What can we say about the 
apparent distance of the 
elevated moon? Because the 
elevated moon is seen through 
empty space, within a 
homogeneous surround, the 
only possible sensory cues we 
have as to its distance would 
derive from the tendency of the 
eye to accommodate for the 
distance of an object until 
sharp vision is achieved and 
from the tendency of the two 
eyes to converge on an object in 
order to maintain single vision 
of it. However, the cues to 
distance of accommodation and 
convergence are ineffectual 
beyond short distances, a few 
feet and several yards, 
respectively. Therefore, based 
on such information, the 
perceptual system might 
localize the elevated moon as 
relatively nearby even though 
we know it is faraway.

    If the moon at the horizon 
appears farther away than the 
elevated moon, then, because 
the visual angle formed by the 
moon remains essentially 
constant regardless of the 
moon’s location, we would 
perceive its size as larger. This 
is the argument that various 
thinkers have advanced over 
the centuries in attempting to 
explain the illusion, and it is 
consonant with what we know 
now as Emmert’s law.