Astro Bob: Emerald fire at dawn — how to see the elusive green flash

I never see enough sunrises. Every time I make the effort to drive down to the shore of Lake Superior to watch my favorite star rise I promise myself to get up for more. Sunrise isn't just a singular celestial experience, but an emotional one. Like getting vaccinated with hope.

Things happen fast in the minutes surrounding the sun's first appearance. Colors intensify, the sky in the sunrise direction brightens to bursting and then it happens — the sun explodes onto the scene like a supernova. That's no exaggeration. The first peep of sunlight is a brilliant point of light that quickly expands into a slab, an oval and finally a disk as the rotating Earth lofts it into the sky. Often, a combination of atmospheric refraction and multiple layers of air at different temperatures deform the rising sun into a wild variety of shapes, making every sunrise a unique natural masterpiece.

I want to focus on a special phenomenon that occurs at the moment of sunrise and sunset called the green flash. At those times, for a second or two at most, the sun’s upper rim can appear intensely green. Where I live only sunrises are visible because that's the only direction with a horizon.

A good horizon is key. You'll need to see down to within about 1° to 1.5° of that fine line, equal to your pinkie finger held against the sky at arm's length. Hills or trees in the distance will block the sight, the reason it's most often reported over large lakes, seas and oceans. Not that you can't see the flash over land, provided the horizon is flat and far away. Clear, clean skies help, too. If the sun is a deep red ball as it approaches the horizon, you can probably cancel your green flash appointment. But if it's too bright to glance at, chances improve.

A friend saw the flash for the first time while watching a sunset along the north shore of Lake Superior. He wasn’t even looking for it. But when it was over a second later, he turned to his paddling partner and asked “Did we just see the green flash?” My first sighting was several years back through a camera viewfinder. I was equally surprised. Then on March 6, I drove down to the lake at dawn to find Jupiter, Mercury and Saturn and stuck around for sunrise. My jaw dropped when the first bit of sun glowed intensely green as it breached the horizon. Pure emerald fire! Pure joy, too.

Green flashes occur because the atmosphere bends or refracts sunlight, especially near the horizon, where our line of sight traverses hundreds of miles of the lower atmosphere. Here, the air is thick and dense compared to looking overhead. If you’ve ever used a prism to spread sunlight into its component colors, you’ll recall that blue and violet lie at one end of the spectrum and orange and red at the other. That’s because different colors of light are bent by different amounts, cooler colors more than warmer ones.

Dense air acts as its own prism and spreads the sun into a compact stack of multiple, overlapping disks, one for each color of the rainbow. The spreading of light according to color is called dispersion. It's most apparent near the horizon where the multi-hued solar disks overlap and produce white light except at the top and bottom edges, which are fringed in blue-green and red, respectively. You can view this phenomenon safely around the time of full-moonrise in a pair of binoculars. Of course, never look at the sun directly with or without optical aid except through a safe solar filter.

Since blue light is bent more than green light, you might wonder why we don't see a blue flash instead. That's because air molecules scatter blue light, which then colors the sky blue. Green is literally the last color left standing. Rarely, violet or blue flashes have been recorded, but emerald green is the most common color observed.

The green flash didn’t get much attention from skywatchers until its mention in the Jules Verne novel Le Rayon Vert (The Green Ray). Verne wrote ecstatically about the color of the flash as “a most wonderful green, a green which no artist could obtain on his palette, a green of which neither the varied tints of vegetation nor the shades of the most limpid sea could ever produce the like! If there is a green in Paradise, it cannot but be of this shade, which most surely is the true green of Hope.”

I used to think the green flash was extremely rare, but after viewing it at least a half-dozen times in the past few years I discovered it was more a matter of "being there". With clear skies and a good horizon, there’s no reason why the flash can’t become more familiar to anyone.

When you seek the green flash, never stare directly at the sun or you'll damage your retinas. Moreover, the solar dazzle can create confusing afterimages that make seeing the phenomenon more difficult. At sunrise, keep your eye on the brightest spot along the eastern horizon. That's where the sun will rise and the brief flash may appear. At sunset, it will jump into view when the moment the last bit of sun dips below the horizon.

Mirages are common over large land surfaces and bodies of water, and they can make the green flash easier to see. An inferior mirage, better known as the “hot road” mirage, can isolate and expand the sun’s green upper rim into a distinct patch of green light that appears to hover a second or two above the departing and before the rising sun.

Have hope.

• The Photographer’s Ephemeris ($9.99) for iPhone, Android and Desktop — Shows you exactly where the sun will rise and set at your location with local landmarks
• Timeanddate sunrise and sunset time calculator — Sunrise and sunset times for anywhere on Earth
• Introduction to Green Flashes — A wonderful compendium of information, diagrams and animations on the topic

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob .