The sun appears larger on the horizon due to an optical illusion effect called atmospheric refraction. When the sun is low on the horizon, its light passes through a greater thickness of atmosphere, making it appear bigger.
The sun on the horizon passes through more of the atmosphere than when it is high in the sky. As a result, sunlight undergoes refraction, meaning it is slightly bent as it passes through different layers of air, particularly due to variations in temperature and density. This refraction subtly flattens the image of the sun, making it often appear more spread out horizontally, which contributes to the deceptive impression of a larger celestial body. This phenomenon is even more pronounced when the horizon is hazy or polluted, as the particles present in the air enhance this visual distortion.
Our brain interprets the size of an object based on its context, comparing it to its surroundings. When the sun is high in the sky, it lacks precise reference points and appears smaller. On the horizon, however, surrounding objects like trees, mountains, or buildings create a contrast that influences our perception. As a result, our brain gets tricked and imagines the sun to be larger than it really is: this is an optical illusion. This visual phenomenon has nothing to do with a real increase in size, because if you were to measure it, you would see that its apparent size remains the same. It’s a matter of interpretation by the eye and the brain, which try a bit too hard to make sense of what they observe.
When the sun is positioned near the horizon, our brain automatically compares it to elements of the landscape: buildings, trees, mountains, or even the skyline of a city. These landmarks strongly influence our perception of size. At the horizon, we tend to unconsciously estimate celestial objects based on the surrounding context, which makes the sun appear significantly larger. In contrast, when the sun is high in an empty sky, devoid of concrete references, this comparison disappears — and it gives us the impression that the celestial body seems very small. In fact, it's exactly the same with the moon: a busy landscape makes it visually more imposing than a clear sky. Our visual perception is definitely very suggestible.
Our brain uses perspective to interpret the sizes of objects in our visual field. When we look at the sun on the horizon, our mind is deceived by a phenomenon comparable to the famous Ponzo illusion: two identical horizontal lines appear to be of different sizes when placed between two converging lines. It's the same with the horizon. The landscape forms natural converging lines, such as roads, rows of trees, or distant clouds. Our brain thinks, "if the sun remains this large despite this apparent distance, it must be enormous!" It then adjusts its judgment and makes us perceive an exaggeratedly large sun. In reality, its apparent size hasn’t changed at all; it is just our perception playing this little cognitive trick on us.
Some photographers sometimes use a telephoto lens to capture stunning photos of the sun on the horizon, but be aware that the lens also enlarges the visual perception of its size, further enhancing the illusion that is already present!
The same illusion that makes the sun appear larger on the horizon also applies to the full moon! It seems enormous when it rises, but it's simply a perception created by our brain.
In reality, the apparent diameter of the sun does not change at all during the day; it is only our visual perception that deceives us.
This solar illusion was already observed and described by Aristotle in the 4th century BC, who mistakenly believed that it was water vapor in the atmosphere that magnified the sun!
The flattening effect is explained by atmospheric refraction. When sunlight passes through a thick layer of the atmosphere near the horizon, this layer acts like a lens, vertically compressing the image of the sun, giving it a flattened or oval appearance.
Yes, this phenomenon is observable everywhere on Earth. It depends more on psychology and the perception of the human brain, as well as atmospheric refraction, which are present everywhere, rather than on your geographical location.
Yes, the Moon, as well as other celestial bodies, can also appear larger when they are near the horizon for the same reasons: optical illusions, visual references, and atmospheric refraction.
Although both phenomena are related to the atmosphere, they remain distinct. The reddish appearance of the sun at sunrise or sunset is due to the increased scattering of blue colors by the atmosphere, leaving mainly the warm colors (yellow, orange, and red). The phenomenon of the apparent increase in size at the horizon primarily involves mechanisms of visual perception and optical illusions.
No, the apparent diameter of the sun remains almost constant throughout the day. The impression of a larger size near the horizon is mainly due to optical illusions and effects related to human perception.
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