The New Moon Illusion B Rogers1, S Anstis2 |
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1Experimental Psychology, University of Oxford, United Kingdom |
| In the traditional moon illusion, the moon appears to be larger when it is near the horizon compared to overhead. Our New Moon illusion can be seen when both sun and moon are visible and is most striking when the sun is setting and the moon is higher in the sky. Under these conditions, the sun does not appear to be in a direction perpendicular to the backward tilt of the terminator (boundary between the lit and dark side of the moon) as must be the case from physics and can be verified by holding up a piece of string to ‘join’ the sun and the moon. There is a cognitive aspect to the illusion, arising from the incorrect assumption that because the terminator is tilted backwards, the sun must be higher in the sky. There is also a perceptual aspect that Walker (1975, The Flying Circus of Physics, Wiley) has attributed to our perception of the sky as a spherical dome. While this may be part of the explanation, it raises the deeper question of how we judge which lines in the world are straight and parallel (Helmholtz H. von, 1910 Physiological Optics; Rogers and Rogers, Perception 38, 2009). |
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