On observing the green flash
On the nature of the true and legendary green flash

The Green Flash and the Blue Flash

The Green Flash is an exceptional , mind-boggling , poorly-understood
sight of Sunrise and Sunset .
The true and legendary Green Flash seems as bright as a flashbulb
and has the same momentarily disturbing effect on vision .
It is so rare that I am unaware of any image which captures it .

Two preliminary matters :

The green segment and the green flash are both remarkable sights .
There is a difference between the green segment and the green flash .
The green segment is of normal photospheric brightness .
The green flash is an immensely brighter burst , seemingly 10 or 100
times brighter .

Mathematical nomenclature :  A chord cuts off an arc from a circle and ,
simultaneously , cuts off a segment from a disk .
Application :  Ideally , a spherical sun rises above a horizontal ,
linear horizon to expose a segment of itself .

Reality :  This ideal sun is actually a range of suns of different
spectral colors , conventionally , the red sun , the orange sun , the
yellow sun , the green sun , the blue sun and the violet sun , although
actually , a much larger range , a more or less continuous range is
present on various occasions .
When near the horizon , these suns are separated by chromatic
differential atmospheric refraction .
The lowest of these is the red sun and the highest above the horizon is
the violet sun .
The yellow sun is typically the brightest .
The violet sun rises first and sets last , but is almost always dimmed
to invisibility by atmospheric aerosols , etc.
A segment of the blue sun is occasionally seen .
More frequently , a segment of the green sun can be seen at
Sunrise/Sunset while the yellow sun is still/already below the horizon .
The green segment , it could be called the green glow , or the green
shift , is infrequently seen , but not really rare .
When the sky near/at the orient/occident is very clear , the green
segment can normally be seen .
It is an easy inference that , from a very favorable location , the
green segment can be seen dozens or hundreds of times per year .
The first color seen will vary over pretty much the whole spectrum .
Sometimes greenish-yellow , sometimes yellowish-green , or
bluish-green , or others .
Quite often , as the first bit of the yellow sun rises , the green
segment will split into two green pieces separated by a yellow segment .
The green segment has been imaged many times .

The idealized picture just presented is severely modified by the
atmosphere .
The shape of the segment(s) is often wildly and greatly distorted .
Quite often , as the first bit of the yellow sun rises , the green
segment will split into two green patches of some shapes or another at
the termini of a yellow bar .
Also quite often , the Sun is dimmed to a greater or lesser extent as it
rises or sets , just as it is less often when higher in the sky .

More reality : Anyone who has expectations of how a Sunrise or Sunset
will appear is almost surely bound to be disappointed .
The range of phenomena is simply too great .

When the Sun is near the horizon , its light takes a long path through
the atmosphere .
This long path quite commonly facilitates a wide variety of
meteorological modifications of the image .
The long path also tends to scatter blue light more efficiently
( thus the blue color of the midday sky ) making the image of the Sun ,
which is formed only from light which survives the trip , redder in
color .

In particular , violet and blue are frequently seen to be missing
altogether .

Consequently , there is no such thing as a unitary , or uniform ,
"green flash" phenomenon , such as you may have read about in less
authoritative sources .
The green flash is not something you either see or don't see , like some
sort of step-function .
There are intergrades between the green segment and the much rarer , and
much brighter , green flash .
Sometimes the green will be much brighter than the usual , maybe a
little brighter , maybe much , much brighter .
Sometimes as bright as a flashbulb .
That's the Green Flash .
Any of the entire range of appearances from green segment to green flash
is a quite remarkable sight , especially on initial viewing or if
unexpected .
But , no one who has seen a true and legendary Green Flash could
possibly confuse that immensely bright flash with the mere color shift
of a green segment .

Sometimes , these green phenomena are actually blue , or as a few
images suggest , violet .
Sometimes , the green segment is detached from the main part of the
photosphere , sometimes not .

The green segment can be brilliant and striking , truly a remarkable
phenomenon , and people who see it may remember it for a long time .
"Ordinary" green segments are still definitely brilliant ,
striking and remarkable .
They are not really "ordinary" .

But the green flash will "knock your socks off" ; it seems to be
as bright as a flashbulb , and momentarily disturbs your vision .
It's a flash , a burst , much brighter than the brightness of the
unenhanced photosphere .
That's why it's called a flash .

                        -       -       -

Notice to imagers and photographers .
Please don't throw away your images .

I worry that , with such wide distribution of so many mislabeled
representations of the green segment , imagers may have thrown
away as useless the greatly overexposed images that would
document the green flash .
This is a highly unfavorable situation .

                        -       -       -

My observations .

Although far from ideal , conditions under which I typically
observe are quite often good enough to allow me to see the green
segment .
I've seen the green segment many hundreds of times .

My observations reveal a wide range of phenomena which seem
well-explained by these basic principles .

I wonder , under my typical conditions , if I may not have been
advantaged in observing these phenomena by :
a) an elevation far above sea level , b) binoculars , c) insomnia ,
and d) consequent convenience of Sunrise .
People who flock to the seashore for its far horizon may be
disadvantaging themselves by immersion in the marine murk .

I have seen the classically described phenomenon , an extremely bright
flash of green light , as "bright as flashbulbs" .
Much more frequently , I have seen a bright green glow ,
lasting a greater or lesser length of time , and assuming a variety of
shapes and sizes .
More frequently than that , I have seen merely bright green specks ,
also of various durations , shapes and sizes .
The large number of these specks I have seen may be inflated by the
preponderance of my observations over a land horizon at Sunrise through
binoculars , where it is easy to catch a "first bit" through leaves of
trees .
Many , many times , there is no green , not really , just yellow , or
orange or red ;  many of these , of every hue .
I generally ignore any dim phenomena at Sunrise , since I can't
distinguish them from isolated illuminated clouds .
And it is only with experience that I can properly anticipate the pace
of events at Sunset and make this distinction then .

The blue flash .

On extreme occasions , the phenomenon is not green , but blue .
I have seen a number of bluish-green and greenish-blue phenomena , of
various hues , mostly very green .
At the equinoctial Sunrise , 1998 SEP 23 , I saw a BRIGHT BLUE FLASH .
In detail :
The Sun rose behind trees , which are far away and not of any great
consequence .
Immediately prior to Sunrise , the whole area of the sky near the orient
was especially luminous , so much so that it raised the question
"Why is the sky already so bright if the Sun isn't up yet ?" .
I saw a definite brightening of the pale orange at the orient .
31.40 seconds after that , at 11h 18m 23.6s UTC (I was too excited to
have the patience to obtain the hundredths , personal equation = about
.25s not applied) , I saw the "first bit" ; it was bright green ,
rapidly evolving , giving way to green and yellow spreading out
horizontally , and then it happened .
A BRIGHT BLUE FLASH , as blue as could be , not more green than violet ,
occurred 6.26s after the "first bit" and high , not directly in front of
the first bit ;  but a fraction of a degree higher , and seemingly big ,
perhaps as big as the , as yet unrevealed , Sun .
If the brightness hadn't been so disturbing , I'd call it a lovely
blue , a sky blue or a baby blue , mostly a sky blue .
After the blue flash , the color continued to evolve , moving left and
right , until the green ended 3.24s after the blue flash .

I spoke with an experienced and enthusiastic meteorologist ,
Andre Bernier , who said there was nothing very unusual about the
weather at sunrise .
He said there was a small inversion of 10 or 15 degrees F for about
200 feet or 1000 feet at the surface .
Apparently , the key to seeing blue phenomena is a clear path , which
can occur even if there are a few clouds , as there were .

As with my 9 observations of the corona of our Sun from within the
umbral shadow of the Moon , but to a lesser degree , the words above
describing the blue flash are rather definitely insufficient to
thoroughly describe the phenomena .
There is something very significant missing .
You have to be there and see them yourself to really understand .
( And this remains certainly true even if right now you are smugly
telling yourself I am surely mistaken in this .
It's an observing thing .

In retrospect , I see that in some ways I did a pretty lame job in
reporting the blue flash I observed :
"a blue flash is better than a green flash" :
Pressured by the demand of e-mail , hastily improvising , I mistakenly
chose a jocular subject header , intended to be light-hearted , which I
now see as troublesome , interfering with explaining the message .

As Michael Feldman might say , "sticklers for the truth" should
substitute the word "rarer" .
With hindsight , I wish I had used a title with less potential for
confusion , since my dominant purpose was to dispel confusion .

And I never quite got around to explaining clearly that the
observing experience of a green flash is qualitatively different
than that of the green segment .

I surmise that favorably situated observers could observe the
green segment 100 or 200 times per year or even more .
Being near sea level or in a moist or dusty environment makes it
difficult , not to mention sitting indoors watching tv or
playing video games .
But the green segment is merely a color shift , not a flash .

Being the color of the clear sky , and hence the very happiest of
colors ;  in the abstract , blue is a very nice color .
I'm so accustomed to hoping for green that blue at Sunrise can have a
slightly surprising aspect , despite being so welcome .

I've seen roughly 2 and a half green flashes .
One was a green flash , one was a weaker green flash and one was a
blue flash .

On a flight between Philadelphia and Washington , I once saw 8 green
segments in fairly rapid succession beyond a very distant cloud .
I offer no explanation about how that could happen , beyond noting that
the airliner was not noticably oscillating up and down .

I posted this before my experience base had expanded sufficiently to
allow me to speak as specifically as I now do :

Newsgroups: sci.astro
Date: 1995/03/31
Subject: Re: Green Flash Revisited

Anyone who has expectations of how a sunrise or sunset will appear is
bound to be disappointed.  The range of phenomena is simply too great.

I'm currently writing an article about the green flash and looking for a
publisher (hint, hint).  Here is a teaser from my log, from this last
Monday; note this is a SUNRISE over a LAND horizon:

1995 March 27 11 21 30, -81.86371, 41.37355, 256 m:
Sky 90% overcast, but not near the orient.
At 11 21 8, an orange billowing began at the orient.  This
roiling-cloud-like phenomenon sometimes lasts for many seconds
prior to the first bright rays reaching over the horizon.  It is
uncommon at sunset.  It expands as it continues.  At 11 21 30 green
twinkles began to appear.  This was the moment of sunrise.  At 11
21 35, a very bright, large burst of green bloomed above the green
and yellow fragments.  At 11 21 40 the green had been completely
overwhelmed by the yellow and pain was imminent immediately
thereafter, due to the high level of illumination.
Sometimes there is no bloom or burst of green, but there will be a
distinct green phase.

Then this :

Newsgroups: sci.astro
Date: 1995/09/24
Subject: Green Flash! Sun rise, land horizon; Venus set; Mars set; Ganymede

Sunrise on a moderately polluted planet

Sunrise this morning from Berea, Ohio, USA, -81.8637 (W), N 41.3735,
256m; very low, land horizon; 7x35 binoculars, stopwatch, WWV 5 MHz.

Bright green flash, preceded by tiny orange roiling cloud.

(Only rarely (except when it is cloudy) is it truly difficult to discern
the location of the orient merely by inspection prior to sunrise, without
considering the possibility of precalculating it, altho the notice is
sometimes only a few seconds).

Illumination above the orient shifted markedly to the right for a couple
of minutes prior to Sunrise.  At 950924 111902.1, the first bit of a very
small, orange, dim (i.e., not bright), roiling cloud appeared at the
orient.  At 111907.1, Sun rise, the first bit of green, which was bright,
appeared.  Within .5s it had expanded to become a nearly horizontal,
somewhat interrupted bar, which rapidly became very bright, really
spectacular.  At 111910.4, the green flash ended as it was overwhelmed by
incredibly brilliant yellow, which terminated observations.

The equinoctial Sun rise the morning before had been in some ways more
spectacular than this morning, but those details will have to follow


Venus set last evening was spectacular.  For a couple of minutes prior to
Venus set, as seen thru a tripod-mounted, 81x89 monocular, the planet was
separated into multiple, highly colored, vigorously vibrating disks.  The
separation was so spectacular that had they not been bouncing around like
crazy; at times, they might have appeared to be stacked on their edges,
one on top of another.  These disks were red, yellow, green, and to a
lesser extent orange, and even less, blue.  The altitude at disappearance,
950923 232823 must have been very low because ICE computes an altitude of
-29.7', with total altitude corrections of -33.6'.  I'm not exactly sure
how to interpret that.


Whenever someone tells you what they saw , what they are certain they
saw , no matter how bizarre , please advise them to make observations
like these for themselves and to carefully compare what they appeared to
see , on the one hand , with what they know to be the physical nature of
the Sun and Venus , on the other hand .
And then to characterize the transparency of the atmosphere , and the
fidelity of images seen through it .

Another thing to keep in mind is that , although they had only 1X
available , our ancient ancestors could see phenomena something like
these .
E.g. , I'd be quite surprised if Galileo was the first human
to see the satellites of Jupiter .
I consider it all but certain that someone had previously taken
advantage of favorable conditions and a very distant rock to see a step
disappearance of Jupiter , revealing one or more of its satellites .
Whether such observers made the proper interpretation is a separate
matter .

Then this :

There are trees low on my horizon where Mars set, so I couldn't follow it
for the last few minutes.  But prior to that, it was very interesting
because the disk was split into various colored disks, tho not as
thoroughly as Venus had been.  In fact, near the time I lost sight of it,
about 950924 005620, it was essentially just one disk.  But what a disk.
It was an incredibly highly saturated, deep red; really spectacular; so
incredibly red.  Often people report having difficulty seeing Mars as red,
characterizing it as orange instead.  On this occasion there wasn't even a
ghost of a doubt; in fact it was a very deep red, way over toward the

[  950924 111902.1 = 1995-09-24 11:19:02.1 UTC  ]

A thought toward a proposal .

NSF should commission an atmospheric survey project to record these
Sunrise/set phenomena every day with a view to creating a baseline ,
documenting the present state of the atmosphere going forward .
This would be primarily meteorological in purpose , to capture
detailed information over the decades about the content and nature
of the atmosphere by imaging , including spectroscopically ,
Sunrises and Sunsets  ;  and Moonrise/set , Venusrise/set ,
Jupiterrise/set , and Marsrise/set .
It would also be capable of capturing images of the green flash .
We have one atmosphere .  One .

Other observations .


It is like a flashbulb from a camera, green, and rare to see. There were
scientists who had never seen it before. They would jump around on the
deck, because they were no longer virgins.


The green flash occurred just as the sun dropped below the horizon. It
was almost a neon green color and gave the appearance of a slow motion
green flashbulb going off. It lasted for a couple of seconds and then
the horizon was back to normal. We were both excited to see it!


One other "first" was when Debbie was on the wheel at sunset, and
witnessed the green flash. This occurs the moment the sun disappears
from the horizon, and in the red and orange glow sometimes you seen a
flash, similar to a green flashbulb going off.


Just like a flash bulb photo from the 1930s, a brilliant blast of light
pulsated immediately after the last edge of the sun dipped below the
horizon.  ...  The flash also seemed to travel along the horizon
outwards from the center. Brilliant and beautiful.


The Green flash at Sunset
Most of the time it is the last one second before the sun disappears,
but on occasion it is like a small green flash bulb.


But sometimes it is actually a flash. Like a small green flash bulb.


No worries, the Green Flash occurs at a precise moment as the sunsets.
There's no guarantee that it will occur, or that you will even see it,
it just kind of happens. It's like a green camera flash bulb going off,
everything gets a green tinge, it's almost blinding.


I've only seen it happen one time and that was in St Kitts. It was
bright like someone taking a picture with a green flash bulb.


The Green Flash
Saw it once really well at 13 degrees N latitude. Never thought it was
anything to speak of - until I saw a really good one. Shorter than a
flash bulb but just as bright and green as green can be.


It is, as has been stated elsewhere- an actually neon green flash, like
a flash bulb.  ...  It's really a flash! I've seen it twice-
The "flash" that looks like a "flash bulb" happens farless often.

                        -       -       -

The main ideas .

                    Model of the Standard Atmosphere
                  for Green Flashes and Blue Flashes

atmospheric refraction :  bending of incoming light by the atmosphere

differential atmospheric refraction :  refraction increases as the
density of the transmitting medium increases .

chromatic differential refraction :  lower frequency light is refracted
more , i.e. , through a greater angle, e.g. , by a prism .

atmospheric refraction :  Celestial objects appear to be closer
to the zenith , i.e. , at higher altitudes .
Sunrises come earlier than geometry suggests ;  Sunsets come later .

differential atmospheric refraction :  refraction increases at lower
altitudes , i.e. , near the horizon , e.g. , the Sun appears highly
elliptical at very low altitudes as the lower part of the image is
raised more than the upper part .

chromatic differential refraction :  the violet sun rises earliest ,
followed by blue , green , yellow , orange , and finally , the red sun .
At Sunset , the red sun sets earliest , followed by orange , yellow ,
green , blue , and the violet sun .

atmospheric extinction :  scattering and absorption reduce the light
from an astronomical object .

The light from an astronomical object is reduced by atmospheric
extinction caused by scattering and absorption .
Rayleigh's Law holds that higher frequency light is scattered more
efficiently .
Often , extinction is augmented by water in the form of haze , fog ,
mist , or cloud or by dust .
This is so effective that violet flashes are virtually unknown and blue
flashes are very rare .

The Green Segment

This atmospheric model explains how the first visible segment of the
rising Sun can sometimes be green , or even blue , instead of yellow ,
and by symmetry , the last visible segment of the setting Sun .
( It also explains why the sky is blue and why the rising or setting
Sun , and light near it or near the horizon , is often orange or red ,
instead of yellow .

A green segment can be a quite remarkable sight , especially on initial
viewing or if unexpected .

I'm not a physicist , so my judgment cannot be definitive , but the
documentation of the green segment by Professor Andrew Young at
seems excellent to me .
That site uses the term green flash to describe the green segment and
does not address the green flash , except in reference to historical
events .
Its images show the green segment .

Documents the green segment very well .

Reversed in time and somewhat obscured by far-away , low trees , the
is very familiar to me as a typical view of the vicinity of the orient
on a clear morning .
Excellent documentation .

Excellent documentation .

Meteorological Modifications

The atmosphere is rarely completely free of disturbances which distort
the view of the rising or setting Sun to a lesser or greater degree .

Refraction varies with temperature , humidity , and pressure .

Sometimes , especially near sea level , disconnected segments or bands ,
i.e. , segments of two bases , of the rising or setting Sun appear ,
often one above the other ;
sometimes , some of these are inverted , or upside-down .
These can be quite remarkable sights .

Sometimes , the meteorological modifications are very complicated ,
making adequate description nearly impossible .
Sometimes , the effects of these meteorological modifications evolve
very rapidly , making adequate description of a visual observation
nearly impossible .

Sometimes , the meteorological modifications enhance the view .
This may be a brightening of a disconnected piece or pieces of the Sun .
It may be a merging of such pieces .

The green flash , the true and legendary green flash , is apparently the
result of a specific , rare type of meteorological modification which
produces a huge increase in brightness .

Other than the observations themselves , I don't claim to know very much
about meteorological modification , so I'm not comfortable making
judgments .

The Green Flash

Rarely , a green flash , or very rarely , a blue flash , may occur when
the brightening is so extreme as to appear to be as bright as a
flashbulb .
The Green Flash is not a piece of any size ;
  it's a "flashbulb areal multiflare" .
Once the flash starts , your eyesight is instantly blasted , and you
would not be watching the gleaming for a half-second or second or so .
Words fail to wholly capture this electrifying experience and it has
become the stuff of legends .

No one has yet described or catalogued all the meteorological
modifications which may occur .

Landscape Modifications

Obviously , the view can be obstructed by objects near and far .
Between and above distant leaves , often only green specks will appear .

Ocular Modifications

Obviously , the view can be distorted by imperfections in the eye and in
any optical system used .
At Sunset , careless viewing of the yellow sun , or yellow photosphere ,
can result in bleaching of photosensitive pigments in the retina ,
specifically erythrolabe , which can result in insensitivity to red
light , which can result in perception of anomalous colors ,
specifically , what is recorded by a camera as yellow light can appear
to be green to the eye .


More high-quality data is desperately needed .

Walter Nissen
preliminary posting : 2013-05-08
minor update        : 2013-05-11