Size of the moon in the sky

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Dear Tom,
I recently was amazed at how large the moon appeared when it was rising over the eastern horizon. Why the large appearance?
Jane Alles
Dear Jane,
It’s an illusion, and that’s the explanation. The sun and moon always appear larger whenever they are positioned near the horizon. It’s such a compelling illusion that it has a name: the “moon illusion.”
The visual diameter of both the sun and moon is always 0.5 degrees of arc, no matter where they are located in the sky. You can prove it by photographing the “large” rising moon, but the result will be disappointing because the camera sees the moon’s size as it actually is. The illusion probably results from the differing ways our minds relate size and distance. The sky at the horizon seems more distant than the overhead sky. When viewed against the “more distant” horizon sky, the moon and sun seem larger than when seen against the “closer” overhead sky.


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