The green flash lasts about a second and is entirely predictable
Refraction splits the setting Sun into coloured images stacked vertically. For a moment only the green one is above the horizon.
For about a second, at the moment the Sun finishes setting, the last visible sliver can turn a vivid green. It is not a camera artefact, a retinal afterimage or a myth. It is refraction, and it is the most direct demonstration of chromatic dispersion available without a prism.
Why the Sun sets as a stack of coloured images
The atmosphere bends light, and it bends blue light more than red. At the horizon the total refraction is about 34 arcminutes — more than the Sun’s own diameter — and the difference between the red and green images is around half an arcminute, with blue another fraction above that.
So when the Sun sets, it is not one disc going down. It is a series of images of different colors stacked vertically, green above red, and blue above green. While the Sun is high, the stack overlaps and you see white. As the last sliver disappears, the red image is already below the horizon while the green one is still above it — and for that moment the topmost visible part of the Sun is green.
The effect also explains why the setting Sun looks flattened: the lower limb is refracted more than the upper limb by about 5 arcminutes, which squashes a 32-arcminute disc into an oval. The green flash is the vertical color separation of the same mechanism.
What improves the odds
- A sea horizon, or a very distant flat land horizon. The Sun must disappear cleanly behind a sharp edge.
- A low vantage point, so the line of sight passes over water for a long distance.
- A temperature inversion — warm air above cooler water. This increases refraction and makes the flash last longer and appear greener.
- Clean air above the horizon. Haze above the sunset kills it; haze below can help.
- Patience. It is not visible most evenings.
Why green rather than blue
Blue is scattered out of the beam over the long horizontal path — the same Rayleigh scattering that makes the sky blue removes the blue from sunlight traveling through 300 km of dense atmosphere. The blue image is also the highest and thinnest of the stack, so there is less of it to see.
That leaves green as the last color above the horizon, and it is why a blue flash, though real, is rare enough to be a specialist’s prize.
The rest of the family
The same geometry produces a set of related effects: the green rim, a green fringe visible along the top edge of the Sun for the last minute or two of sunset even without a flash; the green ray, a beam extending upward from the point of sunset, reported mostly by observers with a very good horizon; and on Venus, which is bright enough and low enough to show a green flash of its own.
How this connects to the rest of the site
The horizon convention matters here more than anywhere: sunrise and sunset are defined on the upper limb, and the 0.833° that refraction and semi-diameter contribute is exactly what makes the green image briefly visible after the red one has gone. A rise/set table that used the geometric horizon would have the Sun disappear before the flash could happen.
The sunrise table gives the moment to be watching, and the equinox page explains where the 0.833° comes from.
Refraction values are the standard published figures. Whether you see it depends on things no table can promise: weather, horizon and luck.
Frequently asked questions
- Is the green flash real?
- Yes, and it has been photographed many times. It is a refraction effect, not an optical illusion or a retinal afterimage. It lasts about one to two seconds at a sea horizon, occasionally longer, and is most often seen at the last moment of sunset or the first moment of sunrise over water.
- Why green and not blue?
- Because blue light is scattered out of the direct beam by the atmosphere over the long horizontal path, and because the blue image sits even higher than the green one and is therefore fainter and thinner. In very clean air a blue flash can be seen above the green, but it is much rarer. Green is the surviving color of the topmost sliver of the Sun.
- Does it need to be a sunset over the sea?
- A distant, unobstructed horizon helps because it gives the Sun a clean edge to disappear behind, and the sea provides both that and a low vantage point. But the effect has been seen over land, from aircraft, and even from high buildings. What it needs is a long atmospheric path and a sharp horizon.
- Why is it not visible every evening?
- Because it needs a nearly perfect horizon, no haze layer above it, and a specific temperature structure. A temperature inversion — warm air over cooler water — increases the refraction and makes the effect last longer and turn greener. Most evenings the last sliver just fades.
Sources
Each source is a primary or official reference, checked before publication. Links open the original.
- Meeus, Astronomical Algorithms, 2nd edition, chapter 16 — differential refraction by wavelength and the vertical separation of the solar image at the horizon
- NOAA Global Monitoring Laboratory — atmospheric refraction at the horizon (about 34 arcminutes) and its wavelength dependence, which is what separates the coloured images
- Royal Observatory Greenwich — the green flash as a real refraction phenomenon, the conditions that favour it, and its typical duration
- U.S. Naval Observatory — the sunrise and sunset definitions and the upper-limb convention that determines the last visible sliver of the Sun (aa.usno.navy.mil)