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If I'm to change my view from "we are not on a sphere" I need something that I experience for myself that would prove the spherical nature of the earth. Else, I am putting what I experience in my own life to the side to believe in what authority and consensus tells me.
For me the boat disappearing behind the curve just doesn't do it for me as with stronger and stronger viewing devices I can bring the ship back until the atmospheric conditions make that impossible. Yes I have really done this as when I was first introduced I tried to disprove it as a science project with my son. Its the same as watching your friend travel away from you a distance. He will start disappearing form the bottom half as he moves out in the distance.
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I think the question you need to ask is why does an object moving further away always disappear from the bottom?
You know from your perspective research that as an object moves away from the viewer it gets smaller and smaller until it reaches an angular size that the naked eye can no longer distinguish from the background. It's at this point that you can use a camera, telescope or binoculars to bring the object back into view if their magnification levels are adequate. At no time does any edge of the object start to vanish before any other edge, this only occurs when another object obscures the original object.
So taking all this into consideration, if an object starts to disappear from the bottom as it moves further away from you, what is obscuring the object?