Sky Wise: The Mysterious Green Flash

Today, our guest John Hicks joins us to explain an interesting and unusual effect known as "The Green Flash". This unusual phenomena can be seen in the right atmospheric conditions as the sun sets. John is an expert who studies the sun in detail with specialized equipment from his observatory in Southern Ontario. His knowledge is sought by many amateur astronomers wishing to study our sun. John will be a presenter at this year's "Starfest" to be held in August.

Please note that it is DANGEROUS to look at the sun without taking the proper precautions. Permanent EYE DAMAGE can result. SUNGLASSES ARE NOT ENOUGH! History's first telescopic astronomer, Galileo went blind from studying the sun so be warned -the eye will not feel pain but your vision will be permanently affected! See John's notes about eye safety!

THE GREEN FLASH AND GREEN RAY PHENOMENA by John Hicks

THE GREEN FLASH EXPLAINED

Occasionally at sunset, the green flash appears just after the sun’s disk sinks below the horizon, appearing as a small emerald green ellipse (click onfollowing color diagram outlining the sequence of events)

The green flash fades, shrinking out of sight in 3 or 4 seconds maximum. A sea horizon is usually essential in that your apparent horizon must be formed by some obstacle lower than your eye, hence the ocean landscape fulfills this requirement. Green flashes are also caught from mountain heights as long as your position surmounts the horizon. The second requirement is clean air (polluted or dust-laden air prevents the transmission of the green light). The third requirement is an optical aid, preferably a telescope or telephoto lens at least 500 mm focal length and more favourably 1000 mm focal length (add a 2X teleconverter to your telescope or telephoto). John Hicks uses a 500 mm TeleVue Pronto telescope which is “quasi-apochromatic”, with a 2X teleconverter lens in the optical train. This instrument has an extra bonus in that a true apochromatic lens has little chromatic aberration, meaning that all the spectrum of colors will focus at an exact point. This aids in resolving the pure green of the “flash”.

Note that it is very dangerous to look at the sun through any optical aid (binoculars, telescope, or telephoto lens) until the sun is resting on the horizon. At this point of contact, it is just attenuated enough by our atmosphere to reduce its luminosity to acceptable levels. Even at this low point, I find it too brilliant to look at through the telescope without using “averted vision”. The infrared and ultraviolet radiation, although weak at such a low altitude, can destroy rods and cones in the central portion of your eye, called the fovia. Damage is irreversible! Beware! Solar photography is for experts.

THE GREEN RAY EXPLAINED

The Green ray is seen rarely at sunset and vanishes as quickly as it appears. About 1/8^th to 1/10^th the diameter of the sun, it lasts as long as a lightning flash only, often appearing like a light from a lighthouse below the horizon throwing a ray of light upward. Often it is confused with the green flash, the green ray being much more elusive and spectacular of the two phenomena. It is every astronomer’s quest to capture the green ray on film, although the green flash itself is reward enough for me. As a bonus, an ancient myth claims that anyone seeing the green flash “will never be deceived in matters of the heart”.

To my knowledge, the GREEN RAY has never been successfully photographed.

PHYSICS OF THE GREEN FLASH

Atmospheric refraction of sunlight is basically the cause of green flashes and green rays. The following diagram illustrates the actual position of the sun at sunset. The upper circle shows where you see the sun’s image at point of contact with the ocean or horizon. The lower circle, outlining the already set sun, shows where it actually is. The displacement

of the sun’s disk is due to atmospheric refraction.

At sunset, as sunlight enters the Earth’s atmosphere it is refracted, bending downward

by 0.5 degrees (exactly the diameter of the sun). The refraction of light depends upon

its wavelength (essentially its color). Sunlight, being composed of many different wavelengths of light is bent at differing angles. Blue light is closest to the resonance

of atmospheric molecules, so it is refracted more than red or green. At sunset, the red light (the usual red image of the sun) will appear below the green image, which will

appear below the blue image. When the sun sets, all images of the sun made by wavelengths shorter than green are blocked by the horizon, leaving only the top edge

of the green image visible. This is the green flash. We don’t see the blue flash normally because our atmosphere scatters it more than green or red. Very rarely the atmosphere is so clear that the blue light will not be scattered as much as usual, and there will be a rare “blue flash”. The “blue flash” has been recorded in Arizona. The following diagram illustrates the color sequence and how the green flash forms.

PHOTOGRAPHIC TECHNIQUE

Photography of the green flash is disappointing. The image is a tiny, oval green patch even at 1000 mm focal length and it takes many attempts to register it on film. After attempting to photograph it for one month, with several film types and speeds, I finally succeeded with a slower film speed and some knowledge gleaned from other investigators. A long telephoto lens or telescope is needed on a tripod (I would not attempt it under 1000 mm focal length). A single lens reflex camera, either film or digital with a view screen magnifier is essential (I use a SLR Pentax 1000, 35 mm film camera with a 90 degree magnifier clipped on the viewfinder). You need the viewfinder to just “see” the flash let alone focus the lens. A cable release is mandatory to avoid vibrations. Exposure speed at the moment of green flash was 1/500^th second using 100 ASA Kodachrome transparency film. Arundles in Sanibel looked after processing the film for me. In most attempts, the “green flash”, although green enough by eye, turned out to be a “yellow flash”. The reason for this is as follows: Staring into the viewer at the sun’s disk, red or yellow in color, reduces your eye’s red sensitivity and often makes the normally yellow stage of the flash appear green. Tripping the shutter at this point will capture the “yellow light flash”. Waiting just a split second more before tripping the shutter will often produce success as the green spectrum arrives. This was the most important key to capturing it. I can now add it to my list of Solar astronomy images.

THE AUTHOR

John’s hobby is astronomy and he has concentrated on the study of the solar surface for 25 years, his main interest being photography of the outer layer, or chromosphere of the sun. His photography has led to many awards and publications in national astronomy magazines and books. He designed and built his own domed observatory, and has fabricated several telescopes, designed exclusively for solar photography. His main objective is to capture flares, prominences, and coronal mass ejections on film. He has well over a thousand color images of solar phenomena, and is an invited speaker at colleges, libraries, and astronomy associations.

Holidaying in Sanibel for the past 14 years, this year he brought down a compact solar telescope to capture the green flash. Sanibel, he claims is an Astronomer’s paradise with no street lights and without the usual mercury-vapor or high-pressure sodium lights. Lucky are Island residents, with a council so forward-thinking that they preserved the night skies of Sanibel.

Thanks John for explaining the mysterious "Green Flash" to us.

Images and words courtesy John Hicks