Irrefutable proof? Don't make me laugh. Oh, wait. You already did.
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Here is irrefutable proof for you:
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Get out and measure the angle between the sun and the moon during the last quarter moon, which is coming up this coming Thursday, May 18th, at 5:35 pm in the west coast USA.
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There should be three measurements, one at around 11 am on Wednesday (because the moon sets at 11:26 am in the west), and a second one around 12 pm on Thursday (moon sets at 12:22 pm), and a third one around 1:00 pm on Friday (moon sets at 1:21 pm).
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If you can only do one in the morning then that's better than nothing. Thursday morning and/or Friday morning would be great. The moon will be high in the sky when the sun rises, so measure the angle between the two, and post it here.
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By measuring the angle between the sun and the moon at the quarter moon, you can observe the reality that moon phases provide for us to know. Of course, flat-earthers don't want to know the truth, so they won't take the measurement.
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You can prove me wrong by taking the measurement.
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