Astronomical Society of Edinburgh
Journal 46

Cover photograph of book

Picture from OUP website

The Mystery of the Moon Illusion:
Exploring Size Perception
By Helen Ross and Cornelius Plug


I have written several times about the Moon Illusion (it appears about 1½ times larger near the horizon than when overhead), most recently in Journal 41 (April 2000). Consequently, I was most interested to read this book. Helen Ross was latterly in the Department of Psychology at the University of Stirling. Cornelius Plug hails from a similar department at the University of South Africa at Pretoria. The book has a short foreword by Prof. Richard Gregory.

The Illusion applies not just to the Moon, but also to the Sun and constellations near the horizon. Consequently it can be called a celestial illusion: a magnification of all astronomical objects seen close to the horizon (or horizontally). It has been known at least since Babylonian times and was first described scientifically by Aristotle 2300 years ago.

Theories of the Illusion fall into three categories : physical, physiological or perceptual. The first two are based on the idea that the image of the Moon does actually increase (angular enlargement), the last on the idea that it is only perceived to increase.

The authors painstakingly explore many explanations for the illusion, concluding either that they do not explain it or only partly explain it. Rejected are the varying distance of the Moon, atmospheric refraction (most people's explanation), aerial perspective, change in pupil size, the perceived flattening of the sky overhead (the sky illusion) and the presence of intervening objects. Partly rejected are relative size, angle of regard, and the vestibulo-ocular (balance) reflex. The authors conclude that the Illusion is due to a combination of the effect of several factors and that about 40% of the Illusion is caused by relative size effects. Another 10% comes from oculomotor commands, angle of regard and posture, while the effects of a light haze and a red colour might contribute another 10%. They observe that the perceived enlargement of the Moon is found to be the same as that of terrestrial objects on the horizon. Consequently, whatever analysis is given to the Moon Illusion should be applied to normal size constancy. They urge more experiments and predict large advances in the neuroscience and psychophysics of size perception in the next few years. Therefore, it is an illusion that we still cannot completely explain. Even though one is not sure where one has arrived, the journey is fascinating and very well documented.

There is an appendix summarizing scientific developments relating to the Illusion, comprehensive chapter notes, references, and name and subject indices.

Steuart Campbell

Bibliographic information:
Ross, Helen Elizabeth, 1935-; Plug, Cornelis.
The mystery of the moon illusion : exploring size perception / Helen E. Ross, Cornelis Plug.
Oxford ; New York : Oxford University Press, 2002.
viii, 277 p. : ill. ; 24 cm.
ISBN 0-19-850862-X
QP495 .R67 2002
152.14/8 21