A visual illusion that causes the moon to appear relatively larger when it is near the horizon than when it is at its zenith, although in both cases it subtends the same visual angle of about one-half of a degree or 30 minutes of arc, and although it does not necessarily appear any further away at its zenith. The sun is much larger than the moon but by remarkable coincidence almost identical to it in angular size, fitting behind the moon almost perfectly in a total eclipse, and it is also subject to the moon illusion, appearing much larger shortly after sunrise or shortly before sunset than at its zenith. The illusion first appeared in print in the Meteorology of the Greek philosopher Aristotle (384–322bc), who described its effect on the sun and stars but oddly omitted to mention the moon: ‘The sun and stars seem bigger when rising and setting than on the meridian’ (Book 3, Chapter 4, Bekker edition, p. 373b). Also called the celestial illusion, especially when referring to the illusion in a stimulus other than the moon. See also Emmert's law, horizontal-vertical illusion, perspective illusion, size-distance invariance, visual angle.