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.

Ballochroy North Kintyre.

Ballochroy
This stone row stands on a commanding position overlooking the Sound of Jura and the mountainous high ground of Jura island.
The most definitive summer solstice site identified by Alexander Thom must be the stone row at Ballochroy, North Kintyre. This dramatic stone row sits on the mainland hillside overlooking the Sound of Jura with it's central stone, a large blade or cleit, having it's two long faces clearly aligned on one of the high peaks, Corra Bheinn, on the mountainous island of Jura to the north west. From these stones, at the summer solstice of 1800 BC, (according to Thom's theodolite surveys), the upper limb of the solar disc, at sunset, would first graze the top of Corra Bheinn then slide down the long north flank before settling into the notch at the foot of the mountain.
This demonstrates that the angle of slope on Bheinn Corra's north bank matches the Angle of Obliquity to the Ecliptic in 1800 BC.
Ballochroy Stone Row

The Green Flash.

At sunset there might be up to three green flashes observable on the flanks of Bheinn Corra from this stance at, or near, a summer solstice @ 1800 BC. If the flashes are seen one evening, when standing at the stone on the right, then, on the following evening, an observer must move 12 feet to the left- to the central stone- to see them again. On the third evening the observer will find that the flashes can only be observed from the original stance- the stone on the right-and will know that the solstice has passed and had occurred on the previous day.

BallochroyThom


Observing procedures at a solstice.

The solstitial sunset no longer reaches the northern flank of Bheinn Corra as seen from Ballochroy. This photograph above is an accurate simulation of the situation at the summer solstice 1800 BC based on Thom's survey and calculations presented in Megalithic Lunar Observatories p.37, Fig. 4.1.
The difference in declination of the Sun from the day before the solstice is 12 arc seconds. The distance to Bheinn Corra is 19.1 miles. For an alignment of this length and altitude the angular displacement on the ground at this stone row which would represent 12 arc seconds is about 10-15 feet, (3.5m). If an observer wishes to see the Sun settle on the same foresights one day before or one day after a solstice they must take up a stance some 12 feet to the north, (right), of the central stone. As we can see the distance between the central stone and the smaller northern stone is 9.25 feet. This northern stone may be intended to mark the necessary extrapolatory stance required to define the day of the solstice reliably.

Ballochroy winter solstice alignment.

SetCaraNet
Ballochroy Stone Row to cist, Cara Island and winter solstice sunset 1800 BC.
The Ballochroy stone row boasts yet another solstitial alignment- to the south west, to Cara Island- at right angles to the Jura foresight. This alignment follows the line of the three stones through a cist- a stone box often associated with Bronze Age mortuary sites.
It is unlikely that this winter solstice alignment would be as reliable as the line to Bheinn Corra in the summer as there is no obliquity bank involved at the foresight. Also the altitude is much lower, actually having negative value. The foresight on Cara Island is 65 ft (20 metres) lower than the stone row so the line of sight must pass through many more miles of atmosphere than a high altitude line such as that to the summit of Bheinn Corra.

The cist as early warning feature.

BallochroyCist
Ballochroy Cist. On the correct evenings,(7 days before and after the solstice), there may be two green flash sightings from the cist- one on the peak of Bheinn Shiantaidth and one in the small shelf in the southern flank of Bheinn Corra.
A further possible use of the cist in this position seems likely when the declination of the setting Sun at 7 days before a summer solstice is examined. The cist marks the place where an observer might stand in order to witness the emerald flashes on the summit of Bheinn Shiantaidh 7 days before the summer solstice and 7 days after. The position at the cist might be an early warning feature for the solstice backsight proper, but it may also be used separately to find the true date of the solstice if poor seeing conditions around the time of the solstice sunset obscures observation.

Finding the date of the solstice in retrospect

If two good observations of the green flash are made from the cist to the crest of Bheinn Shiantaidh, (or the notch in the southern flank of Bheinn Corra), before and after the solstice then halving the count of days elapsed between the two will render the solstice day in retrospect.

The latitude of Ballochroy.

Ballochroyaspect
Where the two alignments meet they form a right angle. This is a feature of the latitude. The megalithic astronomers recognised this phenomenon at other sites in Britain, in particular the Station Stones rectangle at Stonehenge, established in the earliest years of the great monument.
At the latitude of Ballochroy, near to 56 degrees north, the line to the point of the summer solstice sunset makes a close 90 degree angle with the line to the winter solstice sunset. It can be seen at Stonehenge that the builders recognised that, at certain latitudes, alignments to solar or lunar extremes will strike a right- angle at the backsight. At the latitude of Stonehenge, (51 deg. 11 min. north), the summer solstice sunrise line is at 90 degrees to moonset of the Northern Lunar Major Standstill on ideal flat horizons. At Ballochroy, (56 deg. north), solstitial lines to Midsummer and Midwinter sunrises and sets will form a right- angled cross. This is plain from the surveyed plan of the site though the angle is not quite 90 degrees due to slight deflections caused by the foresights being higher and lower than the ideal flat horizon.

Ballochroy Lunar Standstill alignment

BallochroyNE
The rising ground to the north east affords an alignment through the rock outcrop to the upper limb of the rising moon at a Northern Major Lunar Standstill at 1800 B.C.
In the reverse direction from the winter solstice alignment Ballochroy stone row indicates an outcrop of rock on the crest of the steeply rising bank nearby. From the cist, on the same line but some 90 yards, (83m) further back, this outcrop offers a declination of 29.4 degrees,- the rising point of the upper limb of the Moon at the Northern Major Lunar Standstill in 1800 BC. Although, at the latitude of Ballochroy, on flat horizons, no extreme of the lunar orbit may strike a right-angle to a solstice line there is yet an indicaton of a lunar extreme involved in the fundamental design of this backsight. This alignment is only some hundreds of feet in length and might not have been considered a high-resolving astronomical feature but rather another demonstration of the megalith builders intense interest in establishing, or finding fundamental geometric designs which incorporate horizon and astronomical features as at Castle Rigg, Cumbria, and the Stonehenge Station Stones.