The angular size illusion has defied explanation for decades. McCready suggests that the moon illusion is a special case of a lesser-known phenomenon known as oculomotor micropsia, which literally means “looking small”. It is intimately related to oculomotor macropsia, which translates as “looking large”.
It is believed that the way we focus our eyes is affected by changes in the visible patterns near an object. These patterns or secondary objects are known as distance cues.
Oculomotor macropsia is an angular size illusion caused by changes in the activity of eye muscles. When one views the horizon moon, the details in the landscape form distance cues that sends a “very far” signal to the observer. This has the effect of making the eyes focus at a distance much greater than where the object is, called overfocusing. Hence, a smaller angular size is perceived.
Oculomotor micropsia has the opposite effect. The zenith moon offers few distance cues, and the eyes tend to adjust to a relatively near position, known as the resting focus position. This is typically about one or two metres from the face. The perceived angular size becomes smaller. A closely related phenomenon is night myopia, where the eyes tend to underfocus in dark surroundings. This causes temporary nearsightedness, also known as “night blindness”.
It must be strongly emphasized that oculomotor micropsia and oculomotor macropsia are physiological and not geometrical effects. Furthermore, they occur when viewing all objects, not just the moon.