An unusual atmospheric phenomenon, an anticrepuscular rainbow, was captured by a photographer in Presenzano, Italy, last week. This striking optical illusion, resembling a rainbow but appearing in the opposite direction of the Sun, offers a fascinating glimpse into how light interacts with our atmosphere, demonstrating a rare alignment of light, clouds, and atmospheric conditions.
Understanding the Illusion
While appearing as a rainbow, this anticrepuscular phenomenon is fundamentally different. True rainbows are formed by sunlight reflecting off water droplets at a specific angle, with the observer positioned between the sun and the rain. In this case, the Sun, which appears to be low on the horizon just in front of the observer, is actually behind them. The light rays responsible for the anticrepuscular rainbow have traveled across the entire sky after being illuminated by the setting sun, creating an illusion of light convergence in the opposite direction.
Anticrepuscular Rays: The Hidden Sun
The key to this visual trick lies in anticrepuscular rays, which are also known as antisolar rays. These are the celestial counterparts to crepuscular rays (like sunbeams filtering through clouds), but they appear to emanate from the antisolar point – the point directly opposite the Sun in the sky. In this particular photograph, gaps in the clouds allowed the sunlight to refract and scatter, creating the fan-like appearance of the anticrepuscular rainbow. This captivating display was noticed by the photographer while performing a common household chore: hanging laundry.
Source: NASA
