Observational Astronomy
The Green Flash: How Atmospheric Dispersion Splits the Setting Sun
For one to two seconds at the instant the last sliver of the Sun disappears below a sharp sea horizon, the top rim can flare a vivid emerald — the green flash. It is not a trick of a tired retina but a real image: Earth's atmosphere acts as a weak prism, refracting sunlight by roughly ½° near the horizon and dispersing it by about 10 arcseconds between red and blue, splitting the low Sun into overlapping colored disks whose green upper edge is the last to set.
The effect is faint, brief, and geometry-dependent, which is why it earned a reputation as a myth for centuries. In fact it is a predictable consequence of atmospheric refraction, wavelength dispersion, and molecular scattering, and under the right thermal conditions it can be photographed, mimicked with a telescope, and even observed rising at sunrise.
- RegimeAtmospheric optics, low solar altitude (<1°)
- Key number~½° total refraction at horizon; ~10″ red–green dispersion
- Driven byWavelength-dependent refraction (dispersion) + Rayleigh scattering
- First describedPopularized 1882 (Jules Verne); physics by G. B. Airy & others, 19th–20th c.
- Observed withNaked eye, binoculars, video at sharp low horizon
- Duration~1-2 s (up to ~15 s from ships/high latitude/mountains)
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
What the green flash is and why it matters
The green flash is a brief burst of green (occasionally blue or violet) light seen at the upper edge of the Sun — usually at the last moment of sunset or the first moment of sunrise — when it lies almost exactly on a clean, distant horizon such as the open ocean. It is a genuine optical image formed by the atmosphere, not an afterimage or contrast illusion, though a bright sunset can leave a complementary green afterimage that people sometimes confuse with it.
It matters as a textbook demonstration that the atmosphere behaves like a prism and a scatterer simultaneously. The same physics — astronomical refraction and chromatic dispersion — governs how star and Sun positions must be corrected in precise astrometry, why the low Sun and Moon appear flattened, and why celestial objects are displaced upward near the horizon by more than their own diameter. The green flash is where these effects become dramatically, colorfully visible to the naked eye.
The mechanism, step by step
Air is a dispersive medium: its refractive index n is slightly greater than 1 (about 1.0003 at sea level) and rises toward shorter wavelengths. So blue light bends more than red. Near the horizon, sunlight traverses a long, density-stratified air path and is refracted upward by the total amount ≈34′, but by a slightly different amount for each color. The atmosphere therefore builds a set of overlapping colored solar disks, each shifted vertically by up to ~10″.
Because red bends least, the red disk sits lowest and sets first; violet/blue bends most and would set last. But over the long horizon path, Rayleigh scattering (cross-section ∝ λ⁻⁴) and ozone absorption strip out the blue and violet, so the shortest wavelength that survives to the eye is green. As the Sun disappears, the red, orange, and yellow rims vanish in turn until only the thin green upper edge remains — for the last second or two, that edge is all that is left, and it flares green.
Characteristic numbers, scales, and the key relation
The governing quantity is refraction angle R as a function of apparent altitude h. Near the horizon a standard approximation (Bennett's formula) is R ≈ 1 / tan(h + 7.31/(h + 4.4)), with R and h in degrees and arcminutes — giving R ≈ 34′ at h = 0° and ≈ 10′ at h = 5°. Since R exceeds the Sun's ~32′ diameter, the Sun is entirely below the geometric horizon when its lower limb appears to touch it.
Dispersion — the color-dependent part — separates the red and green images by only ~10″, roughly 1/200 of the solar diameter, which is why the flash is a thin rim rather than a fat band. The Sun's vertical setting rate is ~15″ per second at the equator (slower at higher latitude, ∝ cos(latitude)), so a ~10-30″ green sliver is visible for on the order of 1-2 seconds. Anything that magnifies the vertical dispersion — a temperature inversion producing a mock mirage, or an inferior mirage over warm water — can stretch this to 10 seconds or more.
How it is observed and recorded
Requirements are a sharp, distant, low horizon (sea, desert, or mountain-top), clean stable air, and precise timing at the instant of last/first light. Ocean sunsets are classic because the horizon is far (~5 km for an observer 2 m high, farther from ships or cliffs) and unobstructed. Observers must never stare at the still-bright Sun; the flash is watched at the final sliver, ideally with brief glances or through video.
Modern documentation relies on telephoto video and DSLR bursts: frame-by-frame playback resolves the green (and sometimes blue) rim over a fraction of a second and confirms the color is real, not retinal. A small telescope defocused or aimed just above the setting limb can show the split colored disks directly. The phenomenon is also reproducible in the laboratory and in ray-tracing simulations of a standard or inverted atmospheric temperature profile, which predict its several distinct visual forms.
Where it operates, and its several forms
The green flash occurs wherever the geometry allows: any latitude, any season, at sunrise as well as sunset. It is favored over water and at high, clear vantage points, and can be more prolonged at high latitudes where the Sun sets at a shallow angle. Andrew Young's modern classification distinguishes several types: the inferior-mirage flash (an oval green segment, common over warm sea), the mock-mirage flash (a strip detaching from the disk in a temperature inversion), the rarer sub-duct flash, and the green ray (a green beam shooting upward in hazy air).
It is distinct from related but separate effects: general astronomical refraction (position shift), the flattening of the low Sun, the reddening of sunsets (scattering along the line of sight), and the complementary green afterimage that dazzling sunlight leaves on the retina. Only the true flash is an atmospheric image with measurable green photons at the solar rim.
Open questions and significance
The green flash is physically well understood — it is fully reproduced by ray-tracing through realistic atmospheric density and temperature profiles — so the frontier is not the basic mechanism but the details of the lower atmosphere. Which flash type appears, how long it lasts, and whether a rare blue or violet flash is seen depend sensitively on the vertical temperature structure (inversions, ducts, turbulence) within the lowest tens of meters of air. In this sense each flash is a diagnostic of local micro-meteorology, and its form encodes the thermal stratification along the sightline.
Its broader significance is pedagogical and practical: it is a vivid, everyday manifestation of the dispersion and refraction corrections that professional astrometry and geodesy must apply, and the same layered-atmosphere physics underlies looming, towering, superior and inferior mirages, and the Fata Morgana. Historically it moved from folklore — cemented by Jules Verne's 1882 novel Le Rayon-Vert — to a quantitatively predictable, photographable phenomenon.
| Quantity | Value / behaviour | Notes |
|---|---|---|
| Total refraction at true horizon (h=0°) | ~34′ (≈0.57°) | Larger than the Sun's own ~32′ diameter — the Sun is geometrically below the horizon when its lower limb appears to touch it |
| Refraction at h = 5° | ~10′ | Falls off rapidly with altitude |
| Red vs blue/green angular separation at horizon | ~10″ (dispersion) | Small fraction of the 32′ disk; produces overlapping colored rims |
| Blue/violet upper rim | Scattered/absorbed away | Rayleigh scattering (∝ λ⁻⁴) removes blue over long air path, leaving green as the shortest surviving wavelength |
| Typical flash duration | ~1-2 s | Set by the Sun's ~15″ s⁻¹ vertical setting rate near the equator |
| Enhanced (mirage) flash duration | up to ~10-15 s | Inferior/mock-mirage thermal layering stretches and lifts the green segment |
Frequently asked questions
Is the green flash real or just an optical illusion in my eye?
It is a real atmospheric image — actual green photons from the Sun's upper rim, separated out by refraction and scattering. It can be photographed and resolved frame-by-frame in video, which rules out a purely retinal effect. However, a separate green afterimage can appear if you stare at the bright Sun, so people occasionally confuse the two; the genuine flash is watched only at the final thin sliver.
Why green and not blue or violet, since blue light bends the most?
Blue and violet do bend most and would form the highest, last-setting rim, but they are removed by Rayleigh scattering (cross-section ∝ λ⁻⁴) and ozone absorption over the long, low horizon path. Green is the shortest wavelength that survives to reach your eye, so the surviving top rim looks green. In exceptionally clean air a brief blue or violet flash is occasionally recorded.
How long does the green flash last?
Typically about 1 to 2 seconds, set by the Sun's vertical setting rate of roughly 15 arcseconds per second near the equator and the ~10-30 arcsecond thickness of the green rim. It lasts longer at high latitudes (shallower setting angle) and can persist 10-15 seconds when a temperature inversion or mirage vertically stretches the green segment.
What conditions do I need to see one?
A sharp, distant, unobstructed low horizon — the open ocean is ideal — plus clean, stable air and precise timing at the exact moment of last light. A slightly warm sea surface or a temperature inversion enhances it. Higher vantage points (cliffs, ships, mountains) push the horizon farther away and improve the odds. Never stare at the still-bright Sun; watch only the final sliver, or use video.
Can the green flash happen at sunrise too?
Yes. The same geometry applies in reverse: at sunrise the green upper rim appears first, an instant before the rest of the disk. Sunrise flashes are physically identical but harder to catch because you must be watching the precise spot on the horizon before the Sun appears, with no cue for exactly where or when it will rise.
Is the green flash related to mirages and the flattened setting Sun?
Yes — all are consequences of atmospheric refraction through a density- and temperature-stratified atmosphere. The flattening of the low Sun comes from stronger refraction of the lower limb than the upper limb; mirages come from sharp thermal layering that bends rays and creates displaced or inverted images. Andrew Young showed the common inferior-mirage and mock-mirage green flashes are directly tied to those same mirage-forming temperature profiles.