First, that's an artist's depiction of what they think is occurring and second, why do you think it does?
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Yes, it's an artist's depiction, that is, a
graphic artist. The curving line is plotted by observational data.
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It's not "what they think is occurring" it's what has been tabulated and recorded, in fact.
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Remember the total solar eclipse, how all the data was calculated and predicted and the eclipse did precisely what was anticipated?
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The scientists who foretold where the moon's shadow would fall minute by minute were not flat-earthers BTW.
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It depicts a snapshot of where the sun's daylight illumines the earth and where it does not, at one moment in time.
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There is no way to get a photograph of this because it's shown on a Mercator projection, which is artificial, and necessarily so because to make a spherical object appear flat for illustration you have to include particular distortions.
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But the distortions don't have to interfere with the overall shape of the day/night transition profile.
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