Revised November 10, 2004.

Nearly all people will agree that the picture at the right represents approximately how the horizon moon's size looks when compared with how it looks later, with the moon higher up in the sky.

If you wonder why this famous moon illusion occurs, you should read the following article, for, as illusion researchers know, a new explanation is needed: The explanations currently offered by textbooks and the popular media (including virtually all the 'moon illusion' sites on the internet) simply do not explain the moon illusion that most people experience.   

This article reviews a new theory previously presented (since 1983) only in some technical articles in specialized publications for vision researchers (cognitive psychologists).

This article is long because, first of all,  it describes the moon illusion more completely and in a more logical way than do conventional discussions.
Most moon illusion researchers now accept this new description.
Secondly, this article reviews the currently best-known theories, and shows why vision scientists don't accept them.
Thirdly, this article reviews the new theory that the moon illusion is an example of the less familiar, but ubiquitous, "size" illusion known as oculomotor micropsia/macropsia.
Finally, in order to complete the theory, this article reviews  an explanation for oculomotor micropsia.
In other words, the new theory for the moon illusion is not simple: But it currently is the most satisfactory explanation.

The initial version of this article was placed on this web site in May, 1999, and print copies were mailed to most of the researchers listed in the bibliography.
Major revisions were made in December 2002.
This entire article was revised on November 10, 2004.

Extremely important research on visual angle illusions was published in March 2006 in Nature Neuroscience. The article is, “The representation of perceived angular size in human primary visual cortex,” by Murray, S. O., Boyaci, H., & Kersten, D. (2006).
It fully supports the approach advocated here, so it is reviewed in the Introduction and Summary Section, in the "Technical Note added June 7, 2006."
It is analyzed in detail in Appendix B (posted February 5, 2007).

Advanced students also should consult the excellent book, "The Mystery of the Moon Illusion," by Helen Ross and Cornelis Plug, published in September 2002. It offers the most complete and up-to-date review of research and speculation on the moon illusion.