12 November 2004

The science of sunrises and sunsets

Forrest M. Mims III

If you plan to enjoy a sunrise or sunset anytime soon, this column is just for you. So find your sunglasses. We’re going to get ready for the next good sunrise or sunset.

Sunrises and sunsets can be as instructive as they are glorious. Clouds obscure the science, but they often add to the grandeur.

Unless the sky is thick with haze, the rising sun should not be watched after it makes its appearance. This is why you have to take more precaution while watching a sunset.

If the sun stays bright all the way to the horizon, the sky is extraordinarily clear. You need to wear very dark sunglasses to watch this rare kind of sunset. And you should never look directly at the sun on such days until it is mostly behind the horizon.

Often the sun becomes a deeper orange as it descends toward the horizon. This is because natural haze and air pollution absorb the blue colors of sunlight, leaving behind the orange and red.

When the sun is about its diameter over the horizon, its lower edge will touch the horizon in about two minutes. Now is the time to take some pictures.

As the sun continues to set, prepare for the possibility of seeing the elusive green flash. This phenomenon is not an urban legend. It’s quite real, but it is rare when skies are hazy.

If the sky is clear, prepare to see the green flash by looking away from the sun until only a sliver remains above the horizon. Then watch carefully as the last edge of the solar disk slips behind the horizon. If the sky is clear, the last bit of the sun may glow green for a fraction of the second.

While this phenomenon is called a green flash, it’s really a green glow. I’ve seen it many times in Hawaii and across the Southwest, once when the sun set behind the metal roof of an old farmhouse not far from where I live.

My wife Minnie and I once saw several green glows in a row as we drove up and down a road in Arizona’s Painted Desert. Each time we crested a hill, the sun came into view. As we drove down the hill, we saw another sunset and another green glow.

A real green flash is much more stunning than a mere green glow. The only one Minnie and I have ever seen occurred while we were on a cruise ship off Baja California the evening after the solar eclipse of 1991. We were seated at dinner at sunset. Just as the sun slipped below the horizon, it emitted a brilliant green flash as bright as an electronic strobe flash. Seeing a total eclipse and a spectacular green flash on the same day was something we will never forget.

This column has been about watching the sun rise or set. Next time, we’ll look at the twilight glows that occur before sunrise and after sunset.

Forrest M. Mims III and his science are featured online at http://www.forrestmims.org/.

This feature was originally published in Forrest Mims's weekly science column in the Seguin Gazette-Enterprise, Seguin, Texas. The column is written for a general audience.