from the ground. Not only can they be seen in the cases of Iridium Flares, but they also can be tracked and signal to them monitored. GPS signal depends on constant communication with several satellites, all of which must be tracked and connected to independently. So why would these companies who make GPSs decide to make GPSs disconnect for no reason to simulate loss of connection to satellites that don't exist? Why would they worsen their product compared to competitors for the sake of a conspiracy that doesn't even benefit them? Answer- they don't, because the satellites are actually there. And if this signal isn't coming from satellites, where in the (flat) world is it coming from? And why does the signal track to the sky if there is nothing there?
7: Accuracy of maps. If the flat earth world actually existed, langitude and longitude would break down, because they are based on a spherical earth and as such you would not be able to navigate a flat earth with a curved grid. It just wouldn't work. So why does it? Because the earth is curved. And the world itself would be physically broken in the process. If the world really is flat, then how does it narrow towards the poles? How in the (flat) world can you explain the fact that distances on a 2D plane don't line up with the spherical world, and as such distance is often misinterpret elated on flattened maps of large areas because the curvature of the earth is very incompatible with a flat world. In short: a 2D earth would break all of the physical natural laws because it would require the world to widen and narrow to fit it's 2D constraints, and these 2D distances don't line up with the real world.
8: The aether. Oh god why. The real world doesn't line up with your opinions, so you just make something up to explain it. I don't even want to tackle how revolting this piece of non-logic is.
9: Antarctica. In the FE hypothesis', Antarctica is always described as an ice wall encircling the 'borders' of the Earth. Why is this utter lies? Because you can go over and under the ice. We've crossed Antarctica. And came up on, you guessed it, the other side. You can claim its just another conspiracy, and they just magically teleported him around the circumference of the outer circle of the world with a whole bunch of also hoaxed scientific data and photographs. Yah right. We can also fly over it, drive over it, pilot submarines beneath the ice sheet, etc. Antarctica is not a wall.
10: Antarctica again. How could this massive southern ice plate even exist if it weren't for the earth being spherical? It gets colder the closer you get to a pole because of light hitting the earth at a more oblique angle due to the earth's curvature, thus scattering the light more and absorbing less light of what does hit the ground, and thus less heat. So... Is there some mystical source of cold at the ring and center of a flat earth or is there just a round earth?
11: Seasons. The FE hypothesis suggests that the sun is just a big lamp in the sky pointed at us, even though the other planets are lit up by it while they are orbiting around it. So if that were true and the sun is a magic wobbling spotlight floating above the earth, then why would it wobble with variations that produce the seasons? Assuming that there is some unknown force holding the sun a fixed distance from the surface and wobbling it, then how could this force be imprecise enough to cause the sun's wobble to wobble over time, but somehow also contain this second wobble so that it only deviates a certain distance? It comes to mind how rediculous it is to make all this up to prop up the FE hypothesis rather than just admit defeat.
12: Gravity variations. The FE hypothesis requires gravity to not exist, or be weakened in some form, or else the FE would be destroyed by its own weight. So instead they say that the earth is accelerating upwards at 9.8 m/s constantly by some unknown force which is also doing the same to the moon and sun. But this hypothesis does not match up with reality. It sounds, well, sound, unless you think about it a little bit more. Gravity weakens with distance, and as such if you measure gravity from a high elevation, it is weaker than 9.8, and if you measure gravity closer to the earth's core, the gravity is higher. But if it isn't gravity and rather acceleration, then this would mean the higher parts are moving slower and the low parts moving faster. So then we get that Louisiana would break off and fly ahead while the Himalayas would punch out and fall behind the rest of the world. More likely, the intense variation of heights would cause the FE to ripple and curl until it shattered and destroyed itself in a cataclysm of molten rock.
13: The atmosphere. I'd like to think that everyone here will freely admit that we breath oxygen, and that oxygen (as well as all the other major constituents of the air we live in) are gasses. (At least in their atmospheric temperatures) Well then everyone here has just disproved flat earth hypothesis yet again, because if the earth were flat, and not tearing itself apart, then the atmosphere would just blow away to the edges and fall away into space.
I personally can't wait to hear all the replies, and to get the arguments rolling. Everyone play fair, and may the best facts win!