The Natural Moon Illusion: A Multifactor Angular Account

Abstract

It is argued that the failure to explain the celestial illusion results from conceptual confusion about perceived size and from disregard of the observational evidence relating to the natural moon illusion. The evidence shows that the illusion consists of a perceived angular size enlargement of horizon objects, by a factor of about 1.5-2.0 in diameter in comparison with elevated objects. Most measurements of the illusion have been made in terms of angular size, although in some proposed explanations an illusion of linear size is assumed. The magnitude of the illusion varies, particularly with the detail of the horizon scene. The illusion can be explained as the sum of several factors that affect perceived angular size: size contrast, vergence commands and eye or head position, aerial perspective, and colour. The relative contributions of these factors are assessed.

Perceiving the Present and a Systematization of Illusions

“…This distal versus projected distinction has been made often in the visual perception literature (Arend & Goldstein, 1990;Carlson, 1960;Changizi & Widders, 2002;Gibson, 1950;Gilinsky, 1955;Gillam, 1998;Mack, 1978;Palmer, 1999;Rock, 1983;Sedgwick & Nicholis, 1993), and perception of projected size (as opposed to distal size) has been observed a number of times over the history of visual perception (Angell, 1974;Baird, 1968;Biersdorf, Ohwaki, & Kozil, 1963;Carlson, 1960Carlson, , 1962Craig, 1969;Daniels, 1972;Foley, 1972;Gibson, 1950;Gilinsky, 1955;Gogel & Eby, 1997;Jenkin & Hyman, 1959;Joynson, 1949;Kaneko & Uchikawa, 1993, 1997Komoda & Ono, 1974;Leibowitz & Harvey, 1969;Lucas, 1969;Mack, 1978;McCready, 1965McCready, , 1985McCready, , 1986McKee & Welch, 1989, 1992Ono, 1966;Over, 1960;Plug & Ross, 1994;Reid, 1813;Rock & McDermott, 1964;Sedgwick, 1986;Sedgwick & Nicholis, 1993). Researchers have also...…”

“…This distal versus projected distinction has been made often in the visual perception literature (Arend & Goldstein, 1990; Carlson, 1960; Changizi & Widders, 2002; Gibson, 1950; Gilinsky, 1955; Gillam, 1998; Mack, 1978; Palmer, 1999; Rock, 1983; Sedgwick & Nicholis, 1993), and perception of projected size (as opposed to distal size) has been observed a number of times over the history of visual perception (Angell, 1974; Baird, 1968; Biersdorf, Ohwaki, & Kozil, 1963; Carlson, 1960, 1962; Craig, 1969; Daniels, 1972; Foley, 1972; Gibson, 1950; Gilinsky, 1955; Gogel & Eby, 1997; Jenkin & Hyman, 1959; Joynson, 1949; Kaneko & Uchikawa, 1993, 1997; Komoda & Ono, 1974; Leibowitz & Harvey, 1969; Lucas, 1969; Mack, 1978; McCready, 1965, 1985, 1986; McKee & Welch, 1989, 1992; Ono, 1966; Over, 1960; Plug & Ross, 1994; Reid, 1813; Rock & McDermott, 1964; Sedgwick, 1986; Sedgwick & Nicholis, 1993). Researchers have also shown that observers make qualitatively very different “size” judgments when given projected size instructions compared to when given distal size instructions (Biersdorf et al, 1963; Carlson, 1960, 1962; Gilinsky, 1955; Jenkin & Hyman, 1959; Leibowitz & Harvey, 1969): For stimuli with cues to the distal size, projected size instructions lead to judgments closely matching projected size, and distal size instructions lead to judgments closely matching distal size.…”

The moon illusion: Kaufman and Rock's (1962) apparent‐distance theory reconsidered1

“…In a natural outdoor setting, a full moon on the horizon appears often to be more than 1.5 times larger than the moon at the zenith. Many other factors than the state of oculomotor system therefore may contribute to the natural moon illusion (Leibowitz & Owens, 1989; Plug & Ross, 1994; Ross & Plug, 2002). I think, however, that, as shown in my complete darkness and monocular/binocular experiments, the moon illusion per se (i.e., the kernel of this illusion) is principally induced by the binocular state of the oculomotor system.…”

“…Kaufman and Rock called the former unconscious distance the "registered" distance to distinguish it from the consciously perceived distance (Kaufman & Rock, 1962a, 1989. This explanation for the apparent distance of the moon has been often referred to as the further-larger-nearer theory (Plug & Ross, 1989).…”

“…Kaneko and Uchikawa (1997) also demonstrated that the size‐distance invariance hypothesis seems applicable to judgments of linear size (i.e., the actual size of objects), but less applicable to those of angular size (i.e., the apparent retinal size of objects). In general, size constancy concerns the relationship between the actual size of objects and their actual distance; the moon illusion is an illusion of angular size, not one of actual size (Plug & Ross, 1994). This is simply because the moon is not an object located on the earth, and we do not perceive its actual distance (380 000 km) or size (3500 km), but rather only the angular size of the moon.…”