Glossary X
At a lunar backsight where permanent or temporary ancillary extrapolation gear has been set up there should be a length preserved between major stones which is the 4G for this site. This 4G represents, on the ground, the lateral distance an observer must move in order to 'shift' the Moon through the declination range K. K is the change in the Moon´s position during the 24 hours preceding a standstill. At a Major Standstill it is 46.5 arc minutes and at a Minor Standstill 30 arc minutes. See also- K, ANGULAR DISPLACEMENT, EXTRAPOLATION PROCEDURE, LUNAR STANDSTILL.

Sunset 19th. June, 2012 on Silbury Hill, nr. Avebury, Wiltshire.

High precision solstitial alignment from Stance #2 stone gate- post to the north face of Silbury Hill.
The series of images were taken on the evening of the 19th June 2012. When accurately layered they show that this slope follows exactly the Angle of Obliquity to the Ecliptic for this latitude.

Stance S2 to Silbury Hill north face 19th June 2012.

Silbury_Stance_S2_19th_June_2012
The north face of Silbury Hill.
A camera position was adopted where the final spark of the upper limb of the Sun sat on the north flank of the monument. No further movement of the observer position was required to maintain the upper limb on the slope. This means that the north slope of the monument exactly parallels for it's entire length the Angle of Obliquity to the Ecliptic. This situation would allow an observing resolution of better than 3 arc minutes. The diameter of the solar disc is 31.6 arc minutes.

Silbury Hill

The largest man- made mound in Europe stands 39 metres high by the River Kennet in Wiltshire. Although it is an impressive statement in the landscape it cannot be viewed from any aspect at distances greater than 500 metres or so as it sits on low ground by the river.

This enigmatic monument has been the subject of much debate and speculation. No burials nor cavities have been found despite several shafts and tunnels sunk into and through the heart of the construction. No ancillary settings or earthworks with clear associations have been identified in the surrounding territory. See: http://www.english-heritage.org.uk/daysout/properties/silbury-hill/

Silbury Hill underbuilding features.

Block set containment.

However the archaeological excavations have discovered a remarkable sophistication in the internal construction procedures of the ancient builders. In the later development heavy block- built walls with battered outer surfaces and radial internal bulkheads were set in place. The internal compartments of these walls were filled and tamped with chalk ballast- rubble.
Five more tiers, to the same standards, were raised in either terraces or a continuous spiral.

Timber footings.

In the quarry ditch at the foot of the north face additional timber revetment footings were laid and infilled.

Top cladding.

The entire tiered structure was finalliy clad in clay and turf and tamped.
Examination of the material on the north face shows that this surface had been reworked and added to after completion at an early date.

Inner ring ditch.

Prior to the commencement of the wall building phase a straight sided ring ditch with inner bank was dug and backfilled. This ditch was covered by the first walling tier.

Silbury section.

Silbury scantlins
Block built or drystane walling techniques had been perfected in Scotland, Ireland and Brittany several centuries before the erecting of Silbury. Notably at the Bend of the Boyne Necropolis, County Meath, Ireland : Newgrange.
and the Grey Cairns of Camster, Caithness, Scottish Highlands :Grey Cairns.
Also in France with the very fine Carn du Barnenez, Brittany : Bernenez, may be seen very effective retaining walling and resilient chamber construction using the drystane building approach.

However no prior constructions in Western Europe had reached the height of Silbury nor amassed the weight of materials. No other monument is so entirely devoid of evidence as to the purpose or intent of the builders.

An astronomical interpretation of the underbuilding of Silbury Hill.

Of the several unique building features of this monument most can be seen to have the effect of imparting extreme stability to the steep slopes of the profile.

Ditching.

First the inner ditch, which together with the later outer ditch, would, like a modern roadway, serve to relieve the foundations and upper structures of the burden of run- off and standing water.

Block walls.

The careful block building of the terraced outer walls, battered inwards for maximum stability and infilled and tamped.

Terraced soil containment.

The terracing of the block walls holds the outer soil and clay covering in discrete parcels taking most of the weight of each section off the lower slopes.

Timber footings and resurfacing.

Finally the particular attention to the the north slope and it's base, the timber revetment and underpinning with evidence of a later re- covering of this face with added soil and turf.

The northern face and it's location.

It is only this north face that can be used for an astronomical foresight in the Northern Hemisphere as the angle of descent of Sun or Moon is always from top left to down right at a set, whilst the angle of rise of Sun and Moon is always from bottom left to top right, hence the north face can be used for both rise and sets of celestial bodies with stances established on the eastern skirts for sets and rises fixed on the western sides.
Also the choice of placement in the landscape we see here with clear, low, flat surrounds is required in order that observers may locate backsight stances on both sides with the monument standing fully against the sky for the entire length of the northern slopes.

Silbury Hill- a universal foresight.

Silbury_backsights
Backsight stances are practical on most sides of the monument as it is based a few metres above the River Kennet water meadows, however to the south west the ground rises sharply though an examination in the field shows that it is possible to find places along the modern roadway, the A4, where most of the northern slope can still be viewed against the sky.

The backsight stances.

Solar.

S1 = Summer solstice rise.
S2 = Summer solstice set.
S3 = Winter solstice rise.
S4 = Winter solstice set.
E1 = Equinoctial rise.
E2 = Equinoctial set.

Lunar.


L1 = Northern Major Standstill rise.
L2 = Northern Major Standstill set.
L3 = Southern Major Standstill rise.
L4 = Southern Major Standstill set.
L5 = Northern Minor Standstill rise.
L6 = Northern Minor Standstill set.
L7 = Southern Minor Standstill rise.
L8 = Southern Minor Standstill set.

Silbury Hill summer solstice sunset backsights for 2012 AD and 2500 BC.

Stance_S2_to_Silbury_Hill
On the evening of the 19th June 2012 the camera position adopted was beside this abandoned stone gate post still standing by the footpath to the bridge crossing the Kennet leading to West Kennet Long Barrow.

2012 AD.

Stance_S2
This was the best position from which to observe the upper limb riding down the entire length of the north slope at the summer solstice of 2012 AD.

2500 BC.

Stance_2500_BC
At 2500 BC the solstitial sun was some 0.5 degrees higher in declination hence the adjustment in parallax for an observer must make in 2012 AD to locate the stance for 2500 BC would be some 10/15 metres to the south, close to this solitary tree.
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Sunrise 19th. June, 2015 AD on Silbury Hill.

Sunrise_19th_ June_2015_SURVEY
On the morning of the 19th June 2015 this series of photographs of the upper limb of the rising sun were taken from a fixed camera position, now marked Stance S1, with no movement of the camera position throughout. This position was established by moving laterally, when the first flash of the solar disc was seen, bringing the sun down until only a gleam of the upper limb remained visible. As found with the set of photographs secured at sunset on the 19th June 2012 from Stance S2, these verify that the slope of the northern flank of Silbury Hill exactly follows the solar path at the summer solstice both rise and set:
- demonstrating the Angle of Obliquity to the Ecliptic - the angle of tilt of the Earth's axis to the plane of it's orbit.
.

Stance S1.

This position was established by moving laterally, when the first flash of the solar disc was seen, bringing the sun down until only a gleam of the upper limb remained visible on the flank of Silbury. With no further movement of the camera position an hour of still photos were taken at 5 minute intervals. Due to a haze of thin cloud causing camera flare the actual rim of the upper limb was not resolved but it was clearly visible to the naked eye allowing the stance to be established to within centimetres of accuracy.

Silbury_Stance_S1_19th_June_2015
This area was chosen for the photo shoot at sunrise on the 19th June 2015. it is the lowest place where the far horizon intersects with the flank of Silbury hence exposing more of this flank to open sky than anywhere else in the direction of a summer solstice alignment. The bank supporting the A4 highway rises abruptly behind with the road some 4/5 metres higher. The stance could have been located on top of the road works or even across the road into the first few metres of the cornfield. Less open sky is presented as the observer climbs higher until at the top of the field the back horizon coincides with the flat top of Silbury leaving no further flank presented.

Silbury layout pegs.