page_title: Moon illusion
section_title: Relative size hypothesis
hierarchical_section_title: Moon illusion / Possible explanations / Relative size hypothesis
context_page_description: The Moon illusion is an optical illusion which causes the Moon to appear larger near the horizon than it does higher up in the sky. It has been known since ancient times and recorded by various cultures.
  The explanation of this illusion is still debated.
context_section_description: Historically, the best-known alternative to the "apparent distance" theory has been a "relative size" theory. This states that the perceived size of an object depends not only on its retinal size, but also on the size of objects in its immediate visual environment. In the case of the Moon illusion, objects in the vicinity of the horizon Moon (that is, objects on or near the horizon) exhibit a fine detail that makes the Moon appear larger, while the zenith Moon is surrounded by large expanses of empty sky that make it appear smaller.
  The effect is illustrated by the classic Ebbinghaus illusion, where a circle appears larger when surrounded by smaller circles, than it does when surrounded by larger circles.
caption: The Ebbinghaus illusion. The lower central circle surrounded by small circles might represent the horizon Moon accompanied by objects of smaller visual extent, while the upper central circle represents the zenith Moon surrounded by expanses of sky of larger visual extent. Although both central circles are actually the same size, the lower one looks larger to many people.