The sun’s daily performance includes a finale often featuring a distortion as the solar disc touches the horizon. Instead of the familiar perfect circle seen overhead, the sun at sunset frequently appears squashed, flattened, or warped into an oval shape. This change in appearance is not a trick of the eye, but a genuine optical phenomenon caused by the Earth’s atmosphere. This alteration of the sun’s shape is a common effect of light interacting with the air around our planet.
Understanding Atmospheric Refraction
The fundamental reason the sun’s image becomes distorted is atmospheric refraction. Refraction describes the bending of light as it moves from one transparent medium into another with a different optical density. A common demonstration is how a straw appears bent when partially submerged in water, as light rays travel at different speeds through water and air.
Earth’s atmosphere is a vast layer of air that grows progressively denser as it approaches the planet’s surface. This density gradient means that light traveling from space passes through a continuous series of layers, each slightly denser than the last. As sunlight enters the atmosphere, it continuously bends toward the denser air closer to the ground. The atmosphere effectively acts like a giant lens, shifting the apparent position of celestial objects.
The Maximum Effect at the Horizon
This bending of light is magnified when the sun is near the horizon at sunrise or sunset. When the sun is directly overhead, its light travels the shortest possible distance through the atmosphere, entering almost perpendicular to the air layers. In this scenario, the light is minimally refracted, and the sun appears in its true position.
As the sun descends toward the horizon, its light must travel a much longer, more oblique path through the atmosphere’s densest layers. This extended journey subjects the light to the maximum cumulative effect of the density changes, resulting in the greatest degree of bending. Due to this refraction, the sun’s image is lifted significantly, causing it to appear higher than its actual geometric position. When the sun’s lower edge appears to touch the horizon, the entire solar disk is already physically below the horizon.
Why the Sun Appears Flattened
The specific shape change—the vertical flattening—is the result of differential refraction. The sun is an extended object with a measurable diameter, not a point source of light. This means light rays from the top edge enter the atmosphere at a slightly different angle and altitude than the rays coming from the bottom edge.
Light from the bottom edge of the sun must pass through the absolute lowest and densest layers of the atmosphere, subjecting it to the strongest upward bending. The light from the top edge enters the atmosphere at a slightly higher altitude, where the air is less dense. Consequently, the top edge of the sun is refracted upward, or “lifted,” less than the bottom edge.
Because the bottom of the sun is raised significantly more than the top, the perceived vertical distance between the two edges is compressed. The horizontal width of the sun’s image is not affected because the density gradient of the atmosphere is vertical, not horizontal. This unequal bending of light across the sun’s vertical diameter causes the circular solar disk to appear squashed into an oval or elliptical shape.