The Horizon Illusion Explained
The phenomenon where the Sun or Moon appears larger when it's near the horizon compared to when it's high in the sky is primarily a psychological effect, known as the 'Moon Illusion' or 'Horizon Illusion'. Despite how it looks, the actual angular size of the Sun/Moon remains virtually constant as it moves across the sky; in fact, it's technically slightly smaller at the horizon due to greater atmospheric distance.
Role of Atmospheric Refraction
When sunlight enters Earth's atmosphere at a shallow angle, as it does near the horizon, the atmosphere acts like a lens, refracting the light. This refraction slightly flattens the Sun's disc vertically and can make its lower edge appear lifted. While this distorts the shape, it does not magnify the Sun's apparent size; this atmospheric effect is minor compared to the dominant perceptual illusion.
Psychological and Perceptual Factors
The dominant explanation for the horizon illusion involves how our brains perceive objects in context. When celestial bodies are near the horizon, we view them alongside terrestrial objects like trees and buildings, which provide depth and scale cues. Our brain interprets the horizon as being farther away than the zenith (overhead), leading it to compensate by perceiving an object of the same angular size as physically larger when seen against a distant background.
Absence of True Physical Change
Crucially, scientific instruments and photographs confirm that the angular size of the Sun (the angle it subtends in our vision) does not increase at the horizon. The illusion is a fascinating trick of the mind, illustrating how our visual processing can lead to a perception that deviates from objective physical reality, rather than a change in the Sun's actual size or proximity.