If the apparent-distance 
theory is correct, the moon 
illusion should disappear when 
sight of the terrain is entirely 
eliminated. This is precisely 
what happened when we 
occluded sight of the terrain 
with cardboards or when we 
asked observers to view the 
moon in the dark in the Hayden 
Planetarium. In a further 
experiment, mirrors were used 
to create the impression of a 
terrain for the elevated 
"moon," so that, in looking 
upward, the observer saw the 
ground rising in a vertical 
direction. Similarly, mirrors 
were used to eliminate the 
sight of the terrain ordinarily 
visible below the horizon 
moon, so that, in looking 
straight ahead, the observer 
saw only sky. Under these 
conditions, the illusion was 
reversed: The elevated moon 
appeared larger. Although some 
problems of interpretation 
remain, it is plausible to 
believe that the moon illusion, 
like the afterimage illusion, 
can be explained on the basis of 
the same principles that 
explain veridical size 
perception and constancy.