THE MOON ILLUSION EXPLAINED

Finally! Why the Moon Looks Big at the Horizon
and Smaller When Higher Up

Don McCready,    Professor Emeritus
Psychology Department
University of Wisconsin-Whitewater
Whitewater, WI 53190
Send messages to: mccreadd at uww.edu

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 that picture resembles what you usually see, and you wonder why this famous moon illusion occurs, you should read the following article; for, as all illusion researchers know, a new explanation is needed:.

The picture mimics an angular size illusion; the horizon moon looks a larger angular size than the zenith moon. Most moon illusion researchers now accept that the illusion begins as an angular size illusion. However, that idea has not yet reached the general public.

Instead, the explanations commonly found in textbooks and the popular media, including virtually all "moon illusion" discussions on the internet, do not mention the basic angular size illusion. Such articles repeat the ancient idea that the horizon moon looks the same angular size as the zenith moon, but looks farther away so it logically looks a larger physical size. It often is said to look farther away due to a "flattened sky dome" illusion or due to changes in 'cues to distance' as illustrated by the "Ponzo Illusion." These explanations require that the horizon moon must first look farther away. But, all researchers know that very few people see it that way. That minority moon illusion is not mimicked by the picture above.

The picture mimics, instead, the angular size illusion that nearly all people have.
For most people, the horizon moon looks a larger angular size than the zenith moon, and the horizon moon "looks closer" than the zenith moon, because it correctly looks about the same physical size (it appears to be the same moon).
For most of the rest, the horizon moon correctly looks about the same distance away as the zenith moon, so because it looks a larger angular size it also looks a larger physical size than the zenith moon.

The scientific challenge has been to explain why those equal angular sizes look unequal.

This article is long for three reasons.
1. It advances the relatively new idea (1965, 1970, 1985, 1986) that, for most people, the moon illusion begins as an angular size illusion which has several possible outcomes.

2. It reviews in detail the very few explanations of the illusion that vision scientists are paying the most attention to (and still researching). These theories are not simple.

3. It reviews in detail the latest theory (1985, 1986, 1989) that the moon illusion is an example of the less familiar, but ubiquitous, "size" illusion known as oculomotor micropsia/macropsia. Explanations for oculomotor micropsia then are reviewed.
For the moment, it seems to be the most satisfactory explanation.

Introduction and Summary. (Loading these links may take several seconds)
Section I. New Description of the Moon Illusion
Section II. Conventional Versus New Descriptions
Section III. Explaining the Moon Illusion
Section IV. Explaining Oculomotor Micropsia
Bibliography and McCready VITA
Appendix A. The (New) Theory

Appendix B. Analysis of the Murray, Boyaci & Kersten (2006) Experiment

The initial version of this article was placed on this web site in May, 1999 .
Major revisions were made in December 2002.
This entire article was revised on November 10, 2004.


Then, 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 "Technical Note added June 7, 2006., in the Introduction and Summary Section,

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. They strongly advocate that the moon illusion begins as an angular size illusion.
This present article is the only website they refer to (as it was in 2001).

since 12/07/02.