Green Flashes: Sun, Moon, Venus, Mercury

Follow a sunset on a clear day against a distant horizon and you might glimpse green just as the Sun disappears from view. The green flash is caused by refraction of light rays traveling to the eye over a long path through the atmosphere. Shorter wavelengths refract more strongly than longer redder wavelengths and the separation of colors lends a green hue to the last visible vestige of the solar disk.