What is the 'Green Flash'?
by Peter Michaud

Gemini Observatory Information Manager

Lots of people talk about it, but how many have actually seen the much-coveted green flash? If you haven't seen or heard of the green flash, it's simply a fleeting spot of intense green light on the horizon an instant after sunset or immediately before sunrise. Since Hawai‘i is one of the best places to see this phenomenon, let's explore why the green flash happens and how to improve your chances of glimpsing it safely.

Sunset 1

First of all, when trying to spot the Green Flash don't be fooled by what I call, "The Fool's Flash". This is simply the greenish after-image burned into your retina by looking directly at the setting sun. (If you're lucky, this condition will only be temporary.) Remember, even when it's rising or setting it's still dangerous to look directly at the sun's disk. While awaiting the green flash at sunset, look away until only the very top of the sun's disk is about to disappear below the horizon - it's only during this last instant that the green flash will be visible anyway.

Secondly, the green flash is best seen when the sun sets or rises over the ocean and only when it is absolutely clear all the way to the horizon. While this might seem common in Hawai‘i, distant clouds often linger on the Sunset 2horizon, making the green flash much less frequent than one might imagine.

In order to understand what to expect from the green flash, it helps to know how our atmosphere affects sunlight. Coincidentally, the phenomenon responsible for the green flash is also the one that paints rainbows across Hawai‘i 's sky.

Sunset 3A rainbow is created when rays of sunlight enter a raindrop, bounce around inside, and exit. Light from the sun consists of a potpourri of colors that are each bent by a different amount inside a raindrop. This unequal bending of sunlight, called dispersion, ultimately sends a rainbow of colors from each raindrop. It's simply a variation on this theme that makes the green flash possible.

Each day at sunrise and sunset, sunlight passes through the thickest possible layer of our atmosphere and this bends the light slightly. As our atmosphere bends the sun's rays, they are also dispersed or broken up into different colors, just like a rainbow. However, this atmospheric dispersion doesn't result in a rainbow. Rather, this bending of sunlight results in colored arcs of light above and below the bright orange disk of the sun.

Imagine getting a close-up look at the sun just before it sets. Studying the bottom edge of the sun's disk you'd notice a thin rim of pure red light. Above the mixed colors of the sun's blinding disk would be the light that was bent the most - a crest of vivid green light that's often thinly frosted by blue. Just after sunset (or before sunrise) this bright emerald green light at the edge of the sun glimmers over the horizon for about as long as the blink of an eye. So look carefully and don't blink!

It has been calculated that the green flash itself is far too small to be resolved by the unaided human eye. This really isn't so odd when you consider that all of the stars in the nighttime sky also appear too small to be resolved by the naked eye! Like the brightest stars and planets, the intensity of the green flash compensates for its size, producing a brilliant beacon in the twilight sky.

Over the years, some observers have reported seeing slightly different manifestations of the green flash than the one I described above. Under certain circumstances the green flash might actually appear larger, but no satisfactory theory exists to explain why. One of the most common reports describes a green ray of light projecting up from the horizon, but until I see, or better yet, photograph this phenomenon, I'll remain skeptical.

I've also been told that polarizing sunglasses can help enhance one's view of the green flash, but again I've never seen a good theory to explain why. I personally believe that almost any type of filter, polarizing or otherwise will reduce glare and improve your chance of seeing the green flash while reducing your chance of seeing the "Fool's Flash"!

Another way to improve your chances of spotting the green flash is to observe from a location that provides a clear view of the sunrise or sunset. I recommend looking for the green flash at sunset since it is easier to look in the right place at the right time. Here on O'ahu, an unobstructed view of the sunset can be had from Magic Island from the second half of September until early March. During the rest of the year I like to watch sunsets from Electric Beach at Kahe Point Park in 'Ewa.

For green flash watchers on the neighbor islands, simply look for a west facing shore that is not obstructed by any land mass. During some times of the year, our friendly neighbor islands can also interfere. For example from May-July here on O'ahu, clouds over Kaua'i prevent any observations of the green flash. Without going into details, it looks like most of the neighbor islands except Kaua'i and the southern part of the Big Island suffer from this problem to varying degrees, especially from April-August.

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