Here is a simple experiment you can do yourself to find out if the earth could be flat. Make a circle out of cardboard. Also get a small round Christmas ornament (opaque) and a flashlight. Observe that no matter what angle you look at the ball-shaped ornament from, the ball portion of it presents the same round shape. If you take a picture of the ball, the ball occupies a circle on the two-dimensional image. This is true no matter what angle you take the picture from. The circle made of cardboard only looks like a circle from two angles, the direct front and the direct back. When you start walking around it and looking at it from different angles, it does not present a circle. If you take a picture of it at an odd angle, the two-dimensional image of the circle in the picture will not actually look like a circle. Now shine a flashlight at the wall and dangle the ornament in the beam. The ball portion of the ornament creates a round shadow that does not change its shape no matter how the ornament turns. You can even tip the light back and forth a bit and watch the shadow move, but still remain a circle. If you take your cardboard circle, you will find its shadow is like its picture: it is only a circle when the light hits it dead on. It changes size and shape when you turn it to different angles. The simple way to describe this phenomenon is that the profile of a globe is always a circle, but the profile of a circle is only a circle at a perfect right angle. Now, look at the moon. The moon glows because it reflects light from a massive source. The reason that it goes through phases other than full is that something massive is blocking that light, partly or (in the case of a new moon, wholly). We can tell that the light source, light blocker, and/or projection surface (the moon) are moving relative to each other because the shadow changes position. And we can tell that whatever is doing the blocking is a globe, because the shape of the shadow always shows the same curve no matter what angle the moon is at. The profile of a globe is always a circle. The profile of a circle depends on the angle. Now, an invisible object couldn't emit light because we would see the light it is emitting (this is how we see the sun) and an invisible object couldn't block light because we would see the lack of light as it hits it (this is how we see black objects). Therefore the light source must be something we can see and the light blocker must be something we can see. There are no massive objects visible in the sky besides the sun to account for the light, and no other visible massive objects in the vicinity of the earth and the moon that could be casting that shadow. So when the sun is shining on the other side of the earth from the moon, if the earth and moon are lined up the right way for the shadow of the earth to fall on the moon, you see the curve of the earth. And that curve is always circular. And the only object that casts an always-circular shadow is a globe. A circle won't. A cylinder won't. Even other round objects don't cast always-circular shadows on distant objects. Only globes do that. Now, you could say, "That's all very well and good, but you have to accept the sun is shining on the other side of a round earth for that to work. I don't accept that and you can't convince me." You're right. I can't. No one can actually convince anyone of anything. This is the meaning of the saying "You can lead a horse to water but you can't make it drink." People who like to think they are as skeptical as you are are basically waiting for magic: someone says the right words in the right order and, hey, presto, their mind is changed. No one can actually change your mind. You have to do it yourself. The tools to do that are critical thinking, which is not just being critical of everything in the sense of "Pshyeah, that's probably a lie." I've given you the tools to see how a round earth explains the facts that you can observe with your own two eyes. Everything about the situation you can observe on the moon lines up perfectly with the round earth model and with simple experiments you can do yourself. Could there be another explanation? Maybe the moon has this weird luminescent cycle that *just happens* to perfectly mimic what we would expect based on the observable properties of roughly spherical objects orbiting each other, perhaps? If you believe in that level of coincidence, you forfeit the right to ever call yourself a skeptic. It's not just the shadow thing. We could keep going and go through all the things that the round-earth model perfectly predicts and accounts for but which the flat-earth model cannot sustain without adding a bunch of wild, evidence-free suppositions ("maybe there's an optical illusion that happens when you get high up enough that makes the horizon appear curved, maybe earth's magnetic field is distorted in exactly the right way to make instrument readings appear to support a round earth, maybe every single person who has ever worked in the satellite telecommunications industry is part of a global conspiracy, maybe every single airplane that flies around the world happens to get turned around"). For every single thing the round-earth model fits perfectly, you can throw out as many conjectures and objections as you want or need to, and no one can stop you. Not a single person can make you engage your critical thinking, consider the evidence, and weigh the balance of probability. No one has that power except you. And all of this is why people would rather dismiss you than argue with you. This is why people mock your comprehension and knowledge. They know it's not going to change your mind, but neither is anything else. Even if this post and my simple experiments sway you, it's still going to be you who decides to be swayed.

Oh, I should add about shadows: the shadow of a globe will be a stretched-out circle (ellipse) if it's being projected on a wall at a weird angle for the same reason an image from a film stretches when projected at a weird angle, but the key point is, its profile is still round, curved all the way around. The shadow of the circle doesn't show its curves in that same situation. So perhaps I should have said "the profile of a globe is always round", for precision.