Using the Moon as a proxy for the ancient Sun

There's a problem all archaeoastronomers face when they're dealing with properly ancient sites and their possible alignments to the sky, and it's called the change in the Obliquity of the Ecliptic.

The Earth's rotational axis is currently tilted at an angle of roughly 23.4° to the plane of our orbit around the Sun - it's why we have seasons. This angle (the Obliquity of the Ecliptic) isn't fixed, it varies over long periods of time (roughly 41000 years) between about 22.1° and 24.5°.

This angle is critical in determining exactly where on the horizon the Sun rises and sets at the solstices - the greater the angle, the closer to the due north direction the summer solstice sunrise and set appear; and the closer to the due south direction the winter solstice sunrise and set are.

The large sarsens of Stonehenge were erected about 4,500 years ago when Earth's tilt was 24° (in round numbers), and at summer solstice the Sun rose and set about 1° further north than it does today. Similarly, the winter solstice Sun rose and set about 1° further south than today.

All this means is that we can't, today, experience the alignments at solstice exactly the same way as the people who built Stonehenge. Sure, we can do calculations to correct for the offset but we can't ever see what they saw. Frustrating.

The Moon's orbit is tilted to the plane of the Earth's by about 5.1° and the Moon's orbital path itself rotates around the Earth, taking 18.61 years to go around once. This is what gives rise to the so-called Major and Minor Lunar Standstills.

In the couple of years around the Major Lunar Standstills the Moon can rise and set much further around to the north and south than the Sun can ever reach - something like 10° or roughly 20 lunar diameters further at the utmost extremes. It's these extremes that the long sides of the Station Stone rectangle are aligned on, as first reported by Peter Newham (and, independently, Gerald Hawkins) in the 1960s.

We can take advantage of the phenomenon, because while the Moon is in the "Major" half of its cycle there are times when it can rise (and set) at the azimuths that the ancient Sun once did and no longer does.

So we can effectively use the Moon as a proxy for the ancient Sun.

As I've written about before, there is a secondary solstice axis that runs through Stonehenge which points towards the winter solstice sunrise in the SE and the summer solstice sunset in the NW.

There is a sightline, tightly constrained, through Stonehenge that frames the precise spot on the SE horizon where the ancient winter solstice Sun appeared. It makes use of a notch in Stone 58 and the edge of Stone 53 to create a "hole" where you can view this small patch of horizon.

Once upon a time, 4,500 years ago, someone standing here at winter solstice would see the Sun rise just to the left of the electricity pylon you can see in the image above. Due to the change in the Earth's tilt, as explained above, that doesn't happen for us today.

Happily, though, on 11th July 2025 - one day after it was full - the Moon rose at almost precisely the same azimuth that the ancient winter solstice Sun once did (exceeding it by ~9' of arc in declination, so rising fractionally south of perfection) and I was afforded the opportunity by Historic England and English Heritage to be on site to watch it happen.

I set up my camera to point along the sightline through the notch in Stone 58 and told it to take a photo every few seconds from 22:20 UTC+1 to catch the rising Moon.

Remarkably for England in the summer time the sky was clear to the horizon, and at 22:22:27 UTC+1 the first gleam of the Moon appeared.

Over the course of the next several minutes, the Moon gave me the chance to watch with my own eyes something very close to what the builders of Stonehenge would have seen at their winter solstice Sunrises.

Half orb occurred at 22:24:52 UTC+1.

Full orb followed at 22:27:01 UTC+1.

Overlaying several shots together, and with some calculation to produce the true position of the ancient winter solstice Sun, I'm able to show how close this Moonrise was to being spot on.

Ever since Prof. Gordon Freeman first suggested the existence of a secondary solstice axis I've been intrigued by his idea. Back in 2012, I helped confirm the calculations he made based on his 1990s observations near the winter solstice at Stonehenge because we had a perfectly clear sunrise that year but he and I could never actually observe it "truly" due to that pesky change in the Earth's tilt.

Thanks to the Moon's periodic excesses in rising and setting azimuths, I've now been able to see it with my own eyes.

It's hard to convey the sense of excitement I felt in the few minutes before first gleam appeared, and the enormous sense of satisfaction in watching the Moon behave exactly as anticipated.

There's something wondrous about peering through a narrow gap created by two stones erected 4,500 years ago and being perhaps the first person in several millennia to experience a ghostly echo of the spectacle the Stonehengineers intended.

The observations and technical details have been written up and accepted as a research article for the Journal of Skyscape Archaeology so if you fancy you can find that here: https://doi.org/10.1558/jsa.33686

Stonehenge - Access All Areas

Just a short post about the upcoming release of "Stonehenge - Access All Areas", a documentary - perhaps the documentary - about that famous pile of rocks in a field.

Julian Richards has been involved in the Stonehenge story for over 40 years as an archaeologist, writer, broadcaster and educator and has always wanted to make a film that properly explores everything we know (and think we know) about Stonehenge.

During the pandemic lockdown he got the unique opportunity to spend several days filming inside the monument when it was closed to the public and what started out as a plan to put out three 1-hour films has expanded into a massive project that (currently) runs to 11 episodes.

Julian, Luke Winter, Tim Daw and I have spent the last several years collaborating to distill our geeky obsession with Stonehenge into a coherent narrative, all filmed and edited by Steve Shearn.

Episode 1 drops on Julian's YouTube channel on May 3rd 2026.

Expect interviews, road trips, discussions, arguments, experiments, fun, frolics, Druids, festivals and (maybe) aliens.

The Stonehenge Byways and the Threat to their Existence

There are a number of Public Rights of Way (PRoW) that run through the Stonehenge World Heritage site.

You can find them marked on the Ordnance Survey map (Explorer sheet 130 "Salisbury and Stonehenge"), on the Wiltshire Council's Rights of Way Explorer GIS (try searching for AMES12) and see how their routes have remained mostly stable over the last 200 years using the National Library of Scotland old map archive.

Here's the closeup from Wiltshire Council's website.

PRoW include footpaths, bridleways and byways open to all traffic ("BOATs") and afford the public the right to travel along them freely. England and Wales's PRoW network was codified in 1949 in the National Parks and Access to the Countryside Act, where ancient tracks and paths were formally set down in Definite Maps and Statements that described their routes and enshrined them in Law.

If a PRoW is a footpath you can only walk along it, for a bridleway you can walk, cycle or ride a horse. For BOATs you can do all of that, and you can use a motorised vehicle along them as well.

It's the local Council's responsibility to maintain these PRoW in a fit state to be used as intended, and it's an offence to block them up to prevent the public using them (something certain landowners will sometimes do, illegally).

The BOAT that runs past Stonehenge comprises a collection of routes known as (from South to North) Berwick St James 11, Wilsford-cum-Lake 1, Amesbury 12 (these three are the long dotted green and brown line in the map above) and Durrington 10 (the solid brown line continuation at the northern end). For the purpose of this post, this is called "the Drove".

The shorter dotted green and brown line that stops at the A303 comprises (from South to North) Wilsford-cum-Lake 2 and Amesbury 11.

For generations, people have made use of the Drove to get to the vicinity of Stonehenge, stop, and walk up to (and originally into, before it was fenced off) the monument and out into the wider landscape.

Ever since the new Stonehenge Visitor Centre was built about 10 years ago, when the old main road that used to also run right past Stonehenge (the A344) was closed and grassed over, the Drove has been the only way that people in motorised vehicles have been able to approach the area "out of hours".

So if you want to see Stonehenge at dawn, for example, you can drive down the Drove and park on the side of it near where the old Visitor Centre used to be, get out of your vehicle and walk along the "Permissive Path" (not a PRoW, but an informal public path along the route of the old A344) to get quite close to Stonehenge. For free. Whenever you like.

So Far So Good, but...

It came to light recently, quite by accident, that Wiltshire Council have invited a select group of "user groups and interested parties" to offer their comments on "measures they feel would be appropriate and proportionate to address the problems being experienced."

I quote the communication that was sent out by the Council in full, below:

I am writing to you in relation to the Byways Open to All Traffic contained within the vicinity of Stonehenge. For some time, the current level and type of use of these Byways Open to All Traffic has resulted in a number of issues around damage to the surface of the Byways Open to All Traffic and degradation of the environment within the World Heritage Site (WHS). While it was thought that the promotion of the Stonehenge Tunnel Scheme would have addressed a number of these issues, its cancellation means that this opportunity has now disappeared.

Stonehenge, Avebury and Associated Sites World Heritage Site Management Plan (2015) sets out some specific objectives in relation to highways and public rights of way:

· Policy 6a of the Management Plan states the need to identify and implement measures to reduce the negative impacts of roads, traffic and parking on the WHS and to improve road safety and the ease and confidence with which residents and visitors can explore the WHS.

· Policy 6b states the aim to manage vehicular access to byways within the WHS to avoid damage to archaeology, improve safety and encourage exploration of the landscape on foot whilst maintaining access for emergency, operational and farm vehicles and landowners.

Most recent inspection of the Byways Open to All Traffic, carried out in conjunction with the “Winter Solstice” closures, identified the fact that the byway surface conditions had deteriorated to an unacceptable condition. This has necessitated the closures being extended to enable repair works to be undertaken.

Wiltshire Council as the Highway Authority is now seeking to develop a holistic approach that addresses Policy 6a and 6b of the Management Plan. To this end we are now looking to consult with user groups and interested parties for their views on what measures they feel would be appropriate and proportionate to address the problems being experienced.

We would be grateful if you could make any observations and respond to this correspondence by 21st March 2025 (UPDATE: now extended to 31st March - SB).

Responses can be returned to:

stonehengebyways@wiltshire.gov.uk

Thank you for your attention in this matter.

On the face of it, this might seem reasonable.

However, the Council has a history of trying to remove vehicular rights over these routes. The last time they tried to do so (before the Stonehenge Tunnel project attempt) was in 2010/11 when the Order to "Stop Up" the old A344 was subject to an Enquiry by an Inspector to determine whether or not it would be allowed. The Council had included in their application an attempt to remove the BOAT status from the byways around Stonehenge at the same time.

The Inspector denied that attempt and Wiltshire Council were frustrated.

This new invitation to offer observations on "a holistic approach" (whatever that is) seems to have behind it another attempt to remove the rights of motorised vehicles to use these routes.

If you should happen to think that this is wrong, and that there ought to be a way for people to readily access the Stonehenge area by motorised vehicle without relying on the Visitor Centre being open and using the shuttle bus (which means having to buy a ticket - currently £86 for a family of 5), then you have until Friday 21st March 2025 (UPDATE: Now extended to 31st March - SB) to let the Council know by emailing stonehengebyways@wiltshire.gov.uk

I've done so, and you can read my response here. It's quite detailed and digs into the World Heritage Site Management Plan 2015 to find out what Policies 6a and 6b actually say, and also checks out what the last Condition Survey Report from 2012 had to say about damage to archaeology as a result of vehicles on the byways. It turns out there's more to this that what Wiltshire Council have said in their communication.

There is no consultation page on the Wiltshire Council website about this, and a vast number of people and organisations who could rightly be called "user groups and interested parties" were not included in the list of recipients of Wiltshire Council's invitation.

It's almost as if they're trying to keep it secret, but let's not go down that route.

A Change.org petition has been set up (not by me), which you can sign, but even if you do that it's vital that you also directly email Wiltshire Council (stonehengebyways@wiltshire.gov.uk) to give your view.

The Larkhill Sun Gap Revisited

The Summer Solstice Sunrise line as seen from Stonehenge runs across the landscape and over the ridgeline at Larkhill, at the northern end of Wood Road and Tombs Road.

Back in 2014, the MoD were proposing to build a large number of houses as part of their Army Rebasing Program, to provide accommodation for troops who were being returned to the UK from Germany.

Two of the potential sites under consideration were in Larkhill, and would have been built across the alignment to the Summer Solstice Sunrise in a spot called the "Sun Gap".

I objected strongly to this idea, blogged about the issue and a petition (not organised by me) was submitted to the formal consultation which contained almost 2000 signatures from around the world. Happily, the offending candidate sites were dropped from the program.

If you're interested, I've republished my blog posts from 2014 here as the original blog is no longer available.

Sun Gap?

In the early 20th Century, just as aviation was taking off (ha ha!), a number of aeroplane "flying sheds" were constructed on this ridgeline in Larkhill so early aviators could store and work on their machines before testing them on the Larkhill Flying Ground.

Initially just one shed was built (for Horatio Barber), which was followed by two others - one for the War Office, which lay unused for many months before being used by Captain Fulton, and the third for George Cockburn. For simplicity, I'll refer to these as the "northern sheds".

Then the War Office became properly interested and decided they wanted a series of them for their own use.

The problem was that the location they chose for them would have blocked the view of the rising Summer Solstice Sun from Stonehenge. The Larkhill ridge was completely bare of trees then, offering an unobstructed view from the stones.

Sir Edmund Antrobus (artistocratic owner of Stonehenge at the time) evidently carried some clout and the War Office were prevailed upon to leave a wide space between the existing northern sheds and their new ones.

This space is what became known as the "Larkhill Sun Gap".

The history of these sheds has been researched and documented in the excellent book by Terry Grace "Stonehenge, Sheds and Sun-Gaps".

I recently came across this book in Amesbury Library and was interested to find that Terry had included one of my diagrams of the solstice sunrise line from my 2014 Army Rebasing consultation response in the chapter where he dealt with the Sun Gap.

There had been some confusion about where exactly the Sun Gap was - some researchers favouring a more southerly location where a different gap existed between sets of sheds. Terry felt that my diagram indicated the northerly larger gap was more likely.

When I was assembling my argument against the plan to build houses on Larkhill, I used some rough and ready methods to determine the direction of the sunrise line along with a diagram of the flying sheds' locations that I'd come across online (in a PDF of "Cross & Cockade International Winter 2012 GFS-89" - now only available as a purchaseable download via https://greatwaraviation.org/shop/back-issues/volume-43-number-4/)

What I found in Terry's book was a much better diagram of the sheds which he'd overlaid on Google Earth.

I thought it'd be useful to redo my solstice sunrise line determination in conjunction with Terry's diagram and see how close my original rough and ready attempt might have been.

Scratching an Itch

These days I use Stellarium with a digital terrain model landscape for the Stonehenge area installed to produce a horizon that isn't simply a 0° altitude line in all directions. It also has the advantage of a good atmospheric refraction model, plus the ArchaeoLines plug-in works well for showing the ancient positions and paths of the Sun and Moon.

First step was to overlay Terry's plan of the sheds' layout in Google Earth. Here's the original from his book:

Flying Sheds' layout from Stonehenge, Sheds and Sun-Gaps

And here's the result when that's placed as an overlay in Google Earth:

Flying Sheds' layout overlaid in Google Earth

The second step was to determine the azimuths of "first gleam" and "full orb" over the DTM (actual) horizon in the direction of Larkhill, for Summer Solstice Sunrise in 1910 which was the era when the sheds were being constructed. Doing it for 2025 wouldn't have affected things appreciably, but I thought 1910 would be appropriate.

Screenshots from Stellarium follow:

First Gleam over Larkhill in 1910

Full Orb over Larkhill in 1910

I've simply eyeballed the Sun and judged where the first gleam has just appeared, and full orb when the horizon is tangent to the bottom of the Sun's disc. This is accurate enough for my purpose.

Reading off the azimuths, we get First Gleam at 50° 22’ 47.5” (50.379861°) and Full Orb at 51° 15’ 30.1” (51.258361°).

Step 3 is to generate Google Earth linestrings to show these azimuths. I need to anchor one end of each at the centre of Stonehenge on the solstice axis and the other ends about 3 miles distant at whatever coordinates each azimuth achieves at that distance. (Aside: it's quite annoying that Google Earth doesn't have a built-in facility for creating a line from a point on a specific bearing).

Handily, there's a useful online calculator that'll give me the end points based on start point, bearing and distance. It uses the Great Circle method, but for a distance as short as 3 miles this isn't going to cause a problem.

Using starting coordinates for the centre of Stonehenge of 51.178855° N; 1.826209° W (found by me using a consumer level GPS years ago, so it could be up to 10 yards out, but this won't make any appreciable difference at the accuracy I'm using for this), the end point coordinates come out at:

First Gleam: 51.206535° N; 1.772819° W
Full Orb: 51.206019° N; 1.772148° W

There is a ludicrous amount of "precision" in all these numbers, which isn't really precision at all it's just lots of digits after the decimal point. I don't care about rounding these down because it'll not make any significant difference to the gross accuracy of the end result.

Final step is to create a .KML file containing the linestrings, load it up in Google Earth and see what we get.

First the wide view with Stonehenge in the bottom left and Larkhill sheds in the top right (I've added placemarks to indicate which yellow azimuth line is which):

Wide view of Azimuths and Sheds in Google Earth

And here's the closeup - the moment of truth, as it were:

Closeup of Azimuth lines and Sheds

I think it's conclusive - the Sun Gap was in the space between the original three northern sheds and the later War Office set.

The Sun Gap that was created over 100 years ago is fantastically symmetrical either side of First Gleam (upper line) and Full Orb (lower line) so "Half Orb" would have been dead centre of the space. Nicely surveyed indeed.

Comparing side by side with my original work from 2014, I wasn't too far out with the rough and ready approach, which is pleasing.

Side by side comparison of 2014 (L) vs 2025 (R) work

Now all that remains is to get those blasted trees removed from between Wood Road and Tombs Road, as well as some alongside Fargo Road near Strangways and a short stretch just north of the east end of the Cursus to restore the original Sun Gap clear sightline.

That's a project for another day.

The Founding Stones of Stonehenge

I was drafting a summary diagram for use in a presentation I'm giving tonight to the Hampshire Astronomical Society on the Astronomy of Stonehenge.

Building on previous work (see "Is the Altar Stone the founding stone of Stonehenge?" and "Implications of an upright Altar Stone in WA 3639"), I was working on this image:

Red lines are the Station Stone rectangle, yellow ones are the solstice axes

While labelling it up, I suddenly realised that it wasn't complete.

What's missing from this picture?

While both the solstice axes are present, aligned to Winter Solstice Sunrise <-> Summer Solstice Sunset and Summer Solstice Sunrise <-> Winter Solstice Sunset, along with the Station Stone rectangle long sides' alignments for Southernmost Major Moonrise <-> Northernmost Major Moonset, the missing element is the other Major Moonrise/Set axis.

Northernmost Major Moonrise <-> Southernmost Major Moonset axis is not represented at all - it appears as if the designers didn't incorporate this. Except... perhaps they did.

The 81° "twist" of the upright Altar Stone so you can sight along its face towards the WSSR<->SSSS directions has another effect. This is the same 81° "twist" that means it lies along that secondary solstice axis in its now prone position in front of the remains of the tallest trilithon, which also shares the same "twist" (see Tim Daw's "The Twisted Trilithon")

It causes the Altar Stone to face the missing Lunar directions.

This is a consequence of the latitude of Stonehenge, where the Northernmost Major Moonrise occurs roughly 9° further north than Summer Solstice Sunrise, and the angle between Summer and Winter Solstice Sunrises (and Sunsets) is 81°, back when the monument was constructed.

The complete diagram of all the alignments encoded with just five stones - the Station Stones and an upright Altar Stone - is this:

The Moon and Sun completely encoded with five carefully positioned stones

Once again, I'm struck by the elegance and minimalism of this structure.

To paraphrase Picasso, when he first saw the cave paintings in Lascaux, we have learned nothing in 5000 years.

Implications of an upright Altar Stone in WA 3639

In the previous post (Is the Altar Stone the founding stone of Stonehenge?) I wrote that perhaps Atkinson was correct when he suggested the Altar Stone might have once stood upright in the stonehole he found immediately southwest of Stone 56, the one with the Group I Cornish polished stone axe deliberately placed in its primary fill.

I went on to suggest that perhaps the Altar Stone and the Station Stones comprised the first stones on the site, as they would be the minimum that would be necessary to fix the Sky to the Earth and encode all the solar and lunar alignments.

The stonehole that Atkinson suggested is at the base of a feature catalogued as "WA 3639".

Position of WA 3639 and the suggested stonehole, from Cleal et al 1995

It's in an interesting place, being on the primary solar axis (Summer Solstice Sunrise to Winter Solstice Sunset), bang up against (and partly under) the large lumpy fragment of Stone 15, midway between Stone 56 (the tallest stone on the site, one upright of the now-fallen tallest trilithon of 55-56-156) and the massive block that is Stone 16.

I wondered if the position was at all significant in terms of the minimal arrangement of stones, so I reached for the 2012 paper "Stonehenge Remodelled" (Darvill et al) because I knew it had a selection of plans of the various "Stages" of the monument through its life. These were originally in Cleal et al, but were amended to illustrate the consensus view that Darvill, Marshall, Parker-Pearson and Wainwright arrived at.

Overlaying the plans to show just the Station Stones and an upright Altar Stone in the position of WA 3639 was simple enough. I also retained the Heelstone, Stonehole 97, the Aubrey Holes, the banks and ditches and the causeway postholes (since they're all potentially early features).

Altar Stone (green) positioned upright in WA 3639, Station Stone rectangle in red

I then added the interior postholes that Hawley discovered in his 1919-26 excavations, since they're possibly early features too, together with Stoneholes B and C outside the causeway entrance in the NE.

As above, but with Hawley's interior postholes and Stoneholes B and C included

Next step was to draw in the primary solstice alignment line, through the Altar Stone (since WA 3639's stonehole is on this axis) and to the left of the tip of the Heelstone which is where the Sun rose 5000 years ago when the Earth's tilt was 24° rather than the 23.5° it is today.

Primary solstice axis is in orange
Summer Solstice Sunrise (top right) to Winter Solstice Sunset (bottom left)

Last step was to take that primary axis line and rotate it 81°, which was the angle between Summer Solstice Sunrise and Winter Solstice Sunrise 5000 years ago, and drop that over the Altar Stone position.

I was not expecting what happened when I did that.

Secondary solstice axis added in orange
Winter Solstice Sunrise (bottom right) to Summer Solstice Sunset (top left) 

I did that last operation when zoomed in close, so I could get the intersection point of the axes exactly over the Altar Stone's position.

When I zoomed out, I was astonished to see that this secondary axis also intersects Station Stone 93.

Not only that, but it also follows the line of postholes that define the unexplained "arrowhead"-shaped arrangement at the end of the Southern Causeway corridor.

I need to do this again, with the high resolution images from Cleal et al to be certain it's what it looks like, but what it looks like is that there's a sightline that runs from Station Stone 93 via the postulated upright Altar Stone in WA 3639 to the Winter Solstice Sunrise position over Coneybury Hill.

It may not hold up to detailed scrutiny when I do that reworking, but for now this is potentially very exciting - at least for me.

Update: 29th Aug 2024

Doing this again using the vector illustrations of Plan 1 and Plan 2 from Cleal et al, the alignment is not quite as precise as with the ones from "Stonehenge Remodelled" in that the alignment just misses Station Stone 93's SW face.

This in itself is interesting, because it means there'd be a very tightly constrained sightline between that face of the Station Stone and the NE face of the proposed upright Altar Stone which frames the appropriate place on the horizon - Winter Solstice Sunrise to the SE and Summer Solstice Sunset to the NW.

Arguably, given another tightly constrained sightline that makes use of pair of stones to create a viewing portal, this is actually more convincing.

Here's the result:

Same construction but using the plans from "Stonehenge in its Landscape" Cleal et al, 1995

You might notice that there's now a second red "blob" in the interior of the monument. The one to the SW is the proposed upright Altar Stone in WA 3639, and the other in the NE is... well.

While trawling through Cleal looking for anything relevant to this endeavour, I came across the following comment relating to the NE end of the Bluestone oval setting:

This is somewhat difficult to parse but it drew my attention to WA 2730, positioned as shown below, just to the bottom right of the middle of the image:

Feature WA 2730, just inboard of the Bluestone circle in the NE quadrant of the monument

This is an odd feature, it appears completely unrelated to the other settings but is about 1.5m NE of the only grave ever discovered in the interior of the stone circle. What happens if you put a stone here?

Such a stone, together with the proposed upright Altar Stone in WA 3639, lies on the primary solstice alignment. Curious and, although Atkinson's comment about a possible pair to the Altar Stone is probably referring to a stonehole near Stone 55 in the SW rather than at the NE end of the interior, it intrigues me.

Could there have been a second Altar Stone in the interior?

This is a rabbit hole that many people have gone down - starting with Inigo Jones' and John Aubrey's comments about a "supposed Altar Stone" in the middle of the "cell" (the innermost part of Stonehenge) "towards the east" that Philip, Earl of Pembroke, reported had been "carried away to St. James' (Westminster)".

I'm going to leave it to others to deal with that conundrum!

Is the Altar Stone the founding stone of Stonehenge?

You've got to be pretty motivated to bring a 6 ton lump of sandstone from the environs of the Orcadian Basin to Stonehenge (see The Scottish Stone article).

Let's, for the sake of argument, say that it's actually from Orkney. What else do we find there?

The list is pretty impressive, a few highlights are:

The Stones of Stennes - possibly the oldest "henge" monument in Britain - a henge being an enclosing earthwork with a bank on the outside and a ditch on the inside - with a 30m wide stone setting in the interior, oval in form with a central hearth. Roughly 5,100 years old.

The Ring of Brodgar - a perfectly circular stone circle 104m in diameter, within a bank and ditch (but with the ditch outside the bank, so not technically a "henge"). Roughly 4,500 years old.

Maes Howe - a chambered cairn and passage grave 35m in diameter, oriented to the setting Winter Solstice Sun. Roughly 4,800 years old.

Skara Brae - a settlement of ten stone-built houses that was discovered after a winter storm stripped the earth from a knoll in 1850 CE. Roughly 5,200 years old.

The Ness of Brodgar - sited on the spit of land between the Stones of Stennes and the Ring of Brodgar, this is a complex of stone-built structures that were first noticed in 2003 CE. Variously described as a "temple complex" or even a "university"(!), this is remarkable site with decorated, carved and painted walls and a striking artistic iconography. Roughly 5,500 years old, but deliberately closed down and abandoned after a huge feast in about 2,200 BCE.

The people inhabiting this region had to have been competent mariners - the strait separating Orkney from the mainland (the Pentland Firth) has some of the most powerful tides in the world.

Their origins are unclear, but Cunliffe, Renfrew, Collins, James, Prior and Oppenheimer all suggested that the arrival of a culture originating in the Iberian peninsular (perhaps emerging from the Mediterranean basin) and travelling by sea along the Atlantic western fringe of Europe is a possibility.

In Oppenheimer's 2006 book "The Origins of the British" he writes:

"...75-95% of British Isles (genetic) matches derive from Iberia... Ireland, coastal Wales, and central and west-coast Scotland are almost entirely made up from Iberian founders, while the rest of the non-English parts of the British Isles have similarly high rates. England has rather lower rates of Iberian types with marked heterogeneity, but no English sample has less than 58% of Iberian samples..."

If we look at the distribution of European megalithic structures, they closely match the supposed migration route of these people - from Malta via Iberia through Brittany and across to Cornwall, Wales, Ireland, the Isle of Man, western Scotland and the Hebrides to northern Scotland and Orkney.

Cultural artistic influences between Ireland and Orkney have been well documented by Sheridan - with a frequent back and forth of ideas, embellished at one location before returning to their source to re-influence subsequent expressions.

What is particularly interesting is the apparent diffusion of two specific aspects of Orcadian culture southwards through mainland Britain - the concept of "henging" and a style of pottery known as Grooved Ware. Thomas's paper on the topic of ceramics is well worth a read.

There was an obvious interest - perhaps obsession would be a better word - of the builders of these megalithic monuments with pointing them at key directions relating to the movements of the Sun and Moon over a development span of at least 2,000 years from the early 5th millennium BCE (in the case of the Locmariaquer complex) to the middle of the 3rd.

At the northernmost extreme of their range in Orkney, the rising and setting arcs along the horizons described by the Sun and the Moon are much greater than at their southern extreme around the Mediterranean.

For example, in 3000 BCE in Orkney (lat 59°N), the angle between summer and winter solstice sunrises is almost 105°, whereas in Malta (lat 36°N) it's only about 60°. The Moon's range at its Major Standstill is even greater - roughly 137° in Orkney versus 72° in Malta.

Without trying to suggest that people in the Neolithic had a concept of degrees, there's a very obvious difference between "obtuse" and "acute" angles.

... and there's a very obvious state that's exactly "between" these two - the right angle.

In Sweden there's a "stone boat" called Ales Stenar, which is oriented to the Winter Solstice Sunrise and Summer Solstice Sunset. It's at the latitude 55° 22′ 57″ N, which happens to be precisely where the angle between summer and winter solstice sunrise is 90°.

Ales Stenar's alignments

Work by Higginbottom on the astronomical alignments of Orcadian sites threw up some interesting observations, particularly with respect to the Stones of Stennes.

From the press release of Higginbottom's 2016 paper

There's an interest being demonstrated by the builders here in alignments to both the Moon and the Sun at their extreme positions, and the one that's drawn my attention is the Northernmost Moonset versus Winter Solstice Sunset. At this site the angle between these positions is roughly 115° in 3000 BCE.

Is it conceivable that they might have wanted to find a spot where this angle is the special one "between acute and obtuse"?

If so, then after working their way down Britain they'd have found it at the latitude of 51° 10' 44" - Stonehenge.

The Station Stone rectangle alignments

We moderns rediscovered this in the 1960s (see the article C.A. "Peter" Newham and the Station Stone Rectangle).

I realise that I am indulging in a huge amount of speculation here (but why not, it's fun!), but is it possible that a culture having a fascination with the sky and the movements of the Sun and Moon took it upon themselves to embark on the establishment of a monument at a location where they'd be able to bring the Sky to Earth with a supremely elegant design, using small locally sourced sarsen boulders to define the required rectangle?

Would they, perhaps, have brought a founding "anchor" stone from their ancestral homeland with them?

Where might they have installed it? Perhaps in line with the weird ridges and stripes in the landscape that pointed to the directions of the winter solstice sunset and summer solstice sunrise?

Perhaps where Atkinson suggested, in the stone hole immediately southwest of the later tallest trilithon?

Extract from "Stonehenge in its Landscape" Cleal et al 1995

Context for the extract from "Stonehenge in its Landscape"

Perhaps they set it upright but twisted it a bit so that its face could be sighted along in the direction of winter solstice sunrise and summer solstice sunset, while also being lit by the rising summer solstice and setting winter solstice Sun. A neat way to encode four directions with one stone.

Atkinson noted, having excavated around the Altar Stone in its present position, that one end had:

"clearly been dressed to an oblique bevelled outline"

which he felt might facilitate precise adjustment to its position when erected in its stonehole. 

They'd have had to twist it about 81° to the primary solstice axis - that being the angle between summer sunrise and winter sunrise at this latitude 5000 years ago.

If we accept Atkinson's suggestion that the Altar Stone once stood in the stonehole within WA 3639, then it's clear that it was subsequently moved to its present location - laying flat in front of the tallest trilithon (Stones 55, 56, 156 - now partly collapsed), most likely by the builders of the sarsen phase of the monument.

That new location is with its long axis directly under the secondary solstice alignment and with its centre being touched by the tip of the shadow of the Heelstone at summer solstice sunrise (see articles The Secondary Solstice Axis and The Shadow of the Heelstone)

We could imagine that relocating a stone of such pre-eminence on the site would have been controversial, even if its intended new position and orientation was chosen to retain that special significance.

The movers may have felt the need to appease their ancestors with an offering of some kind, and Atkinson found - within the primary fill of the empty stonehole, deliberately placed - just such a candidate, a large fragment of a polished Group I Cornish-provenanced greenstone axe.

In the small hollow in the stonehole to the bottom left of the ranging pole,
the polished greenstone axe fragment found by Atkinson

The polished greenstone axe fragment found in the stonehole of WA 3639 (2nd from left)

The authors (E.D. Evens et al) of the 1962 "Fourth Report of the Sub-Committee of the South-Western Group of Museums and Art Galleries on the Petrological Identification of Stone Axes" said, of the Group I samples:

"It is most closely matched by outcrops in the Mount's Bay region near Penzance, and the rock source might well be from a land surface now submerged. The possibility of locating this site by the methods of underwater archaeology should not be overlooked."

If the Stonehenge site was chosen by the Orcadians as the ideal spot to build their ultimate soli-lunar monument, and these five stones were then established (Altar Stone and Station Stones), the possibility exists for this having been the very earliest stone settings of Stonehenge - predating both the large sarsens, the small bluestones and perhaps even the surrounding earthwork bank and ditch and its associated cremation burials.

And so it could be that it was first established prior to 3100 BCE.

These five stones are sufficient to fix the Sky to the Earth.

Everything that came later simply reinforced and enhanced the original design.

Et in Orcadia, ego.

Update: September 12th 2024

In chasing down some references, I re-found Rodney Castleden's work "The Making of Stonehenge" (Routledge Press 1993, ISBN 0-415-08513-6) in which he identified the positioning of an upright Altar Stone in WA 3639 on the primary solstice axis. I felt I ought to acknowledge that he published and developed this idea first.