A remarkable photo showing the summit of a volcano in Spain's Canary Islands includes something odd: a shadow of the flat-topped peak forming a perfect triangle.
Photographer Juan Carlos Casado captured the image of Mount Teide, or Pico de Teide, a dormant volcano on the island of Tenerife with a peak that towers 12,000 feet (3,700 meters) above sea level. The image was shared on June 9 on NASA's Astronomy Picture of the Day (APOD) website.
Though the shadow resembles a pyramid, Mount Teide doesn't have a pointed peak. Rather, the shadow appears triangular because of how our eyes perceive it relative to the distant horizon, according to APOD. [The Most Amazing Optical Illusions (and How They Work)]
When artists draw parallel lines extending off into the distance along a single plane, the lines draw closer to each other until they converge at the horizon; this spot where they meet is known as the vanishing point. This happens when you observe real-life parallel lines, too. For instance, if you observe train tracks that continue straight ahead, they appear to draw closer together and meet at a central point on the horizon.
A similar effect is taking place in the photo, APOD explained. In the image, Mount Teide is clearly no pyramid; the Pico Viejo crater flattens the mountain's peak. Nevertheless, as Mount Teide's shadow extends into the distance, it tapers gradually. In doing so, the shadow eventually forms a triangular peak, even though the object casting the shadow isn't a triangle.
A person standing on a summit during sunrise or sunset (when shadows are at their longest) will peer down a corridor of shadow; from that perspective, the shadows of mountains or volcanoes nearly always resemble pyramids, according to the U.K. website Atmospheric Optics. This is because the shadows are so long that a viewer can't possibly see where they end, so their termination appears as a point on the horizon rather than a true representation of the mountain's shape, Atmospheric Optics explained.
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Originally published on Live Science.