The blue beads of sunset | Astronomy.com (2024)

A word of caution

If you’re planning to look visually, it’s imperative to take safety precautions to avoid inadvertently staring for extended periods directly at the Sun, no matter how small the area of sunlight.

I suggest using safe solar filters, available from reputable dealers, to view the Sun as the horizon begins to cover it. Then, just before the Sun’s upper rim slips out of view, quickly remove the filter from your eyes. If you’re using binoculars, make sure you have safe solar filters that fully cover both front objectives — not the rear eyepieces where the focused light can burn through or crack the filters.

Gone in a flash

The “flash” (a term that encompasses all aspects of the phenomenon, which can vary in length, breadth, color, and duration) results from light bending as it passes through the densest part of Earth’s atmosphere near the horizon. As with light shining through a prism, the blue region of the spectrum bends more than the red part. As the atmosphere effectively scatters blue light, the color most often visible in a flash is green. Under exceptional conditions, however, the upper rim of the Sun can appear blue or even violet.

Whatever color you see, the phenomenon lasts just a few seconds. Thus, timing is critical. The photographic blue flash sequence shown at top, taken from an altitude of about 5,000 feet (1,500 meters) on Mauna Loa, Hawaii, spans a mere two seconds. Of it, “green flash” expert Andrew T. Young of San Diego State University in California comments, “The height above sea level is very conducive to getting the blue and violet colors because of the reduced atmospheric reddening.”

Now it’s your turn. Be safe, and send your results to someara@interpac.net.