Psychophysical study of the moon illusion in paintings and landscape photos

The moon illusion is a visual deception when people perceive the angular diameter of the Moon/Sun near the horizon larger than that of the one higher in the sky. Some theories have been proposed to explain this illusion, but not any is generally accepted. Although several psychophysical experiments have been performed to study different aspects of the moon illusion, their results have sometimes contradicted each other. Artists frequently display(ed) the Moon/Sun in their paintings. If the Moon/Sun appears near the horizon, its painted disc is often exaggeratedly large. How great is the magnitude of moon illusion of painters? How different are the size enlargements of depicted lunar/solar discs? To answer these questions, we measured these magnitudes on 100 paintings collected from the period of 1534–2017. In psychophysical experiments, we also investigated the moon illusion of 10 test persons who had to estimate the size of the lunar/solar disc on 100 paintings and 100 landscape photographs from which the Moon/Sun was retouched. Compared to the lunar/solar disc calculated from reference distances estimated by test persons in paintings, painters overestimated the Moon's size on average Q = 2.1 ± 1.6 times, while the Sun was painted Q = 1.8 ± 1.2 times larger than the real one, where Q = r painted / r real is the ratio of the radii of painted ( r painted ) and real ( r real ) Moons/Suns. In landscape photos, test persons overestimated the Moon's size Q = 1.6 ± 0.4 times and the Sun was assumed Q = 1.7 ± 0.5 times larger than in reality, where Q = r test / r real is the ratio of the radius r test estimated by the test persons and the real radius r real of Moons/Suns. The majority of the magnitude of moon illusion Q = 1.6, 1.7, 1.8, 2.1, 2.8, 2.9 measured by us are larger than the Q -values 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.8 obtained in previous psychophysical experiments due to methodological differences.

The Moon Illusion Explained by Visual Motion Perception

We designed two virtual space missions to the moon, namely, two virtual rockets were launched to the horizon moon and to the zenith moon. These virtual rockets carry exactly the same physical characteristics (length, width, contrast, and speed). Our observers had noticed when the rocket travelling horizontally to the horizon moon, it appears to move way slower; deceleration that may trigger the brain to pull the moon towards the observers to compensate the delay of the aforementioned virtual space mission.

The Sun/Moon Illusion in a Medieval Irish Astronomical Tract

The Irish Astronomical Tract is a 14th–15th century Gaelic document, based mainly on a Latin translation of the eighth-century Jewish astronomer Messahala. It contains a passage about the sun illusion—the apparent enlargement of celestial bodies when near the horizon compared to higher in the sky. This passage occurs in a chapter concerned with proving that the Earth is a globe rather than flat. Here the author denies that the change in size is caused by a change in the sun’s distance, and instead ascribes it (incorrectly) to magnification by atmospheric vapours, likening it to the bending of light when looking from air to water or through glass spectacles. This section does not occur in the Latin version of Messahala. The Irish author may have based the vapour account on Aristotle, Ptolemy or Cleomedes, or on later authors that relied on them. He seems to have been unaware of alternative perceptual explanations. The refraction explanation persists today in folk science.

The Moon Illusion Explained by Wundt Illusion

This paper illustrates how the moon illusion complies to Wundt illusion.

The New Moon Illusion

When the sun is near the horizon and the moon is visible and higher in the sky, there is a compelling illusion that the sun is not in a direction perpendicular to the boundary between the lit and dark sides of the moon. This New Moon illusion has been observed and discussed previously but without a complete explanation. This chapter argues that both perceptual and cognitive factors contribute to the illusion and that it arises from the fact that the straight line joining the sun and the moon describes a great circle over the flattened dome of the sky. The New Moon illusion also raises the question of how we are able to judge the straightness of extended straight and parallel lines.