I don't have any old issues of Sky and Telescope
handy, but I did find the following article on the New
Scientist web site.
Cheers,
janet lowry
"Question
Whatever happened to the famous but rare "green flash"
we used to hear about so often twenty or so years ago?
Observers reported a momentary but brilliant green
appearance of the Sun as it sank below the horizon,
usually over the sea in tropical climates. Sometimes
the whole sky turned green as well. Was this green
flash real, and if so, what caused it? Why is it so
rare, has it been recorded on film and where are the
best places for seeing it?
Tim Conway , Upwey Victoria Answers
The green flash is caused by different amounts of
refraction in the Earth's atmosphere. Rays of blue
light are bent more than green, rays of green are bent
more than yellow, and rays of yellow are bent more
than red.
This refraction raises the apparent altitude of a
celestial object such as the setting Sun, because the
difference between red and green rays is about a
minute of arc at the horizon. Therefore the upper part
of the setting Sun has a green fringe. If the air is
very clear and the horizon is sharply visible, the
green fringe will be all that is visible of the
setting Sun for a second or two as the Sun vanishes
over the horizon, causing the distinct green flash
referred to above. The flash can also be seen at
sunrise if you know exactly where the Sun will appear.
The air must be completely free of haze and cloud
between the observer and the setting Sun in order to
witness the phenomenon, a condition that is rarely
met, which is why few people have seen the flash.
Under very favourable conditions, the even rarer blue
flash can be seen in which the green shade appears to
slide swiftly up the spectrum into a pale blue speck
just before the Sun disappears completely over the
horizon. Blue light is bent more than green light and
so this phenomenon is witnessed immediately after the
green flash and just before the Sun vanishes.
I saw several green flashes in 1995, either from a
mountain-top observatory (where a momentary blue flash
was visible) or from sea level looking out over the
ocean. Reports that the whole sky turned green are
unlikely to be true; the flashes are not especially
bright, and afterimages from the dazzling pre-flash
setting Sun can make it hard to see the flash well.
The total light from the green flash itself has been
estimated to be about the same as from the planet
Jupiter at opposition.
Green flashes are reported more frequently in tropical
regions or coastal areas because this is where the
likelihood of a long clear air path is the greatest.
But, given the right weather conditions, they can be
seen anywhere.
A few years ago volcanic dust from the Mount Pinatubo
eruption reduced the clarity of the atmosphere to such
an extent that green flashes were rarely seen, but
most of the dust has since dispersed and they are
visible once again.
Taking photographs of the green flash requires some
special camera equipment, such as a telephoto lens and
a tripod, but not many people bother and this explains
why photographs of the phenomenon are rare. An example
was published in the astronomy magazine Sky and
Telescope in 1994, and I have also found a number of
earlier ones.
Mike Dworetsky , Department of Physics University
College London"
_______________
>
> Message: 16
> Date: Fri, 13 Feb 2004 20:36:00 +1000
> From: Syd Curtis <>
> Subject: Saint Who's fire?
>
>
> Doug,
>
> Please grant me the indulgence of one
> off-topic question.
>
> In my posting about Lord Howe Island and its so
> friendly birds, I mentioned
> seeing St Anthony's Fire from a LHI restaurant.
> Martyn Stewart very rightly
> asks,
> >
> > Lastly, what the hell is St Anthony's fire? Is
> this the equivalent of our
> > northern Lights?
> >
> I confidently turned to my Scientific Dictionary to
> give him an
> authoritative answer only to find that I've got the
> wrong name. The
> Dictionary refers to something completely different
> for St Anthony's fire.
> And worse there was no indication of the right name
> among the Saints.
>
> Somewhere years ago, I read of the phenomenon of a
> flash of green light,
> just at the moment when the upper edge of the sun
> finally disappears below
> the horizon at sea. My recollection is that one had
> to be in fairly high
> latitudes to get this, and therefore I was both
> surprised and delighted when
> watching the sun setting from the restaurant at
> Capella South on LHI to see
> a very definite green flash.
>
> Now it will annoy me until I can find the right
> name. PLEASE, someone on
> naturerecordists, which Saint's name should I have
> been using? And if you
> can tell us what causes it, so much the better.
>
> TIA
>
> Syd Curtis (Brisbane, Australia)
>
>
>
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