  We present another explanation for the moon illusion, the phenomenon in which
the moon looks larger near the horizon than near the zenith. In our model of
the moon illusion, the sky is considered a spatially-contiguous and
geometrically-smooth surface. When an object such as the moon breaks the
contiguity of the surface, instead of perceiving the object as appearing
through a hole in the surface, humans perceive an occlusion of the surface.
Binocular vision dictates that the moon is distant, but this perception model
contradicts our binocular vision, dictating that the moon is closer than the
sky. To resolve the contradiction, the brain distorts the projections of the
moon to increase the binocular disparity, which results in an increase in the
perceived size of the moon. The degree of distortion depends upon the apparent
distance to the sky, which is influenced by the surrounding objects and the
condition of the sky. As the apparent distance to the sky decreases, the
illusion becomes stronger. At the horizon, apparent distance to the sky is
minimal, whereas at the zenith, few distance cues are present, causing
difficulty with distance estimation and weakening the illusion.
