What Is This "Green Flash" Coming from Venus?

In the frosts of the Swedish dawn, a fleeting spectacle has amazed observers: Venus, our planetary neighbor, has emitted a green flash. This phenomenon, rarely observed, is reminiscent of the green glimmers occasionally produced by the Sun at dawn or dusk.


On January 8, as Venus was rising on the horizon of Stockholm, photographer Peter Rosén captured this fleeting moment. The green flash, lasting for about one second, turned out to be an "unexpected bonus" during this morning observation. But what caused this colorful display in the nocturnal sky?

Visible light, composed of different colored wavelengths, generally appears white to us. When sunlight passes through the Earth's atmosphere, these wavelengths are partially refracted by gas molecules. This phenomenon is the origin of the daytime sky's blue color, due to the predominant scattering of blue wavelengths.

Under specific conditions, especially during clear skies and when the Sun or a nearby planet is close to the horizon, light must travel through a larger part of the atmosphere. This results in a more pronounced separation of the different wavelengths. Short waves, like blue and violet, are more scattered, while longer waves such as red, orange, and yellow are more readily absorbed. Consequently, sometimes only the medium-length waves, like green, reach the observer. However, the angle must be perfect for this phenomenon to be visible, hence the brevity and rarity of these "green flashes."

In the case of Venus observed in Stockholm, green was the most evident color, but other hues like yellow, orange, red, and blue were also perceptible. These colorful flashes are not limited to the Sun: similar phenomena have been observed around Mercury, the Moon, and even, under certain circumstances, through ice crystals in the atmosphere, producing rainbow clouds and luminous halos around the Sun or Moon.

Unlike solar green flashes, more frequent above the ocean, planetary light displays occur more through extremely cold air, amplifying their effect. Thus, these planetary luminous phenomena are not strictly limited to the horizon.