We often imagine that the sun’s rays at sunset can only come from one place: the horizon where the star disappears. Yet the sky sometimes holds a surprise exactly opposite to this familiar image. Tonight, if you look up at just the right moment, you can see beams of light seeming to emanate not from the west, but from the east. A optical effect as striking as it is little known, worthy of a moment’s attention before you draw the shutters.
- Clouds, even very distant ones, cast shadows that traverse the entire atmosphere and create alternating bands of light and darkness.
- These bands, called anticrepuscular rays, appear to converge at the horizon opposite the sun, even though they are actually parallel, like railroad tracks.
- To observe this phenomenon, turn toward the east just after sunset, keeping in mind that this visual display cannot be used to predict tomorrow’s weather.
- This evening, the sky plays a trick on our eyes accustomed to the setting sun
- How distant clouds sketch shadows that cross the entire sky
- Why these rays seem to originate from the horizon opposite the sun
Tonight, the sky plays a trick on our eyes used to the setting sun
Everyone knows the scene: the sun sinks toward the horizon, and broad rays appear to fan out from it like a bouquet of light. This image is so familiar that we forget it’s only part of the overall spectacle. For at the very moment these crepuscular rays illuminate the western sky, a twin phenomenon can appear on the opposite side, in the east. The common misconception is to believe that the sunset’s light can only manifest where the sun disappears. In reality, the entire sky participates in the display, and that is precisely what makes watching twilight so rich for anyone who takes time to look in every direction.
How distant clouds sketch shadows that cross the entire sky
To understand this atmospheric magic, you must look toward the clouds. When a cloud lies between the sun and you, it blocks part of the sunlight and creates a shadow. But that shadow does not stay fixed next to the cloud: it projects across the entire atmosphere, made visible by the particles suspended in the air. The result is a sequence of bright bands and dark zones that can stretch across a large portion of the sky. A crucial point to note: these clouds do not have to be directly overhead. Distant cumulonimbus can cast their shadow far away. MeteoSwiss reports an instance observed on August 11, 2023, where cumulonimbus clouds in eastern France produced shadows visible from Switzerland, a phenomenon confirmed by satellite imagery from that time. A concrete example illustrating how this spectacle can unfold at great distance, without a direct weather link to the observation site.
Why these rays seem to originate from the horizon opposite the sun
When clouds intercept part of the sunlight, they can produce an alternating pattern of illuminated bands and shadowed regions made visible by the particles in the air. These bands sometimes span across the sky and appear to converge away from the sun: these are anticrepuscular rays. The important point to understand is that these beams are in reality nearly parallel to each other, even though they give the impression of diverging near the sun and then meeting far away. MeteoSwiss likens this effect to railroad tracks that seem to converge at the horizon while remaining strictly parallel along their entire length. It’s simply a perspective illusion, the same trick played by any straight line receding from our gaze. A good approach if you want to try observing is to turn east just after sunset and look for that convergence point.
This dance of light and shadow remains, above all, a visual pleasure, not a forecasting tool. These rays alone cannot predict tomorrow’s weather, so do not rely on them to anticipate a change in conditions. But next time the sky blazes at sunset, take a moment to glance behind you: the opposite horizon sometimes has something to show you as well.
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