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Horizon Astronomy 4

Identifying the precise days of the solstices. 1.4.1   Finding the day of a solstice by direct observation of the Sun's movements is an exacting feat even for a trained surveyor using a modern theodolite. The Sun changes declination by only 12 seconds of arc in the 24 hours preceding or following a solstice. With the diameter of the Sun at an average of 32 arc minutes this tiny movement represents 0.00625 of the solar disc or 1/160th. For an observer without sophisticated equipment the Sun's rise/set positions on the horizon appear to standstill for several days around the solstice. 1.4.2   Some astronomers do not believe that the unaided human eye can resolve parts of the Sun to this precision, however it should be noted that the often witnessed phenomenon at a solar eclipse termed Bailey's Beads has been seen by thousands of untrained observers world wide. This is where the last, (or first), flash of the Sun, as it is obscured by the Moon, strikes between mountain tops on the edge of the lunar disc presenting sharp points of light distinctly seen from Earth without optical aids.

The tallest mountains on the Moon exceed the height of Mt. Everest above sea level, (5 miles - 8km). The distance from Earth to the Moon is an average of approximately 250,000 miles, (400,000 kilometres). If we assume that the apertures presented to the Sun's light during the Bailey's Beads phenomenon are 5 miles tall, (and some will probably be smaller than this), then, by calculation, we are seeing a flash of only some few seconds of arc in height. On the 26th Jan. 2002 the planet Jupiter was in conjunction with the Moon. At the latitude of Edinburgh- 52 deg. north, the planet disappeared behind the edge of the lunar disc. At the latitude of Greenwich- 52 deg. north, Jupiter remained visible. At the latitude of Liverpool, half- way between, observers without telescopic aids, witnessed the light from the planet twinkling in and out between mountains on the lunar rim.

Bailey's Beads.

1.4.4   This phenomenon also occurs in terrestrial environs when the correct conditions are present.

	Sunrise on Rhos Crug at 5.5 mls (8.8km) from S1, Llananno.
		Bailey's Beads through ground cover. This photograph was taken some seconds before sunrise on a clear, frosty, morning in February. The Bailey's Bead type flashes persisted for a few seconds allowing this photograph to be secured. The top of this hill is entirely covered with close cropped winberry maintained by sheep grazing it down to a height of 18 inches to 2 feet. The dense canopy resulting allows little light to fall on the lower stems of the plants which carry sparse or no leaf. The rising rim of the Sun does not show through the thick leafy canopy but will penetrate the lower region of the cover over the crest of the hill and shine brightly through in the same manner as light penetrates between peaks on the Moon at a solar eclipse. The aperture defined by the winberry stems in this photograph is about 1 foot tall, (0.30m). The distance from the camera to the hill crest is 5.5 miles, (8.8 km), hence we are observing a flash of sunlight several arc minutes in breadth but only about 7 arc seconds tall.

Data on the human eye. 1.4.5    In the ophthalmic literature on the resolving power of the average human eye it is stated that under special conditions magnification effects of x30 may be attained;

OPTICS Smallest visible object. The resolving power of the human eye is 0.0003 of a radian or an arc of 1 minute (1/60th of a degree), which corresponds to 100 microns at 10 inches distance. A micron is a thousandth of a millimetre, hence 100 microns is 0.003937, or less than four thousandths, of an inch. The human eye can, however, detect a bright light source shining through an aperture only 3 to 4 microns across.

1.4.6   3 to 4 microns at 10 inches equals about 2 arc seconds. The megalithic astronomers were well acquainted with this aperture phenomenon and took advantage of it when choosing and constructing their highly sensitive sites. When the bright rim of the Sun slides behind a clear-cut horizon just such an aperture is presented.

Here is a 2 arc minute slice or segment of the solar disc standing over the flank of a hill. The depth at the middle, (the sagitta), is 2 arc seconds.

When positioning their backsights so as to allow the Sun to roll along a hill flank which matched the angle of the disc's movement, or settle into a well defined notch, the horizon astronomers knew that, although the small slice of the Sun's disc retained in view might be several arc minutes in width, and perfectly visible, they were, in fact, resolving to several seconds of declination. For an example of a solstitial site arranged to use this technique see html page Ballochroy, N. Kintyre. For two examples used at lunar standstill sites see html pages S1, Llananno/ Glog hill. & Drygarn Fawr. Pop Up Glossary and paragraph links. Move mouse over text in this colour and Click <<<Back Next>>> Horizon Astronomy - 1/ 2 / 3/ 4/ 5/ 6 Interactive Pop Up Glossary. Move mouse over Text and Click with comments or queries- powys@megalithicsites.co.uk