This page explains the role the Isles of Scilly played in researching backbone and longer-distance alignments.
Long distance alignments and the Scillies
Some might doubt and question the validity of finding alignments crossing from Penwith to the Isles of Scilly, because of their length and the absence of verifying sites in the sea - the distance is 22-25 miles. It makes some sense that such alignments might exist, but from an accuracy viewpoint it’s rather a head-scratcher. (An alignments map of the Scillies is found here.)
The standard norms and rules of geomancers of recent decades, keeping alignment criteria tight, give cause for quandary about the verifiability of long-distance alignments, and there are map-projection issues too (factoring in earth-curvature).
The Isles of Scilly, as seen from near Tregeseal on the Penwith mainland
However, on examination, when it was found that quite a few sites on the Isles of Scilly were definitely aligned on major mainland features such as Cape Cornwall, Chapel Carn Brea, Maen Castle and Carn Lês Boel - all prominent mainland bumps as seen from Scilly - things got more interesting and antennae started twitching.
Eventually, a number of alignments were found. What made this significant was not just their existence, but the actual sites that they linked. To give an example: in the picture above, on the left in the far distance is Samson Hill on the island of Bryher, to which two significant alignments go, one (78) from St Micahel's Mount, Boscawen-ûn and Maen Castle and the other (95) rom Cape Cornwall, Chûn Castle and a number of other sites, including the Knackyboy cairn on Tresco.
Scillonians and Penwithians
The ancient people in the Scillies and Penwith will undoubtedly have had emotional, marital, ancestral and traditional connections.
In those times Sennen beach was the main mainland harbour for the Scillies, and today's island of Great Ganilly - then part of the Scilly mainland - was the main Scillonian harbour. An ancient land trackway leads across Penwith from Sennen to Crows-an-Wra, Brane, Sancreed and Madron, heading eastwards, which will have been the main path for Scillonians when passing upcountry, or for people heading through Penwith heading for the Isles of Scilly.
The ancients were travellers and mariners - much more than is commonly thought. Megalithic culture extended from Portugal to southern Scandinavia and these people were capable sailors and internationally connected, at least during summer months, West Penwith being a key node and stopover place in this maritime network - as also were the Isles of Scilly, useful when sailing from Brittany or the English Channel to Ireland.
The boat traffic across the 25-mile strait
between Penwith and Scilly did not involve regular commuting, since these were challenging seas, but over time there will have been substantial traffic and trade. People didn't go by timetable - they awaited their time and sailed when the going was good.
Ancient site alignments connecting the two thus make a lot of sense. The people of Penwith and Scilly were related, if not by origin, then certainly by intermarriage. And they were connected geomantically, sharing the same localised culture.
The Islands of the Dead
The Isles of Scilly have exercised a magnetic pull on mainlanders for millennia, and the islanders naturally had their connections with the mainland. The Scillies have traditionally been known of as the islands of the dead - and, judging by the number of cairns and cairnfields in Scilly, this probably was so.
However, a Scilly archaeologist has recently questioned this, saying that the large number of grave sites could simply have served Scillonians. This isn't quite plausible, and if it were true, we might expect to see far more such grave sites in West Penwith. Nevertheless, being buried on the Scillies, or going there to die, would probably have been a special honour. My own feeling is that the 'isles of the dead' tradition has substance.
The islands are named after the goddess Scillina (also associated with Aquae Sulis, the city of Bath). Burial in the Scillies might have been regarded as a return to the embracing arms of the goddess Scillina at death.
But death is also a part of life and, to an ancient culture believing in an afterlife and in the influence of the ancestors, the dead (or at least the privileged) might have gone there for the benefit of the people as a whole, to get closer to the Source, to what is far beyond, in the land of the setting sun.
Navigation and signalling
Then there’s the question of signalling and ritual fires spanning the straits between Penwith and Scilly. This was important in ancient times.
There's even a chance that some of the chambered cairns on the islands or on the mainland coast might have been used for watching for fires and signals from the other side of the sound. Screening out ambient light, a distant light from across the sea becomes more visible. This theory needs testing out.
For sailors coming from the Scillies, to take a fix on Sennen beach it was a matter of triangulating to Cape Cornwall and Carn Lês Boel on each side of Sennen. Sail toward a point halfway between them and you’ll come into sight of Sennen beach.
Similarly, sailing to the Scillies, keep these two headlands equidistant behind you and you'll find the islands. This is important because, when in a boat on the sea, the islands become visible only five miles away, even though they can be seen from up on the cliffs of Penwith - this is an earth-curvature issue. However, at times, and due to atmospheric light-bending, the islands sometimes appear to rise out of the sea, as seen from Penwith's cliffs.
In the bronze age, what now are many islands were once one main island with a few outliers - sea levels have risen in the Scillies relative to the land since the bronze age.
Regarding alignments over the sea, it was alignment 78 that clinched it: an alignment from St Michael’s Mount to Boscawen-ûn which, when extended, went through Maen Castle near Sennen to Bonfire Carn on Samson Hill, Bryher - a beacon hill. Also on the alignment were the Chapel Downs cairns on St Martin’s, the Longships Rocks off Sennen, and Ennis Farm menhir on Penwith.
The matter was sealed. This alignment was too significant, and the sites connected by it chimed well with each other. This is not accidental. Maen Castle, Chapel Downs and Bonfire Carn are all beacon sites, signalling and sighting points.
For a map of the Scillies and their alignments, click here. For an online resource on maritime life in prehistoric times, particularly Scilly, click here.
Backbone circuitry emerges
On further research, more long-distance alignments emerged. Gradually a pattern of major lines formed. These were named 'backbone' alignments (using an internet term for the main 'pipes' in the worldwide web). Doubts in these long-distance alignments were removed by their very accuracy. They mostly conformed to the three-metre accuracy rule, even over long distances.
Though backbone alignments have fewer and further-apart sites than more ordinary alignments (comparably to motorways), their accuracy and the significance of the points on them verifies them. After all, an alignment such as 80, linking Carn Brea to Trencrom Hill, then Lanyon Quoit and the Tregeseal complex, is not at all insignificant.
More on backbone alignments here.