Megalithic Studies Mid- Wales.



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*Megalithic Calendar 3 *


	**2.3.1  The light phenomenon known as the green or emerald flash
occurs when the first, or last, speck of the Sun's disc stands on the
horizon. It requires very clear air and a crisp horizon line to appear.
The colour of this bright flash may have a range from green or emerald
to bright blue- very similar to a modern halogen vehicle headlight for
which it may be mistaken. It lasts only a few seconds. For more on the
green flash and terrestrial refraction phenomena see **_Horizon
Astronomy p5_ <horast5.html>* *


	*2.3.2 * 		Thom, /Megalithic Lunar Observatories/ p38;
To understand how accurate this technique can be (were it not bedevilled
by refraction changes from evening to evening) one can look at the green
flash. When the Sun sinks behind a clean-cut horizon in a clear sky the
last visible part of the disc to be seen is very small and is usually a
bright emerald green. If we assume its width to be 2 arc minutes, and it
is perhaps less, the depth (the sagitta) is (1x1)/32 minutes, or about 2
arc seconds. In the clearer skies of Megalithic times the observers
would see and perhaps use the green flash as a criterion that they were
in the correct position. They could thus obtain an accuracy of a few
seconds.



	*2.3.3 * 	*S2, Llananno to Warren Hill tumulus. 3rd October 2001.
GflashNet * 		*This is a good example of the green flash and it's use
secured on film. This alignment, S2, Llananno to Warren Hill tumulus,
operates on the days of Calendar Intervals 9.5 & 16.5.
On the morning this photograph was taken the declination of the rising
Sun at the moment of sunrise was within 1 arc minute of the ideal
required declination for these CIs. We can see how the notch at the base
of the right flank of the tumulus contains the flash perfectly. More
details on this alignment in html page _S2, Llananno/Warren Hill._
<WarrenHill.html>*


	*/"Greenfire at sunrise"/* Chinese verse. **


	*The photograph of the green flash was taken about 1 second after the
first appearance and the colour had changed to bright yellow by two
seconds. It takes four minutes for the entire solar disc to clear the
horizon and the total diameter is 32 arc minutes, hence, in the first
second the Sun rises 8 arc seconds into the sky and in the first quarter
second the sagitta would be 2 arc seconds, as Thom estimates, and still
be clearly visible to the unaided eye.
In this instance the phenomenon was transitory due to the near
horizontal line of the hill, giving little time for manoeuvring should
it be so needed, but if the foresight had been a hill flank which
paralleled the track of the setting Sun, such as at _Ballochroy_
<Calendar4.html>, then the duration of the emerald flash may have been
prolonged even to the length of 15 or 20 minutes. This would allow time
for an observer, by stepping to one side or the other, to displace the
Sun angularly in order to maintain the finest pip of the flash and
'lead' it into a sharp notch. When this had been achieved the observer
would drive a stake into the ground marking this spot. If this procedure
were repeated each evening approaching a solstice <javascript:void(1)> a
line of stakes would develop. When this line is seen to reverse in
direction then the observer knows that the solstice has passed and that
the furthest stake in the line marks the place where a solstitial
backsight might be erected. For further on stake- setting and angular
displacement see html page _ Horizon Astronomy 3._ <horast3.html>

Alexander Thom has located many examples of solstitial sites in Scotland
and Wales <javascript:void(1)> and this author has verified an important
one of these, photographically, on the island of North Uist in the Outer
Hebrides. See html page _Leacach an Tighe Chloiche._ <nuist1.html> *


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