APOD October 20, 2026: The Anticrepuscular Rainbow Fan Explained by NASA Science
A Rainbow With an Illusion
The featured image shows a rainbow limited to a fan of sunlight. That partial shape is not especially rare, but the apparent position of the Sun makes this scene unusual.
The Sun seems to sit ahead of the viewer, just below the hills. Yet the rainbow’s appearance does not mean the Sun was actually there.
How The Light Appeared
Rainbows form when light reflects backward from falling raindrops, which means the Sun must be behind the person viewing one. In this photograph, the apparent forward direction is an optical effect.
Near the setting Sun, openings between clouds let rays travel across the sky. These anticrepuscular rays seemed to converge in front of the viewer, creating the impression of a reversed rainbow.
Captured At Sunset
The photograph was taken with a phone in Presenzano, Italy, at sunset last week. The unusual-looking rainbow was spotted while someone was hanging up laundry. NASA’s Astronomy Picture of the Day presents a different image or photograph of the universe each day, accompanied by an explanation from a professional astronomer. The next featured image is identified as “open space.”
Key points
- The rainbow appeared in a sunlit fan rather than as a complete arc.
- Rainbows form with the Sun behind the viewer.
- Anticrepuscular rays made sunlight seem to converge in front.
- The phone photograph was taken at sunset in Presenzano, Italy.