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Where did all the circles in apod.nasa.gov/apod/ap181109.ht come from? Some of them really are circles, traced by stars across the night sky. They form an elliptic pencil of circles (en.wikipedia.org/wiki/Apolloni) converging to the north and south celestial poles. Others come from the latticework of a lookout tower, transformed into more pencils of circles. The horizon also appears, less clearly, as the small dark circle near the center of the image.

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Mathstodon

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