Explaining the moon illusion

An old explanation of the moon illusion holds that Various cues place the h orizon moon at an effectively greater distance than the elevated moon. Alth ough both moons have the same angular size, the horizon moon must be percei ved as larger. More recent explanations hold that differences in accommodat ion or other factors cause the elevated moon to appear smaller. As a result of this illusory difference in size, the elevated moon appears to be more distant than the horizon moon. These two explanations, both based on the ge ometry of stereopsis, lead to two diametrically opposed hypotheses. That is , a depth interval at a long distance is associated with a smaller binocula r disparity, whereas an equal depth interval at a smaller distance is assoc iated with a larger disparity. We conducted experiments involving artificia l moons and confirmed the hypothesis that the horizon moon is at a greater perceptual distance. Moreover, when a moon of constant angular size was mov ed closer it was also perceived as growing smaller, which is consistent wit h the older explanation. Although Emmert's law does not predict the size-di stance relationship over long distances, we conclude that the horizon moon is perceived as larger because the perceptual system treats it as though it is much farther away. Finally, we observe that recent explanations substit ute perceived size for angular size as a cue to distance. Thus, they imply that perceptions cause perceptions.