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. Proof of illusion 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. Photographs of the Moon at different elevations also show that its size remains the same. A simple way of demonstrating that the effect is an illusion is to hold a small pebble (say, wide) at arm's length () 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, because the pebble will still cover the Moon. Between different full moons, the Moon's angular diameter can vary from 29.43 arcminutes at apogee to 33.5 arcminutes at perigee—an increase of around 14% in apparent diameter or 30% in apparent area. This is because of the eccentricity of the Moon's orbit. Possible explanations The size of a viewed object can be measured objectively either as an angular size (the visual angle that it subtends at the eye, corresponding to the proportion of the visual field that it occupies), or as physical size (its real size measured in, say, meters). Perceived size is only loosely related to these concepts, however. For example, if two identical, familiar objects are placed at distances of five and ten meters, respectively, then the more distant object subtends approximately half the visual angle of the nearer object, but it is normally perceived to be the same size (a phenomenon referred to as size constancy), not as half the size. Conversely, if the more distant object did subtend the same angle as the nearer object then it is normally perceived to be twice as big. One question concerning the Moon illusion, therefore, is whether the horizon Moon appears larger because its perceived angular size seems greater, or because its perceived physical size seems greater, or some combination of both. There is currently no consensus on this point.