For a long time the green flash had the reputation of a sailor's tall tale — a vivid spark of emerald light that appears on the very top edge of the sun in the last second before it sinks below the horizon. Jules Verne built a novel around it. Sailors swore that anyone who saw it would never again be deceived in matters of the heart.
It is, in fact, completely real, thoroughly photographed, and explained by ordinary optics. It's just very fast — typically one to two seconds — and it demands conditions most people never bother to look for.
Two effects work together to make it happen.
The first is refraction. When sunlight enters the atmosphere at a shallow angle, the air bends it, and it bends different colors by different amounts — shorter wavelengths (blue, green) bend slightly more than longer ones (red, orange). The atmosphere is acting as a very weak prism. Near the horizon, where light is skimming through the greatest thickness of air, that bending is at its strongest, and it effectively splits the sun into a stack of slightly offset colored disks: the red disk sitting lowest, the green and blue ones a hair higher.
As the sun sets, the red disk disappears below the horizon first, then orange, then yellow. For a moment, the only sliver still poking above the edge of the Earth is the top of the green one.
The second effect is scattering — the same process that makes the sky blue. That long, low path through the atmosphere scatters violet and blue light away almost completely, which is why sunsets are red in the first place. So the blue disk, which should be highest of all, never makes it to your eye. Green is the last color standing, and for a second or two you see it alone.
Green flashes are far more common than their mythology suggests. People who watch every clear sunset from the right spot see them regularly. Here's what actually matters:
| Condition | Why it helps |
|---|---|
| A distant, flat horizon | An ocean, a big lake, or a desert plain — you need the sun's true final edge, not a hill or rooftop |
| Very clean, clear air | Haze, smoke, or humidity scatters the green away before it reaches you |
| No clouds on the horizon | Even a low cloud bank hides the last critical second |
| Elevation helps | A cliff, a ship's deck, or a beach dune extends your horizon and lengthens the effect |
| Watch until the very last instant | The flash only happens as the final sliver disappears |
| Try sunrise too | It works in reverse — green appears first — but you must know exactly where to look |
Cold water helps as well. A temperature inversion — cold air trapped under warmer air, common over chilly seas — bends the light more strongly and can stretch the flash out for several seconds or lift the green sliver into a distinct floating shape. The longest observed green flash lasted 35 minutes, seen from Antarctica in 1929 as the sun crawled along the horizon.
No — observers and researchers describe several types. The inferior mirage flash is the most common from a beach, where the sun appears to flatten and its bottom merges with a reflected image, and a green sliver detaches from the top. The mock mirage flash, seen from higher elevations, makes the sun's upper edge appear to be sliced by thin strips as green fragments blink out of a notched rim. Rarer still is the blue flash, which happens when the air is so exceptionally clean that even the blue disk survives its trip through the atmosphere.
The green flash is real optics, not folklore: the atmosphere refracts sunlight like a weak prism, stacking slightly offset colored images of the sun, while scattering strips out the blue — so at the moment the sun's last sliver drops below a clean, distant horizon, green is the only color left to see. Give yourself an ocean horizon, clear dry air, and the patience to watch every sunset to the final second, and you'll catch one sooner than you think. Then, per tradition, you'll never be deceived in matters of the heart again.