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Horizon Astronomy 5

The emerald flash.

"Greenfire at sunrise" - Chinese verse.

1.5.1   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 usually lasts only a few seconds. It probably results from intense refraction of the first, (or last), pip of the solar disc caused by an envelope of warmer air clinging to the hill line. The blue component of white light is known to be displaced furthest by prismatic refraction so this colour will show first at sunrise and last at sunset.

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.

1.5.2   S2, Llananno to Warren Hill tumulus. 3rd October 2001.

This is a good example of the green flash and it's use secured on film. This alignment, S2, Llananno to Warren Hill tumulus, is arranged to observe 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.

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.

1.5.3   In the instance of the Warren Hill sunrise 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 then the duration of the emerald flash may have been prolonged even to the length of several tens of 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 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. Alexander Thom has located several examples of high- resolving solstitial sites in Scotland and Wales 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.

Two Tumps Warren Hill

1.5.4   Horizon bobbles and toes. Whether conditions are right or no for the appearance of the green flash another distinct phenomenon may often be seen on a clear cut horizon. This is a brief lumping of the light as the smallest flash of the disc of Sun or Moon sits on the horizon. It is a prominent magnification effect and may appear on any unmarked horizon but, if deliberately arranged, a containing notch such as the lower flank of a tumulus, as with Warren Hill, or the fine cleft between Two Tumps, will enhance the image. This lumping or 'bobble' effect is probably caused by the same conditions that produce the green flash. Here the true diameter of the Sun has been montaged behind the hill.

S1/ Drygarn Fawr

1.5.5   In this photograph of the Moon settling into a notch the rise of the upper limb in the last moments of it's disappearance can be seen as a 'toe' to the path. Here the change in declination this causes is about 0.75 of an arc minute. The sensitivity of these high- resolving alignments strongly suggests that the refraction phenomena on the horizon were understood and catered to in the setting up of the backsights.

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