Watch the Sun's own light bend on its way to your eye. Scrub its true altitude down toward the horizon and see atmospheric refraction lift the apparent disk, squash it into a flattened oval, and split its colors into bands - the same optics behind a real green flash.
Atmospheric Refraction Sunset 3D Explorer
Scrub the Sun's true altitude down to the horizon and watch two things separate: a faint marker showing where the Sun geometrically is, and a bright disk showing where atmospheric refraction actually bends its light into view - flattened, and briefly split into color bands.
Refraction bends starlight and sunlight more the closer an object sits to the horizon, because the light path cuts through a much thicker, denser slice of atmosphere. At the horizon itself the bending is close to its maximum published value, which is why the setting Sun looks noticeably squashed and sits measurably higher than its true position for the last few minutes before it geometrically disappears.
- Slider control: move the Sun's true (geometric) altitude from a few degrees above the horizon down to well below it
- "Play sunset" animates a full descent automatically
- The faint dashed marker is the Sun's true direction - not actually visible on its own, since only refracted light reaches an observer's eye
- The bright disk is the apparent Sun: its position, vertical squash, and top-to-bottom color banding all update from the same refraction figures live
- "Clean, stable air" toggles a wider green-flash window, matching how mirage-friendly conditions make the flash easier to catch
- Runs fully in the browser with the vendored three.js engine - no account, no upload; your last settings are remembered on your device
| Measurement | Published value |
|---|---|
| Refraction at the horizon | about 34 arcminutes |
| Sun's true angular diameter | about 32 arcminutes |
| Standard sunset/sunrise altitude (disk center) | -50 arcminutes (34' refraction + 16' solar radius) |
| Upper-limb lift near the horizon | about 29 arcminutes |
| Lower-limb lift near the horizon | about 35 arcminutes |
Why the disk looks flattened and sits too high
Refraction lifts the Sun's lower limb (about 35 arcminutes near the horizon) more than its upper limb (about 29 arcminutes), because the lower limb is closer to the horizon and passes through even more atmosphere. That six-arcminute gap, out of a disk only about 32 arcminutes across, is what squashes the setting Sun into the oval shape long noticed by naked-eye observers. The same bending is why the astronomical definition of sunset places the Sun's center 50 arcminutes below the true horizon - 34 arcminutes of refraction plus the 16-arcminute solar radius - so the Sun you watch touch the horizon has, geometrically, already set.
What sets up the green flash
Refraction bends blue and green light slightly more than red, so the setting Sun's disk is really a stack of color-tinted images: a flattened red-orange base with fainter green and blue slivers riding just above it. Blue and violet are usually scattered out of the direct beam before they reach an observer, which leaves green as the color most likely to flash briefly from the last visible sliver of the upper limb - typically for only a second or two, and far more reliably when the air is clean, stable, and the horizon is unobstructed (open sea or desert views improve the odds).
Twilight bands (civil, nautical, and astronomical) describe how dark the whole sky gets once the Sun is several degrees below the horizon - a much wider angular range than the sub-degree bending covered here. See the Twilight Types 3D Explorer for that global sky-brightness picture; this page focuses on the optical bending and color-splitting of the Sun's own disk right at the horizon crossing.
Everything renders on your device with WebGL. The 3D engine loads once (about 0.7 MB) and is cached, and nothing about your visit is sent to a server.
This is an educational geometry approximation (a standard low-altitude refraction model), not a ray-traced atmosphere or tonight's actual sky - the light-bend path is a stylized two-segment bend, and the color banding is simplified. Rayleigh scattering (roughly proportional to 1 over wavelength to the fourth power) is why the direct disk also reddens near the horizon; it is described here, not separately ray-traced. This is not a physical simulation.
Frequently Asked Questions
Why does the setting Sun look flattened?
Atmospheric refraction lifts the Sun's lower limb (about 35 arcminutes near the horizon) more than its upper limb (about 29 arcminutes), because the lower limb passes through even more atmosphere. That gap squashes the roughly 32-arcminute disk into a visible oval.
Is the Sun really already below the horizon when I see it "touch" it?
Yes. Refraction at the horizon is about 34 arcminutes, and the standard astronomical definition places sunset at the Sun's center 50 arcminutes below the true horizon (34 arcminutes of refraction plus a 16-arcminute solar radius) - so by the time the disk appears to touch the horizon, it has geometrically already set.
What causes the green flash?
Refraction bends blue and green light slightly more than red, splitting the setting Sun's disk into faint color-tinted layers. Blue and violet are usually scattered away before reaching an observer, leaving green as the color most likely to flash briefly from the last visible sliver of the upper limb - typically for only a second or two.
Why does toggling "Clean, stable air" change the green-flash window?
Mirage-friendly conditions (a clean, stable, unobstructed horizon, such as over open sea or desert) make the color-separated slivers easier to resolve and widen the practical window in which a flash can be seen - the toggle represents that real-world sensitivity, not a different refraction value.
Is this the same as civil, nautical, and astronomical twilight?
No. Twilight bands describe how dark the whole sky gets once the Sun is several degrees below the horizon - see the Twilight Types 3D Explorer for that wider-angle sky-brightness picture. This page covers the much smaller, sub-degree optical bending and color-splitting of the Sun's own disk right at the horizon crossing.
Why does the disk change color as it nears the horizon?
Rayleigh scattering, which scales roughly as 1 over wavelength to the fourth power, removes more blue light than red from the long, low path sunlight travels near the horizon, reddening the direct disk - on top of the finer color-layering from differential refraction that this scene shows.
Is this a real ray-traced atmosphere simulation?
No. It is an educational geometry approximation using a standard low-altitude refraction model. The light-bend path is a stylized two-segment bend and the color banding is simplified - but the refraction, disk-diameter, limb-lift, and sunset-definition figures shown are the real published values.