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Positional astronomyThe astronomy of the Megalith Builders in Britain was not a quest to discover what a celestial body was nor how far away it might be, nor what it is made of, but rather what it's position is, was or will be. This is termed positional astronomy and it is still the bread-and-butter work of modern astronomy. positional astronomy is largely concerned with maintaining strict time- keeping of the movements of Sun and Moon.
DeclinationIf we imagine a strong light at the centre of the Earth then lines of declination would be the shadows of lines of latitude cast upon the screen of the sky. These are the tracks along which the Sun and Moon travel. From the declination of the Sun the time of year can be read. From the declination of the Moon the current condition of it's orbit may be deduced. Until Alexander Thom's work it was thought that meridional astronomy was the only positional form developed to any degree of practical use in the ancient world.
The GnomonThe Gnomon of Alexandria was used by Eratosthenes, the Greek astronomer, to calculate the circumference of the Earth in 240 B.C. However meridional astronomy using the shadow cast by a pillar, does not give the necessary resolution for accurate measurement of the solstices nor limiting positions of the Moon. Only by employing long distance alignments observing to a distant hill horizon could the early astronomers hope to differentiate among the very small movements at the extremes of the orbits of the Sun and Moon.
Megalithic horizon astronomyInstead of marking the length of the shadow from a vertical pillar the british bronze age astronomers choose to use marks on the great circle of the horizon to define the positions of Sun and Moon as they rose or set. The rise/set points move along the horizon daily- faster near the equator at the equinoxes, slower close to the extreme points north and south which are termed the solstices (of the Sun), and standstills, (of the Moon).
Required declinations for megalithic astronomy in BritainHere, observing south, are the required declinations given by alexander thom for the Bronze Age 16 interval calendar and
The treble blue declinations are the Lunar extremes at the Standstill positions. The three close set lines denote the extremes of the cyclic perturbation a small wobble in the path of the Moon with a period of about 7 months. Cyclic perturbation could only be observed by a horizon astronomer at a lunar standstill once every 9.3 years. Horizon astronomy is a system of positional astronomy used to time the extremes of solar and lunar orbital cycles which the early British astronomers seem to have brought to an unparalleled standard of perfection.
Megalithic astronomical alignmentsHorizon marks of sufficient impressiveness were erected on high ground at or near the point where a required declination would intersect with the horizon as viewed from lower ground. A suitable place, at lower levels, would be found from which the Sun or Moon,
ForesightsTumuli are Bronze Age round barrows often, but not always, containing a burial, with and without a stone box, (cist). They are often carefully constructed of many alternate layers of turf and clay.
Cairns are large piles of boulders and pebbles. Again they may often be built in separate layers of graded size stones or even different geological material. Some may have contained burials but they are of a different period to the similar looking chambered cairns of the Early neolithic Period, circa 3000 BC, though often the much older monuments are incorporated into astronomical alignments dating from after 2000 B.C.