Last Six Seconds of the Setting Sun
March 20, 2023
Photographer: Meiying Lee
Summary Author: Meiying Lee
The last 6 seconds of a sunset I observed on January 31, 2023, from the Hehuan Mountains in Taiwan, are shown above. The lingering yellow, green, and blue lights line the horizon like pearls! How does the Sun disappear in the last few seconds of a given sunset?
When the Sun has fallen below the horizon, we continue to see sunlight because of atmospheric refraction. But before sunlight completely disappears, because of the different refractive indices of various colors of light (atmospheric dispersion), we can detect specks of yellows, greens, and blues arranged like pearls. Finally, they vanish one by one in front of the camera. Note that my altitude on Mt. Hehuan (about 3,000 m or 9,843 ft) permitted a clear view of this phenomenon in relatively clean air.
On light sensitive photographic film, the light with the smallest refractive index (yellow) disappears first, then the green light, and the blue light, having the largest refractive index, disappears last. Because the intensity of blue light is the weakest, and our eyes are more sensitive to yellow-green light, the naked eye can only see the green color disappearing in the sky. We all cheered when we saw this magical emerald, green light flash in front of our eyes just before it vanished.
Click here to see a video of this sunset. The first segment of the film shows the last 28 seconds of sunset at normal speed, and the second segment presents the last 8 seconds of sunset at slow speed. How the pearls of different colors disappear can be seen more clearly in the video.
Mount Hehuan, Nantou, Taiwan Coordinates: 24.1814, 121.2814
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Sunset Mock Mirage and Green Flash