ABSTRACT
The moon illusion undercuts this neat formulation. The moon over the horizon subtends the same visual angle at the eye as the moon high in the sky, but the horizon moon looks much larger than the zenith moon. According to the traditional formulation of the SDIH, if the visual angle is constant, an object that appears larger should also appear to be farther from the viewer; an object that appears to be small should also appear to be closer to the viewer. The problem is that a large proportion of viewers report that the horizon moon appears to be both larger and closer than the zenith moon. This puzzling situation is sometimes called the size-distance paradox.
