Why the sun briefly turns emerald green just before it vanishes from view

Nature · Published 3 October 2026 Swipe more like this →

In short: The green flash is a fleeting optical event occurring at sunrise or sunset. While a subtle green rim exists at every sunset, it is usually invisible to the naked eye. When atmospheric conditions align to create a mirage, this faint color is magnified into a distinct, brilliant flash of light.

The phenomenon occurs because Earth's atmosphere acts as a prism, refracting sunlight into its constituent colors. As the sun sinks toward the horizon, its light travels through an increasing volume of air. This process separates the light into a spectrum, with red appearing at the lower rim and green, blue, or violet at the upper edge. While blue light is refracted most strongly, it is typically scattered out of the observer's line of sight, leaving green as the most prominent color visible.

A green rim is technically present during every sunset, but it is far too thin to be perceived without aid. To witness a genuine green flash, the atmosphere must be layered in a way that produces a mirage. This mirage serves to amplify the green light, stretching its duration from a mere fraction of a second to several seconds. Stable, clear air is essential, as it allows more light to reach the observer without being obscured by atmospheric pollutants or scattering.

Most reported flashes are categorized as inferior or mock-mirage effects, though some rare variations remain poorly understood. While the phenomenon is most commonly associated with the sun, it can also be observed with the moon, Venus, and Jupiter. Airline pilots flying westward often have the best vantage point, as their speed can effectively slow the sunset, extending the window for observation. Although the green flash is a purely meteorological event, it has long captured the human imagination, appearing in literature, film, and folklore as a symbol of hope, return, or transition.

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