Western Slope Skies
1:06 pm
Mon January 14, 2013

Moon Illusion

On Saturday night, January 26th, a full moon will rise in the east just after sunset.  Look at the full moon when it’s still near the horizon.  Does it appear especially large to you?  Then, look at the moon later this same night, when it’s higher in the sky.  Does it appear smaller?  For most people, the moon looks much larger when on the horizon.  This is the "Moon Illusion."


Even though the moon looks bigger near the horizon, the apparent size of the moon is actually very slightly smaller when it is near the horizon than when it’s high in the sky!  When the moon is overhead, it’s about 4,000 miles closer to us than when it’s near the horizon.  This distance corresponds to the Earth’s radius. 

Why, then does the moon look SO big when it’s near the horizon?   This question has been debated for centuries.  Consensus among scientists is that the moon illusion is a psychological phenomenon.  However, the exact nature of that effect is vigorously debated.  One theory is that our brains expect an object on the horizon to be farther away from us than the same object when seen overhead.  So our brains compensate for that by making objects on the horizon seem larger. 

To me, the rising full moon still looks huge, even when I know it should appear very slightly smaller than when it’s high in the sky later that night.         

Western Slope Skies is produced by members of the Black Canyon Astronomical Society. 

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