That gorgeous arc that paints the sky is not an arc after all. What appears to be an arc is actually a circle, AccuWeather explains.
The horizon obscures the parts of the circle that we can't see. As the sun gets closer to the horizon, more of it becomes visible. At sunset we can see a full semi-circle. As the sun gets farther from the horizon, we see less and less of the circle, and therefore it appears only in an arc shape.
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There is a way to see the full circle, according to AccuWeather: Go up in a plane.