When the sun appears surprisingly large or “close” near the horizon, it’s a common visual experience. This phenomenon is an optical illusion, not a change in the sun’s actual distance or size. The sun’s angular size, the angle it subtends in our field of view, remains virtually constant regardless of its position in the sky.
Light’s Journey Through Our Atmosphere
Sunlight bends as it travels through Earth’s atmosphere, a phenomenon known as atmospheric refraction. The atmosphere’s density varies with altitude, causing light rays to bend as they pass through layers of differing density, similar to a lens or prism. When the sun is low on the horizon, its light travels through a significantly greater amount of atmosphere. This extended path means the light encounters more atmospheric particles and varying air densities, leading to a more pronounced bending effect.
This bending causes the sun’s image to appear higher than its actual position in the sky. The lower edge of the sun’s disk experiences more refraction than its upper edge due to denser air closer to the ground. This differential bending of light effectively lifts the bottom of the sun’s image more than the top, which can make the sun appear flattened or distorted. While atmospheric refraction primarily makes the sun appear displaced, this distortion contributes to the perception of it being larger or closer when seen through the thick atmospheric lens near the horizon.
How Our Brain Interprets Size and Distance
The apparent increase in the sun’s size near the horizon is largely a cognitive illusion, often referred to as the “Moon Illusion” because it applies to both celestial bodies. Our brain constantly uses depth cues from our surroundings to gauge the size and distance of objects. When the sun is low in the sky, it is seen in context with familiar terrestrial objects like trees, buildings, or distant landscapes. These reference points provide our brain with cues that suggest the sun is farther away than it would be perceived if seen in an empty sky.
The brain then applies a mechanism called “size constancy,” which allows us to perceive an object as maintaining a consistent size regardless of its distance from us. If the retinal image size of the sun remains constant but our brain perceives it as being farther away due to surrounding depth cues, it compensates by interpreting the sun as physically larger to maintain constancy. This misinterpretation of distance translates into a perception of increased size, and by extension, a feeling of the sun being “closer” in its scaled appearance. When the sun is high in the sky, there are typically no such reference points, leading the brain to lack cues for distance comparison, and thus, the sun appears smaller by comparison.
Environmental Factors at Play
Various atmospheric conditions can amplify the illusion of the sun appearing close and large. The presence of increased dust or aerosols in the atmosphere can scatter sunlight more significantly. This scattering can make the edges of the sun appear softer and less defined, which may contribute to a perception of greater size. The light absorption and scattering by these particles also redden the sun’s appearance, a visual characteristic often associated with its low position.
Humidity in the air can also play a role by increasing the amount of light scattering and creating more distinct atmospheric layers. These layers can further affect how light bends and distorts the sun’s image. Temperature inversions, where a layer of warmer air traps cooler air near the surface, create stable atmospheric conditions that can enhance this effect. Such inversions produce layers of air with different refractive indices, causing additional bending of light and potentially making the sun’s image more distorted and seemingly larger. These environmental factors do not cause the illusion but rather modify the visual input, making the sun’s apparent size and proximity more pronounced.