When the sun dips low towards the horizon, it often appears significantly larger than when it is high in the midday sky. This common observation, occurring at both sunrise and sunset, has puzzled people for centuries. The perceived increase in the sun’s size near the horizon is an optical illusion.
The Apparent Size Phenomenon
The perceived difference in the sun’s size near the horizon is an optical illusion, not a physical change. The sun’s actual angular size, the amount of space it takes up in our field of vision, remains constant whether it is high overhead or low on the horizon. This can be verified by holding a small object, like a pea or an aspirin tablet, at arm’s length; it will perfectly cover the sun at any point in the sky, demonstrating its visual angle does not change. The Earth’s distance from the sun also remains constant over the course of a day, meaning no physical factors cause the sun to expand or contract.
If you photograph the sun at midday and again at sunset with the same camera settings, the images will show the sun to be the exact same size. This photographic consistency supports that the change in perceived size is entirely subjective. The apparent enlargement is a testament to how our visual system processes context rather than absolute measurements.
The Brain’s Visual Tricks
The primary explanation for the sun appearing larger at the horizon lies in how our brains interpret size and distance, a concept related to size constancy. When the sun is high in the sky, there are typically no nearby reference points, so it appears as a distant disc in an empty expanse. Our brain tends to perceive the sky overhead as a flattened dome, making objects near the horizon seem more distant.
As the sun approaches the horizon, it is viewed with familiar foreground objects like trees or buildings. These objects provide visual cues for depth and distance. Our brain assumes that objects appearing smaller are further away, and conversely, that larger objects are closer. Since the sun’s actual angular size remains constant but it appears behind distant, familiar objects, our brain “corrects” for the perceived greater distance by making the sun seem larger. This misinterpretation occurs because the brain attempts to maintain size constancy, perceiving objects as having a consistent size regardless of their distance.
What’s Not Causing It
Several common misconceptions do not explain why the sun appears larger at the horizon. It is not due to atmospheric magnification, where the atmosphere acts like a lens to physically enlarge the sun’s image. The Earth’s atmosphere does not function as a magnifying glass for celestial bodies. Nor is it caused by the Earth being physically closer to the sun at certain times of the day, as our planet’s orbit and rotation do not result in a significant change in distance.
While atmospheric refraction does occur and affects the sun’s appearance, it does not make the sun look bigger. Refraction causes light rays from the sun to bend as they pass through varying densities of air, especially near the horizon. This bending primarily affects the sun’s apparent position, making it appear slightly higher than its true astronomical position. It can also cause the sun’s disk to appear distorted or flattened, particularly at the bottom, but these physical effects do not contribute to the perception of increased size. The apparent enlargement remains a perceptual trick of the brain, not a physical alteration of the light or the sun itself.