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Archaeoastronomy sections


Dating construction periods of megalithic alignments

In close studies of refined astronomical observations in prehistoric architecture it has become clear that dating estimates of the construction period of high resolution solstitial or lunar standstill sites seems entirely possible.
Dating establishment of alignments is most feasible when they are constructed to observe to the limits of solar or lunar movements- that is the solstices of the Sun or the standstills of the Moon- and where the foresight is established to better than 1 arc minute precision.
At a solstice the declination of the Sun is exactly equal to the Angle of Obliquity. This angle changes slowly with time. Calculations based on the rate of this declination change may be applied to solstitial or lunar standstill foresights
The change in Obliquity is around 0.7 arc minutes per century, but there is disagreement as to the exact value beyond 3000 years before the present.
Over 4000 years the declination of the solstitial Sun has decreased by about half a degree.

Angle of Obliquity to the Ecliptic

Obliquity is the angle of tilt of the Earth to the plane of it's orbit. It is read as the angle between the plane of the Earth's equator and the plane of the Earth's orbit around the Sun- the Ecliptic, usually termed e .

From Haluk Akcam's site- 'In general, Obliquity is the angle between the equatorial and orbital planes of a body, which can be defined as the angular distance between the rotational and orbital poles. In the case of Earth, Obliquity to the Ecliptic is the angle between the planes of the equator and the ecliptic, and due to above summarized effects, the axial tilt of the Earth oscillates between round 22.0- 24.6, with a period of round 41,000 years.'

Rate of Change of the Angle of Obliquity to the Ecliptic

The dating calculations depend on estimating the change in this angle over time. When Thom published 'Megalithic Lunar Observatories' in 1971 he depended on S. Newcomb's estimates from 1906. Since then 13 fresh calculations have been offered.
Haluk Akcam- 'Since the definition of Simon Newcomb remarkable progress has been achieved in one century. Some of the historical monuments are the works of Androyer, de Sitte, Brouwer, Woolard, Clemence, Lieske, and Fricke. Here, the expressions of Lieske (L77), which are adopted by the IAU (1976) General Assembly, are taken as the first sample of a series of its kind until the end of 2003. As a reminder, Saturn has almost completed one orbital period since that time.'

Graph of Change in Obliquity

This graph is centred on the year 2000 AD.
The estimates agree until 3000 years ago...from then distinct divergence occurs.
The last peak of Obliquity occurred around 7000 years ago.
'Thirteen models for numerical expressions for General Precession and Obliquity of the Ecliptic are examined for long-term validity, and only seven of them found worthy to consider in this sense. Among these seven models, only two (La86 and B03) seem to be more accurate and meticulously framed than the others, and one (W94) is theoretically remarkable.
Among the four models, which appeared after the IAU 2000 resolutions B03 seems to be the only referable one for long-term applications. Yet, it seems quite possible that a more precise model can be developed in the future, by merging the theories of Laskar and Williams, and applying the new VLBI and LLR hresults, to build accurate numerical expressions for precession and the Obliquity which can be valid for a few ten thousand years.'

Obliquity banks, or slopes, natural and artificial, employed as high resolution foresights.

This image is synthesised from A.Thom's survey of the Ballochroy to Ben Cara, Jura alignment and photographs taken from behind the standing stone row.

Alexander Thom first estimated, by close survey, that the northern slope of the mountain Bien Cara on the island of Jura followed the descent of the summer solstice sunset precisely. This phenomenon, the glancing track of the sun's upper limb, may still be seen today, from across the Sound of Jura, from the rising ground above the mainland shore at an appropriate place 19 miles, (30.4km), distant. By spherical calculation Thom examined a stance on the banks of the mainland near a farm called Ballochroy from where the glancing phenomena on the Jura mountain would have been observed at about 1800BC. Here Thom found an impressive three standing stone row. This style of standing stone is sometimes described as 'blade stones','cleits',or 'feathers'. From this high resolution alignment Thom calculated that the Obliquity shift from 1800 BC to mid 20th century AD was 27 arc minutes.

Today the experience of watching the sun flow down Bien Cara's northern flank and settle in the notch at the foot may be repeated by taking a stance further along the mainland coast to the north. The modern road will make a convenient extrapolation track and there should be plenty of time to 'bring down the sun' by moving along the road until the sun is positioned appropriately on the higher slope of Bien Cara. This is the appropriate stance for an observation today and it will be seen that no further movement is necessary to witness the sun locked to the entire length of this hill flank.

In these studies any calculatory procedures examining the antiquity of an alignment will use the same value for Rate of Change of the Angle of Obliquity to the Ecliptic- 0.72 arc minutes per century.

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