The visual angle that the 
moon forms at the eye remains 
essentially the same regardless 
of its location, but our 
perception of its size varies. 
The taking-into-account 
theory could explain this 
illusion if distance cues cause 
us to see the moon as farther 
away at the horizon than when 
it is high in the sky. We would 
then perceive its size to be 
greater at the horizon because, 
according to Emmert’s law, 
perceived size is directly 
proportional to perceived 
distance.

    Do we perceive the distance 
to the moon to be greater at the 
horizon? When seen over the 
terrain, the moon seems to be 
located roughly at the distance 
of the horizon. Therefore it 
must look as large as an object 
on the ground at the horizon 
whose visual angle equals that 
of the moon. The terrain 
provides information about 
distance so that, over a 
relatively flat landscape, the 
horizon may appear to be very 
far away indeed, and 
consequently the moon appears 
very large. Contrary to 
widespread belief, the moon 
illusion does not require 
comparison of the moon with 
familiar objects on the ground, 
such as houses or trees. The 
illusion is quite compelling on 
the ocean or a desert.