Why does the Sun look flattened at sunset?

The flattening, or oval shape, occurs because refraction is stronger near the horizon. The bottom edge of the Sun's disk, being deeper in the atmosphere, is lifted more than the top edge. This differential refraction vertically compresses the Sun's apparent shape.

Does this model explain the green flash?

This simulator models the basic refraction that makes the green flash possible, but not the phenomenon itself. The green flash requires an additional effect: atmospheric dispersion, where different colors (wavelengths) of light are refracted by slightly different amounts, briefly separating them at the horizon under very clear conditions.

How accurate is the layered atmosphere model?

The layered model is a useful simplification for visualizing the cumulative effect of refraction. In reality, the atmosphere's density and refractive index change continuously, not in discrete steps. More precise predictions for navigation or astronomy use complex integral equations that account for this continuous gradient.

Does refraction affect stars and planets the same way?

Yes, all celestial objects are affected by atmospheric refraction when observed near the horizon. The amount of lift depends on the object's altitude angle, not its distance. For very precise observations, astronomers must correct for this shift to determine an object's true position.

Sunset Atmospheric Refraction

Geometric vs apparent horizon: lift of the solar disk from a layered atmosphere model.

About this model

Atmospheric refraction fundamentally alters our view of the sky, most dramatically at sunrise and sunset. This simulator visualizes how Earth's atmosphere bends, or refracts, sunlight, making the Sun appear higher above the geometric horizon than its true geometric position. The core principle is Snell's Law of refraction, n₁ sin θ₁ = n₂ sin θ₂, which describes how light changes direction when passing between media of different refractive indices. Here, the atmosphere is modeled as a series of concentric, uniform-density layers. As a light ray from the Sun enters each successive layer, its path bends slightly toward the denser air, creating a curved trajectory. The simulator calculates the cumulative bending angle, showing the resulting 'lift' of the solar disk. A key simplification is the use of a layered, spherically symmetric atmosphere, ignoring local variations like temperature gradients and turbulence. By interacting with the model, students learn to distinguish between the geometric horizon (a straight line from the observer) and the apparent horizon (where the sky meets the Earth as seen through the atmosphere). They can explore how the Sun's apparent position changes with atmospheric parameters, directly observing why we see the Sun before it geometrically rises and after it sets.

Who it's for: High school and introductory undergraduate astronomy or physics students learning about atmospheric optics, geometric optics, and observational astronomy.

Key terms

Atmospheric Refraction
Snell's Law
Refractive Index
Geometric Horizon
Apparent Horizon
Layered Atmosphere Model
Bending Angle
Solar Disk

How it works

Refraction is wavelength-dependent (green flash at extreme grazing cases) and varies with temperature profiles. This sim separates geometric altitude from a single effective lifting angle for teaching.

Frequently asked questions

Why does the Sun look flattened at sunset?: The flattening, or oval shape, occurs because refraction is stronger near the horizon. The bottom edge of the Sun's disk, being deeper in the atmosphere, is lifted more than the top edge. This differential refraction vertically compresses the Sun's apparent shape.
Does this model explain the green flash?: This simulator models the basic refraction that makes the green flash possible, but not the phenomenon itself. The green flash requires an additional effect: atmospheric dispersion, where different colors (wavelengths) of light are refracted by slightly different amounts, briefly separating them at the horizon under very clear conditions.
How accurate is the layered atmosphere model?: The layered model is a useful simplification for visualizing the cumulative effect of refraction. In reality, the atmosphere's density and refractive index change continuously, not in discrete steps. More precise predictions for navigation or astronomy use complex integral equations that account for this continuous gradient.
Does refraction affect stars and planets the same way?: Yes, all celestial objects are affected by atmospheric refraction when observed near the horizon. The amount of lift depends on the object's altitude angle, not its distance. For very precise observations, astronomers must correct for this shift to determine an object's true position.

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