<P> The Moon illusion is an optical illusion which causes the Moon to appear larger near the horizon than it does higher up in the sky . It has been known since ancient times and recorded by various cultures . The explanation of this illusion is still debated . </P> <P> A popular belief, stretching back at least to Aristotle in the 4th century B.C., holds that the Moon appears larger near the horizon due to a real magnification effect caused by the Earth's atmosphere . This is not true: although the atmosphere does change the perceived colour of the Moon, it does not magnify or enlarge it . Atmospheric refraction also makes the image of the Moon slightly smaller in the vertical direction . </P> <P> The angle that the full Moon subtends at an observer's eye can be measured directly with a theodolite to show that it remains constant as the Moon rises or sinks in the sky (discounting the very small variations due to the physical effects mentioned). Photographs of the Moon at different elevations also show that its size remains the same . </P> <P> A simple way of demonstrating that the effect is an illusion is to hold a small pebble (say, 0.33 inches or 8.4 millimetres wide) at arm's length (25 inches or 640 millimetres) with one eye closed, positioning the pebble so that it covers (eclipses) the full Moon when high in the night sky . Then, when the seemingly very large Moon is on the horizon, the same pebble will also cover it, revealing that there has been no change in the size of the Moon . </P>

What is the best explanation of the moon illusion