The Moon does not undergo large physical changes in size during its monthly cycle, but its apparent angular size changes slightly because the Moon follows an elliptical orbit around Earth. The Moon is sometimes closer to Earth and sometimes farther away, so its angular diameter changes correspondingly. When the Moon is near perigee, the closest point of its orbit, it appears somewhat larger than when it is near apogee, the farthest point. The difference is real but modest.
Much larger apparent changes in size can occur because of the Moon illusion, in which the Moon near the horizon can appear larger than a Moon high in the sky even though its angular size is nearly the same. These two effects should therefore be distinguished.
The Moon follows an elliptical orbit rather than a perfect circle. Its average distance from Earth is about 384400 kilometres, but the actual distance varies substantially over each orbital cycle. Near perigee, the Moon can be roughly 363000 kilometres from Earth, while near apogee it can be roughly 405000 kilometres away, with exact values varying between orbital cycles because the lunar orbit is influenced by the Sun and other gravitational effects.
Because apparent angular size is approximately inversely proportional to distance for a fixed physical object, the Moon appears larger when it is closer and smaller when it is farther away. This physical variation can produce a noticeable difference in the size of the full Moon as seen from Earth.
The Moon illusion is a separate perceptual phenomenon. When the Moon is close to the horizon, human observers often perceive it as substantially larger than when it is high in the sky. Photographs taken with the same optical magnification and appropriate framing demonstrate that the Moon's angular diameter does not suddenly increase when it rises or sets. The apparent enlargement is therefore largely associated with how the visual system interprets the Moon in relation to surrounding landscape and environmental features. The exact mechanisms behind the Moon illusion involve complex interactions among visual perception, depth cues, size constancy, and contextual interpretation. There is no single universally accepted explanation that accounts for every aspect of the phenomenon.
Several perceptual theories have been proposed to explain the Moon illusion. One idea is that objects near the horizon provide distance cues, allowing the visual system to interpret the Moon as being farther away than the sky overhead while maintaining an apparently similar retinal size, which can produce an impression of greater physical size. Another explanation emphasises relative size and contextual comparisons with buildings, trees, hills, or other objects near the horizon. Other models involve the geometry and perception of the sky dome and how observers interpret the apparent distance of celestial objects. Modern research generally treats the illusion as a complex perceptual effect rather than a simple optical enlargement.
The Moon's physical size is essentially constant on the timescale of a single night. Its mean radius is about 1737 kilometres, and its diameter is about 3475 kilometres. The surface does not expand and contract enough to explain the apparent changes seen during normal observation. The Moon can, however, undergo genuine changes in orientation and illumination as it orbits Earth. Its phases are caused by changing viewing geometry between the Sun, Earth, and Moon, not by changes in the Moon's physical dimensions. A full Moon and a crescent Moon therefore have nearly the same physical size, although their illuminated shapes and apparent brightness differ.
The apparent size of the Moon can also be affected by atmospheric conditions, although these effects should not be confused with the main causes of the perceived size change. Atmospheric refraction can slightly alter the apparent position and shape of the Moon, particularly near the horizon. Turbulence and atmospheric scattering can blur or distort its appearance. Near the horizon, the Moon may also appear reddish or orange because its light travels through a longer atmospheric path, during which shorter wavelengths are scattered more strongly. These colour and distortion effects can make a horizon Moon seem visually different without substantially changing its actual angular diameter.
The term supermoon is commonly used when a full Moon occurs relatively near perigee, causing it to appear somewhat larger and brighter than a full Moon occurring near apogee. The opposite comparison is sometimes called a micromoon. The difference in apparent diameter between extreme lunar distances is measurable and can be photographed under controlled conditions, but it is much smaller than the dramatic enlargement often perceived during the Moon illusion. A supermoon therefore represents a real orbital distance effect, whereas the impression that any horizon Moon is enormous compared with a high Moon is primarily a perceptual phenomenon.
The apparent size of the Moon is consequently controlled by several different factors that must be separated carefully. Its actual angular diameter changes because the Earth Moon distance changes as a result of the elliptical lunar orbit. Its perceived size can change dramatically because of the Moon illusion when it is viewed near the horizon.
Atmospheric refraction and scattering can modify its apparent position, shape, sharpness, and colour, while lunar phases alter its illuminated appearance without significantly changing its physical size. Distinguishing physical angular size from visual perception explains why photographs can show nearly identical lunar diameters at different elevations even when observers strongly report that one Moon looks much larger than another.
