The perception that the Sun appears significantly closer or much larger when it rests on the horizon at sunset is a common experience shared by nearly everyone. This dramatic visual effect is not due to any physical change in the Sun itself or its distance from Earth. Instead, the phenomenon is a powerful type of optical illusion, a disconnect between the constant reality of the heavens and the way human vision processes size and distance. The illusion, which also affects the Moon, is a demonstration of how the brain actively constructs our sense of the world using context and comparison.
The Reality of Solar Distance and Size
The Sun’s distance from Earth and its actual size in the sky remain virtually unchanged whether it is high overhead or low on the horizon. The angle the Sun occupies in our field of vision, known as its angular size, is a constant value of approximately 0.5 degrees. In fact, the Sun is theoretically at its maximum distance from an observer at sunset compared to midday. This slight increase in distance, equal to about one Earth radius, actually causes the Sun’s angular size to be minutely smaller at the horizon. This physical reality stands in direct opposition to the illusion that the Sun looks larger or closer during this time.
How the Brain Interprets Distance
The illusion of a colossal sunset Sun is rooted in how the visual system interprets distance on a massive scale, a concept often referred to as the Apparent Distance Hypothesis. When the Sun is high in the sky, known as the zenith, it is surrounded by a vast, empty expanse of blue. The brain lacks nearby terrestrial objects to use as reference points for scale.
Without these visual cues, the brain perceives the sky as an untextured, flattened dome where the zenith appears relatively close to the observer. Conversely, when the Sun is near the horizon, it is visually integrated with familiar landscape elements such as trees, buildings, and mountains. These horizon cues provide an abundance of depth information that makes the horizon appear much farther away.
The brain maintains a mechanism called size constancy, which attempts to accurately gauge the physical size of an object regardless of distance. Since the Sun’s retinal image size is the same at both the zenith and the horizon, the brain follows a simple rule: if an object maintains the same retinal size but is perceived as being much farther away, it must be physically larger. This overcompensation for the perceived distance at the horizon results in the illusion of a massive Sun. The flattened appearance of the sky, where the distance to the horizon is overestimated, is the primary psychological driver of this magnified visual experience.
Atmospheric Filtering and Visual Distortion
While the illusion of size is purely psychological, the atmosphere does introduce real physical changes that alter the Sun’s appearance. As sunlight travels toward an observer at sunset, it must pass through a far greater thickness of Earth’s atmosphere than it does at noon. This long path through the air causes a process called Rayleigh scattering, where shorter wavelengths of light, such as blue and violet, are scattered away by atmospheric gas molecules.
The removal of the blue end of the spectrum leaves behind the longer wavelengths, namely reds, oranges, and yellows. This selective filtering is why the setting Sun takes on its characteristic warm colors. This change in color makes the Sun more visible, which may contribute to the illusion by drawing more attention to the celestial body.
The atmosphere also causes a visual distortion known as atmospheric refraction. The air acts like a lens, bending the light rays, with the light from the bottom edge of the Sun being bent more than the light from the top edge. This uneven bending often causes the Sun’s disc to appear slightly squashed or oval-shaped when it is very low on the horizon. While atmospheric effects change the color and shape of the Sun’s disc, they are not the main cause of the illusion that makes the Sun appear larger.