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| Jeffrey M. Silverman |
A: The quick answer is that it's an optical illusion. Further explanations can become quite involved, but honestly no one seems to be totally sure why we observe this effect!
It has been disproved that the moon appears larger near the horizons simply because there are earth-bound objects nearby to which the moon's size can be compared. This is a common, but incorrect, explanation. The effect does appear to be related to how humans judge distances and sizes, but it is also related to how we perceive the sky itself.
Since the surface of the earth is curved, the sky we observe appears curved as well. It seems that humans perceive the sky above us as shaped like a flattened bowl or saucer as opposed to a perfect hemisphere. This perception of the shape of the sky causes us to perceive the moon as stretched out, or bigger, near the horizons.
Jeffrey M. Silverman is a UC Berkeley Astronomy Graduate Student, and the astronomy department's public liaison.
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