1 The Moon Illusion ∗ Babu Thaliath Centre of German Studies Jawaharlal Nehru University, New Delhi https://babuthaliath.com Orcid code: https://orcid.org/0000-0002-4557-7765 Abstract The celestial illusions, in particular the moon and sun illusion, are among the enduring problems of visual size and depth perception, the attempts to solve which go back to the antiquity. It is perhaps the longest- standing aporia in the history of the theory of perception, which has been tried in vain in various ancient cultures – in Egypt, China and Greece – and continued to be tackled in the Middle Ages and modern times by many renowned philosophers, mathematicians and scientists of optics. The following essay is an attempt to outline the historicity of this hitherto unresolved problem and to re-examine its topicality. The insolubility and historically incessant persistence of this aporia in visual space perception can be traced back, on the one hand, to the dichotomisation between the intromission and extramission theories of vision that was already undertaken in ancient times and, on the other hand, to the almost philosophically and paradigmatically established internalism in the theory of perception in the Cartesian-Kantian modernity. The study elucidates the topicality of the flattened dome theory as proposed by Alhazen (Ibn al-Haytham) in the 11 th century, which, how I would argue, presupposes a unique externalism in visual perception, or the objective externalisation of the process of vision, rather than an internalism that has been tacitly assumed from the antiquity and Middle Ages to the present day. The moon illusion is a well-known and centuries-old aporia in the theory of visual perception of size, for which no satisfactory explanation appears to have been proffered. The moon appears considerably larger near the horizon than it does higher in the sky at the zenith. Between these positions of the moon, we experience a gradual reduction in the size of the rising moon. However, this visual illusion applies not only to the moon, but also to the sun as well as to the constellations of stars and planets in the sky. The apparent horizon enlargement of the moon is usually called the moon illusion, a term which became popular in the twentieth century. It has also sometimes been called the horizon illusion, but this is clearly less appropriate. A similar illusion can be observed for the sun, and it is normally called the sun illusion. This illusion is less well known to most people than the moon illusion, perhaps because the sun is usually too bright to observe with naked eye. However, the sun illusion was more frequently discussed than the moon illusion in the early literature. Although two different celestial bodies are involved, there seem to be no fundamental difference between the two illusions: they are therefore generally considered to be two examples of the same phenomenon. A third example is the apparent enlargement of the constellations and of the distances between the stars near the horizon. In the words of the astronomer Paul Stroobant (1884, p. 719): ‘The same phenomenon exists for the constellations; thus the Great Bear and Orion, close to the horizon, appear enormous.’ This form of the illusion is probably the least ∗ The following essay is the translation of an excerpt from a chapter of my monograph titled Die Verkörperung der Sinnlichkeit, which was published by Verlag Karl Alber in Freiburg in 2017. The chapter is titled: Die Ausdehnung der Sinnlichkeit. The English translation of the monograph – with the title: The Embodiment of Senses – will be published in the summer of 2025. This translated excerpt has been revised to some extent, but its implicit references can be found in the monograph: Thaliath, Babu: Die Verkörperung der Sinnlichkeit, Verlag Karl Alber, Freiburg / München 2017.