THE MOON ILLUSION 
This diagram shows how the 
perceived shape of the sky
could affect the moon illusion. 
The two arcs illustrate how the
perceived flattenting of the
sky affects judgments of the
half-way point between the
horizon and zenith.

Since antiquity, it has been 
observed that the moon appears 
much larger when it is over the 
horizon than when it is high in 
the sky. Through the ages, 
many writers have speculated 
that the illusion has a physical 
basis, locating its source 
variously in the horizon 
moon’s relative dimness, or 
redness, or some process of 
magnification based on 
refraction because, when the 
moon is at the horizon, the 
light rays pass through more of 
the earth’s atmosphere. A little 
thought will make clear, 
however, that explanations of 
this kind make little sense. To 
the contrary, measurements of 
photographs of the moon in 
different locations in the sky 
reveal no illusion. In fact, the 
image of the moon at the 
horizon is actually slightly 
smaller, not larger than its 
image high in the sky because 
the moon is closest to the 
observer at its zenith. 
Therefore, the illusion must be 
perceptual in origin.