The Sun looks flattened near the horizon because Earth’s atmosphere bends light from its lower edge more strongly than light from its upper edge. Both edges can appear higher than their geometric positions, but the bottom is lifted farther. Their apparent vertical separation shrinks, making the disk look squashed.
The Sun has not changed shape over the few minutes of a sunset. The changing image is mainly a consequence of the light’s path through the air. A dim or reddened solar disk is still not safe to stare at or inspect through unfiltered optics.
Why the bottom moves upward more
Air’s refractive index changes with conditions such as density. In a smoothly stratified atmosphere, a ray traveling through those changing conditions follows a curved path. An observer interprets the light’s final direction as a straight line back into the sky, placing its apparent origin higher than its geometric direction.
Close to the horizon, that correction changes rapidly with altitude. Rays from the Sun’s lower edge pass through a different atmospheric path from rays at its upper edge, producing a larger upward displacement at the bottom. The original explanatory text accompanying Atmospheric Optics’ flattened-Sun example traces that differential bending.
If every part of the disk were lifted by exactly the same angle, the whole image would move upward without becoming flatter. The difference across the disk is what changes its apparent shape.
A calculated disk makes the geometry visible
Consider a simplified Sun with a geometric vertical diameter of 32 arcminutes. One arcminute is one-sixtieth of a degree. For this arithmetic illustration, suppose refraction lifts the lower edge by 30 arcminutes and the upper edge by 25 arcminutes.
The apparent vertical diameter is then:
32 + 25 − 30 = 27 arcminutes
The bottom has gained five arcminutes on the top, compressing their separation. The disk can remain much closer to its original horizontal width, creating the familiar flattened outline.
These refraction values are chosen to explain the subtraction. They are not measurements, a universal sunset correction, or a prediction for a particular evening. Calculating a real profile requires an atmospheric model and the observer’s geometry.
The US Naval Observatory’s rise-and-set definitions use an average horizon refraction of about 34 arcminutes for routine computations. That conventional value helps define sunrise and sunset; it is not one displacement that should be applied to every point on the solar disk.
Why some sunsets are much more distorted
Temperature and pressure affect the air along the path. When the vertical structure departs from a smooth profile, the simple oval can acquire ledges, separated-looking sections, or other mirage distortions. A single surface temperature is not enough to specify the entire path.
Height above the ground and the shape of the horizon also matter. Buildings or a ridge can cut off part of a circular Sun without producing atmospheric flattening. A photograph’s optical distortion or processing introduces additional possibilities. Identifying a shape in an image therefore requires more context than the word “sunset.”
USNO notes that actual rise and set times can differ from calculated values because refraction is variable. Local terrain and observer height add further differences. The familiar appearance of a low Sun should not be confused with a precisely predictable standard oval.
The effect can also be seen from orbit
A spacecraft does not have air all around it, yet its line of sight toward the Sun can graze Earth’s atmosphere. Light passing through that atmospheric limb can form a highly distorted solar image.
NASA’s 1962 Mercury report, in the Space Science Report section titled “The Flattened Sun,” compares a refraction model with photographs from the early orbital flights. It records an observed effect and a model of the viewing geometry, rather than a change in the Sun itself.
An orbital photograph is not a direct template for the view from sea level. The light paths differ. It nevertheless supplies a useful check on the explanation: distortion can arise along a path through Earth’s air even when the observer is above that air.
Appearance, orientation, and physical shape are different questions
Atmospheric flattening changes the relative directions from which different parts of the disk reach an observer. It differs from simply rotating an image, as happens when the Moon appears tilted or upside down relative to different horizons.
Neither effect means the celestial body rapidly changes its physical form. Their explanations depend on different parts of the observing geometry: the path through air for flattening, and the chosen reference directions for orientation.
This is also why an image needs a clear description. A real photograph, a simulated refracted disk, and an artist’s sunset scene can look similar while making different claims about evidence.
Inspect the explanation without risking your eyes
Use published observations or a clearly identified simulation to examine the disk’s outline. Sunset color and apparent dimness are not safety standards. NASA’s solar-viewing guidance states that ordinary sunglasses are inadequate and that eclipse glasses must not be used to look through a telescope, binoculars, or a camera lens.
This explanation is not a solar-observing procedure. The useful physical test is whether the lower edge has been displaced more than the upper edge. That differential refraction accounts for the ordinary flattened appearance without requiring an unusual change in our star.
AI-generated illustration for Galileo Whispers of atmospheric flattening of the setting Sun. Shape and angular scale are illustrative, not a photograph or atmospheric measurement. Never look directly at the Sun without proper solar protection.
