The Hunt for the Altar Stone Source - the video

The Youtube video

Altar Stone diagnostic toggle map

The Altar Stone diagnostic toggle map is an interactive screen of candidate source areas for the Stonehenge Altar Stone. Each published diagnostic is a layer that can be switched on or off, so the effect of any one test on the list of surviving candidates can be seen directly. The code and data are in the GitHub repository.

What it plots

The map covers 85 candidate places: Ba/Rb geochemical clusters across Britain and Ireland, plus named sites with published measurements. An Old Red Sandstone underlay from the BGS Geology 625k dataset shows where the outcrop lies. Fourteen layers score each place:

• L0–L9: the baseline screen, covering barium and Ba/Rb geochemistry, mineralogy, zircon ages, thermal maturity and related published criteria.
• L10 and L11: a strontium ceiling and the Sr/Ca ratio, applied only where rock Sr has been measured.
• U1 and U2: chrome spinel (CZi) and illite/smectite expandability, from Pirrie et al. (2026). Both start off.

How to read it

A place is a match only if it fails none of the layers switched on. Each layer has strict and relaxed settings where the evidence allows. Untested is not failed: a place with no data for a layer stays open on that layer. Presets switch one test or group off at a time (leave-one-out), which shows how much each test contributes, and the preset "CZi + I/S ON (Pirrie et al. 2026)" applies the new chrome spinel and clay results.

Several things are deliberately not scored: the ORS between the Midland Valley and the Welsh Borders, the Lanark Basin, Carboniferous sandstones such as the Clifton Rock, and the Sr–Nd isotope result, because the evidence is missing or has no comparison data. The Latheron–Niandt cool sliver is drawn as schematic only.

Limits

The map is a screening tool. A place that survives is untested or consistent with the evidence so far, not identified as the source, and exclusion by elimination covers only the ground that has been sampled. Corrections and additional data are welcome.

The Pirrie, Bevins, Ixer et al paper that prompted the latest update is discussed in a separate post.

Paper: Pirrie, D., Bevins, R.E., Ixer, R.A., Pearce, N.J.G., Turner, P., Power, M.R. et al. 2026. Searching for the origin of the Stonehenge Altar Stone: new sedimentological, petrographical, mineralogical, geochemical and isotopic constraints. Journal of the Geological Society 183, jgs2026-090. doi:10.1144/jgs2026-090

Searching for the origin of the Stonehenge Altar Stone

A paper now published in the Journal of the Geological Society reopens the question of where the Stonehenge Altar Stone came from. Pirrie, Bevins, Ixer and ten further co-authors give the most detailed description of the stone to date: sedimentology, petrography, heavy minerals, clays, geochemistry and isotopes. Their conclusion is that no outcrop of Old Red Sandstone (ORS) examined so far matches it on every count, and that its provenance "has yet to be determined with confidence".

Paper: Pirrie, D., Bevins, R.E., Ixer, R.A., Pearce, N.J.G., Turner, P., Power, M.R. et al. 2026. Searching for the origin of the Stonehenge Altar Stone: new sedimentological, petrographical, mineralogical, geochemical and isotopic constraints. Journal of the Geological Society 183, jgs2026-090. doi:10.1144/jgs2026-090
Data: figshare doi:10.6084/m9.figshare.33993488.v1

Fig. 1. (a) Map of the UK showing the location of Stonehenge and the distribution of non- marine sediments of the Old Red Sandstone. (b) Plan showing the position of the Altar Stone within the Stonehenge monument (c) Photograph looking towards the SW; the Altar Stone (arrowed AS) is lying mostly underneath the two large sarsen stones. (c)
Source: photograph by Adam Stanford.

Where the question stood

The Altar Stone is a block of about six tonnes (4.9 × 1 × 0.5 m) lying flat beneath fallen sarsens 55b and 156. For a century it was assigned to the Cosheston beds of West Wales, following H.H. Thomas in 1923. Bevins and colleagues refuted that in 2020, and in 2023 they excluded the Anglo-Welsh Basin as a whole: of 58 samples, all but four failed on geochemistry and the remaining four on mineralogy. In 2024 Clarke et al. (Nature) dated zircon, apatite and rutile grains from Altar Stone debitage and proposed the Orcadian Basin of NE Scotland. Clarke et al. (2026) later suggested Caithness or the Inverness–Black Isle area as the most likely sources.

What the new paper adds

The study combines previously published in-situ pXRF on the three exposed areas of the stone with new laboratory work on three museum fragments (MS1–MS3): automated SEM-EDS mineralogy, X-ray powder diffraction of bulk rock and clay fractions, cathodoluminescence, C–O isotopes and colour. Sr–Nd isotopes were measured on MS1 and MS2. The characteristics that emerge are:

• A bedding plane. Low-angle illumination in 2024 showed climbing ripples and slumped foresets on the exposed surface. If it is a bedding plane, the bed was at least 0.5 m thick and extended at least 4.9 m. There is no cleavage, and no petrographic sign of palaeosols or near-surface soil formation.
• Chrome spinel. The chrome spinel:zircon index (CZi) is 35.9–38.6 in the three fragments. In ORS comparison data it averages 1.3 on Orkney (n = 35), 1.4 on the Tarbat peninsula, Inner Moray Firth (n = 42), 0.4 on Foula and Shetland (n = 13) and 0.2 in the Lower ORS of Pembrokeshire (n = 32).
• Clay minerals. The clay fraction contains illite, mixed-layer illite/smectite, kaolinite, chlorite and tosudite. The illite/smectite is about 20% expandable in all three fragments. Expandability falls with burial and heating, and the ORS of Caithness and Orkney is below 10%, which excludes them. The clays alone would not exclude parts of the Moray Firth.
• Barium. Dispersed baryte cement gives pXRF Ba of about 1000–5500 ppm (mean 2758 ppm), linearly related to Sr. This separates the stone from ORS analyses in Wales and Orkney.
• Sr–Nd isotopes. ⁸⁷Sr/⁸⁶Sr is 0.7163–0.7169 and ¹⁴³Nd/¹⁴⁴Nd 0.5119–0.5121. Published Carboniferous sandstones (Leng et al. 1999) have either higher Nd or higher Sr isotope ratios, so there is little overlap. The Nd range of the ORS overlaps closely, but no ORS Sr isotope data exist, so the comparison cannot be taken further. The authors conclude that an ORS source is more likely than a Carboniferous one.
• Diagenesis. Pore-filling kaolinite pre-dates significant compaction; baryte post-dates compaction; both pre-date the dominant poikilotopic burial calcite cement.

What is excluded

• West Wales and the Anglo-Welsh Basin: excluded on earlier geochemistry and mineralogy, and in this paper by the absence of chrome spinel in West and South Wales and Somerset. Welsh Borderland tosudite occurrences are excluded on Ba.
• Orkney and Caithness: excluded on clay expandability. Chrome spinel is also scarce in the Orkney, Tarbat and Shetland samples; Caithness itself was not sampled for it.
• Moray Firth / Tarbat: compatible on clay, excluded on chrome spinel.
• Midland Valley ORS: excluded on abundant lithic grains (Phillips 2007). In the NE Midland Valley, the Stonehaven and Arbuthnott groups have too little I/S expandability, and the zircon populations of the Dunnottar–Crawton to Strathmore groups differ. The Strathmore Group clays are comparable, but its zircons and the texture of its tosudite are not.
• Southern Uplands: chrome spinel is common in the Lower Palaeozoic sandstones, but the Altar Stone contains kaolinite, which indicates it has not undergone low-grade metamorphism.
• Late Carboniferous–Triassic of south-central Britain: no Variscan-age zircons in the Altar Stone.

What remains open

• The ORS between the Midland Valley and the Welsh Borders, where modern heavy-mineral and zircon data are virtually absent.
• Areas with Ba anomalies in stream-sediment geochemistry, including the Lanark Basin and the Fintona Basin of Northern Ireland. The paper reports no chrome spinel, zircon or clay data for the Lanark Basin.
• Carboniferous sandstones recycled from Caledonide sources, such as the chrome-spinel-rich Clifton Rock of the Pennine Basin. Its zircon pattern differs (a large Early Palaeozoic peak, no Neoproterozoic group) and the Sr–Nd data favour the ORS, but the authors do not exclude it.

The diagnostic toggle map

These tests can be switched on and off in my Altar Stone diagnostic toggle map. Nothing to do with the paper or authors, I have just updated it with their data.  Each test is a layer: Ba/Rb screening, K-feldspar, zircon, clay and thermal maturity, strontium (L10, L11), and the chrome spinel (U1) and clay expandability (U2) results of Pirrie et al. The Pirrie layers start off; the preset "CZi + I/S ON (Pirrie et al. 2026)" switches both on. A place survives only if it fails none of the layers switched on, and untested is not the same as failed. Leave-one-out presets show how much each test contributes. The areas listed above as open, and the Sr–Nd result, are not scored on the map.

Relation to the NE Scotland proposal

The paper accepts the zircon, rutile and apatite ages of Clarke et al. and a Laurentian source for the heavy minerals. The argument is that the same age pattern can be shared by several basins, so ages alone cannot identify an outcrop, and that once chrome spinel, clays and barium are added the Orcadian candidates fail. Bevins and Ixer co-authored the 2024 Nature paper, so this is in part a team revising its earlier interpretation. The authors also regard the glauconite reported in one fragment by Clarke et al. (2026) as probably a misinterpretation of SEM-EDS spectra, and caution against relying on a single youngest zircon age.

Caveats

• Three fragments. The new analyses are on three small museum fragments, and the Sr–Nd data on two. Their attribution to the Altar Stone rests on earlier pXRF and petrographic comparison, because the stone itself cannot be sampled.
• Comparison data. The Pembrokeshire CZi value is from unpublished data, and the Orkney, Tarbat and Shetland values come from earlier compilations.
• Elimination, not identification. "No match found" covers only the ground that has been sampled. Large areas of ORS have no modern heavy-mineral or zircon data.
• Coarse Ba screening. National stream-sediment Ba maps are low resolution and can be distorted by historic mining.
• Sr/Ca. Surface pXRF Ca on the in-situ stone is lower than on debitage, which the authors attribute to weathering of the calcite cement. Sr/Ca from the surface pXRF data is therefore biased upward and should be treated with caution.

Why it matters

The origin of the Altar Stone determines the journey and the contacts it implies. A Moray Firth or Caithness source suggests a long coastal voyage; a Lanark Basin source, routes via the Clyde and the Irish Sea; Fintona, a crossing from Ulster. Until a source is identified, the origin is a variable in route modelling, not a fixed point.

The characteristics reported in the paper define the tests a candidate source must pass: high Ba with baryte cement, CZi of about 36–39, illite/smectite about 20% expandable with tosudite, plagioclase far exceeding K-feldspar, and an ORS-type Nd isotope signature. Those are the tests for the next field campaign.

Figures and short quotations are from Pirrie et al. (2026); see the paper and its supplementary tables for the data.

Four Young Officers, Stonehenge 1938

This post is the companion to the short film Four Young Officers, made for Remembrance Sunday 2026: the four Royal Artillery subalterns who painted Stonehenge green in June 1938, and what became of them.

The night of 16 June 1938

By the summer of 1938 the four were serving with anti-aircraft and coast-defence units — Sherrard and Skinner at the School of Anti-Aircraft Defence, Biggin Hill; Peirce with the 22nd Anti-Aircraft Battery at the Royal Artillery experimental camp at Watchet; and Shearme at the Coast Artillery School, Shoeburyness — and had gathered at the School of Artillery, Larkhill, three miles north of Stonehenge, for a gunnery course. On the night of Thursday 16 June, after a mess guest night, they took green line-marking paint from the Larkhill tennis courts and daubed the monument, several of the upright sarsens and the Heel Stone among them, together with road signs nearby. The damage was found on Saturday 18 June. The Times of 20 June reported that Stonehenge “was found on Saturday to have been daubed with green paint” and that “signposts in the vicinity were similarly daubed”. Mike Pitts assembled the fuller sequence from the contemporary press.

Summons and sentence

The four were identified and summoned. The Times reported in July that “four Army officers were summoned for damaging Stonehenge on June 16” and that the hearing “was adjourned until August 9”. On 9 August they pleaded guilty at Salisbury County Petty Sessions. Each was fined £1 and ordered to contribute towards the repair and costs, and the General Officer Commanding, 3rd Division, admonished them. The American weekly TIME, reporting the case on 22 August under the heading “Druidical Sacrilege”, quoted the prosecutor's warning that “It may take 1,000 years for Stonehenge to regain its old weathered appearance”.

Four young officers

They had been commissioned as second lieutenants in the Royal Regiment of Artillery from the Royal Military Academy, with seniority dated 27 January 1938 (London Gazette 34477, p. 587). The Gazette gives the surnames as Peirce and Shearme, where the 1938 newspapers printed Pierce and Shearne:

• William Laurence Sherrard
• William Howard Skinner
• John Edward Passingham Peirce
• John Lambert Shearme

They stood among fifty-two gunner cadets in a single passing-out list. Sixteenth on it was Robin Horace Walford Dunn, winner of the Academy's Sword of Honour, who would be evacuated from Dunkirk, fight in the Western Desert and Normandy, be wounded three times and appointed MC, and in civilian life become a Lord Justice of Appeal; his memoir Sword and Wig records the war into which the cohort was commissioned. Dunn and Sherrard had both come to Woolwich from Wellington College.

What became of them

Captain William Laurence Sherrard served with 12 Battery, 6th Heavy Anti-Aircraft Regiment, Royal Artillery. In 1942 the regiment was among the anti-aircraft units defending the oil refineries at Palembang, in southern Sumatra. On 14 February 1942 Japanese paratroops were dropped on the refineries and the nearby airfield, and the gunners, the only regular troops in the area, fought them at close range. Sherrard was killed that day at the Pladjoe refinery. He was 24. He was mentioned in despatches, is buried in Jakarta War Cemetery, and is named on the war memorial at Ditchling, Sussex, where his parents lived. His batman wrote to his mother that his son “gave his life at a place called Pladjoe … taking on odds of fifty to one” (Ditchling History Project).

The other three survived the war. The London Gazette records that Peirce, Shearme and Skinner were all promoted major on 27 January 1951, thirteen years to the day after their commission (London Gazette 39132). Shearme was placed on retired pay on 29 September 1958 (41508). Peirce rose to lieutenant-colonel and was appointed OBE in the New Year Honours of 1962 (42552, 1 January 1962). Their wartime postings have not yet been traced.

Notes on the film

The newspaper reports are retyped in the film rather than shown. The photographs of the Heel Stone and the 1938 snapshot (photographer unknown) are from the author's collection; the map of Sumatra is drawn from the NOAA GLOBE land mask. The music is original.

Sources

The London Gazette, issue 34477, 28 January 1938, p. 587 (commission); issue 39132, 23 January 1951 (promotions to major); issue 41508, 30 September 1958 (Shearme's retirement); issue 42552, 1 January 1962 (Peirce OBE).

The Times, 20 June 1938, p. 16, “Stonehenge daubed with green paint”; The Times, July 1938, “Officers charged with Stonehenge damage”.

TIME, 22 August 1938, “Foreign News: Druidical Sacrilege”. time.com

“A night out” (London, 11 August 1938), Waipawa Mail, 14 September 1938, p. 1. Papers Past

Commonwealth War Graves Commission, casualty record of Captain W. L. Sherrard, 12 Bty, 6 HAA Regt RA.

Ditchling History Project, “For the Fallen”. ditchlinghistoryproject.org

Dunn, R. 1993. Sword and Wig: Memoirs of a Lord Justice. Quiller Press. (Dunn's place in the commission list: London Gazette 34477, p. 587.)

Moody, F. 2024. Bygone Salisbury: History of protest at Stonehenge. Salisbury Journal, June. salisburyjournal.co.uk

Pitts, M. 2016. How the Magpie Musicians came to stand for free Stonehenge. Digging Deeper, 5 April. mikepitts.wordpress.com

LiDAR Lens V.2

 

Durrington Walls and Woodhenge through the lens, with the What's here? card showing the nearby records and photos.

I have been building LiDAR Lens for a while now, starting with Wiltshire. It now covers the whole of England and Wales, and it is live at lidarlens.sarsen.org. It is free, there is no login, and it works on a phone. The code is open source on GitHub, so you check it and build your own.

What it does

A circular lens slides over ordinary aerial imagery. Inside the lens you see the bare ground from LiDAR, with the trees and buildings taken away, so banks, ditches, barrows and old field systems stand out. You can switch between relief views such as hillshade and sky-view (on a phone these are the mode buttons 1 to 8), turn the sun round and change its height, add vertical exaggeration, pick a colour ramp, or let auto-contrast do the work.

You can search by place name, postcode or grid reference, or start typing the name of one of about 24,000 scheduled monuments from Historic England and Cadw and pick it from the list.

What's here?

Click anywhere and the What's here? card shows:

• nearby Historic England or Cadw records;
• a measurement of the shape from the LiDAR: its size, height, profile and slope;
• nearby Geograph photos, with their photographers credited;
• a Look here up button, which opens Google AI Mode to ask about the archaeology and history of the spot.

There is also an elevation profile tool, and every view has a link you can share, so someone else sees exactly what you are looking at.

One caution: the measurements come from the terrain model, not from anyone standing in the field. They tell you the shape of the ground, not what it is.

The data

England uses the Environment Agency LiDAR Composite DTM at 1 m, and Wales uses Welsh Government LiDAR, both under the Open Government Licence. My thanks to both for making it open.

County lenses

The county versions are still there too, including the Wiltshire LiDAR Lens, along with others.

The code is open source on GitHub. If you find something interesting, or something that does not work, do let me know in the comments.

On the knee: Stoker Moody and the Portsmouth Barracks riot, 1906

Not the usual fare here, but it's an incident I wanted to research.This post is the companion to the short film On the knee, on the riot at the Royal Naval Barracks, Portsmouth, in November 1906. The sources, quotations and caveats follow.

The navy in 1906

In 1906 the Royal Navy was the largest in the world, held by policy to the Two-Power Standard: a fleet equal to the next two navies combined. Its last great fleet action, Trafalgar, was a century past. Germany's Navy Laws of 1898 and 1900 had begun a battle fleet aimed at Britain, and a further law in 1906 added six large cruisers (Massie 1991). On 10 February 1906 Edward VII launched HMS Dreadnought at Portsmouth Dockyard: a turbine battleship whose eighteen boilers, like those of every ship in the fleet, burned coal shovelled by hand (Roberts 1992). The men who fed them were stokers.

HMS Dreadnought, c. 1906–07. US Navy Historical Center NH 63367.

 

The stokehold of a battleship. C. J. Staniland, before 1916.

Sunday 4 November

At evening quarters some 390 stokers were kept standing longer than usual in cold weather; when a heavy shower came on, some broke ranks and ran for cover. All were mustered again in the gymnasium, where the senior gunnery officer, Lieutenant Bernard St George Collard, ordered them "On the knee", a command used in gunnery training but unfamiliar to stokers. Many refused and shouted "no", and went down only when the order was repeated with a threat of punishment (Carew). The Admiralty later listed among the causes of that night's disorder "a feeling of resentment on the part of the stokers caused by the misuse of the drill order 'On the knee'" and "the retention of the stokers on parade on the Sunday afternoon during rain" (Hansard, 17 December 1906).

A year earlier

In November 1905 Collard had ordered a single stoker, Acton, on the knee for answering "Here" rather than "Here, sir" (Carew). On 28 November 1906 Hugh Lea MP asked the Secretary to the Admiralty about "the allegation that Lieutenant Collard on one occasion addressed a stoker with the words, 'Down on the knee, you dog'". Collard denied using abusive language, and his court martial accepted the denial (Carew).

Monday 5 November

The gate of the Royal Naval Barracks, Portsmouth. Postcard, c. 1903–10.

Disorder resumed in the barracks the next evening. Outside, a crowd of over a thousand, locals and former sailors among them, gathered at the locked gates, threw stones and broke windows; a hundred armed Royal Marines were called out from Eastney Barracks (Carew). The Admiralty Minute calls it "a contemporaneous riot on the part of civilians and other persons" outside the barracks.

The courts martial

Eleven stokers were court-martialled. Edwin Moody, Stoker First Class, was sentenced on 26 November to five years' penal servitude (Hansard, 28 November 1906). The Secretary to the Admiralty told the Commons that "Stoker Moody was not charged with refusing to obey the 'On the knee' order, but with having on two occasions incited others to take part in disturbances." Moody's written statement presented him as little more than a spectator; the court preferred a report that he had urged stokers to "stick together" (Carew). Of the others, Day received 18 months, McMichael and Leigh nine months, Fachney and Brown six, Colclough and Beddard three, and Reddish 42 days; two were acquitted (Carew).

The sentences as passed, bars to scale.

Collard was court-martialled too. The Admiralty Minute records that he was "convicted of an act to the prejudice of good order and naval discipline, and the sentence of reprimand has been noted."

The Admiralty's verdict

The Minute, read in the Commons on 17 December 1906, relieved Commodore Stopford of his appointment, superseded Commander Drury-Lowe, and relieved Commander Mitchell "by allowing a drill order to be used for other than drill purposes". It directed that "the drill order 'On the knee' is not to be used for other than drill purposes", and reduced Moody's sentence "from five years to three years' penal servitude". Asked in February 1907 whether the sentences would be reconsidered, the Secretary to the Admiralty had "nothing to add".

The outcry

The affair, in Carew's words, dominated the press at home and abroad. In Parliament it came up at least 22 times between 8 November 1906 and 18 February 1907: 21 questions and statements in the Commons and one in the Lords, found by reading the Hansard index for every sitting day of those months. The Humanitarian League protested at the severity of Moody's sentence, and protest meetings were held (Carew). On Sunday 20 January 1907, between one and two thousand people gathered in Trafalgar Square under the banners of the Social Democratic Federation, hoisted on the plinth of the Nelson monument. The Evening Express of Cardiff reported that the speakers "intended to 'let the Lords Commissioners of the Admiralty know that they would not have the stokers of the Navy go 'on the knee' to mortal man.'"

Evening Express (Cardiff), 21 January 1907. National Library of Wales, Welsh Newspapers Online.

What lasted

With the order confined to drill, "to all intents and purposes it now disappeared" (Carew). The wider campaign against degrading summary punishment, led by Lionel Yexley's lower-deck paper The Fleet, bore fruit in 1912: a committee under Rear-Admiral Brock, set up by Churchill as First Lord, led to the end of the "10 A" routine of standing facing the bulkhead and eating under a sentry, to cells being limited to three days, to officers below the rank of Commander being barred from awarding heavy punishment without approval, and to a copy of King's Regulations being carried for ratings to read. 1912 was the first year in which summary punishments were fewer than the men of the lower deck (Carew).

Royal Navy recruiting poster, "Stokers Required", September 1913. Imperial War Museums, Art.IWM PST 0805.

In the summer of 1933 Edwin Moody, out of work, knocked on the door of a retired naval officer, Lieutenant Commander Charles Drage, near Kensington Palace, looking for a job (Carew).

Notes on the film

Hansard text is shown as published, reset in type, with omissions marked. Moody's defence, the crowd figures and the eight lesser sentences come from Anthony Carew's history, which draws on the court-martial record; the court-martial papers themselves were not consulted. Carew states that Collard's reprimand was for the 1905 punishment of Acton and that no penalty followed for the order in the gym; that is not shown in the film until a primary source confirms it. Hansard gives Moody's sentence as penal servitude; Carew describes all the sentences as with hard labour.

Sources

• House of Commons Debates, 28 November 1906, vol. 166 c35, "The Disturbances at Portsmouth". Historic Hansard.
• House of Commons Debates, 17 December 1906, vol. 167 cc1008–10, "The Disturbances at Portsmouth — Admiralty Minute". Historic Hansard.
• House of Commons Debates, 18 February 1907, vol. 169 cc522–3, "Sentence on Stoker Moody". Historic Hansard.
• Evening Express (Cardiff), 21 January 1907 (third edition), "'On the Knee, Dog!' Lurid speeches under Nelson's statue". Welsh Newspapers Online.
• Carew, A., "The Stoker, the Gunnery Lieutenant and Issues of Naval Discipline around the 1906 Portsmouth Barracks Mutiny". anthonycarew.org.
• Massie, R. K. 1991. Dreadnought: Britain, Germany and the Coming of the Great War. New York: Random House.
• Roberts, J. 1992. The Battleship Dreadnought. London: Conway Maritime.

Contains Parliamentary information licensed under the Open Parliament Licence v3.0.

Has Stonehenge drifted off its line?

This post is the companion to the short film Has Stonehenge drifted off its line? Stonehenge was laid out on the solstice axis about 2500 BC. Three slow movements could have shifted that line since: the wobble of the Earth's axis, the easing of its tilt, and the drift of the ground itself. Only one of them has moved the solstice sunrise by a measurable amount. The numbers, methods and caveats follow.

The question

The main axis of Stonehenge runs from the midsummer sunrise in the north-east to the midwinter sunset in the south-west, at 49.95° true (49.81° on the National Grid; Thom et al. 1974). The sarsen structure is taken here as built about 2500 BC. The question is whether anything has since moved the line between the stones and the Sun.

Stonehenge in 2500 BC (red) and today, about 100 m to the north-east. The two axes are parallel. Plan from T. Daw's 3D model.

1. Precession: the stars change, the sunrise does not

The Earth's axis wobbles like a spinning top, tracing a cone about once in 25,800 years. The north pole of the sky moves with it. Thuban, in Draco, "passed within 0.1° of the pole in 2800 BC" (Rawlins & Pickering 2001); today the pole lies near Polaris. Using the long-term precession model of Vondrák, Capitaine & Wallace (2011) and Thuban's J2000 position (proper motion ignored), the closest approach comes out at 0.05° in 2791 BC, consistent with that figure.

Precession changes which stars stand behind the Sun at the solstice, and the calendar date of the solstice. It does not change where the solstice Sun rises. At a solstice the Sun's declination equals the obliquity, plus or minus, and the rising azimuth depends only on declination, latitude, the altitude of the horizon and refraction (Meeus 1998; Ruggles 1999).

The path of the north pole of the sky from 2800 BC to 2026 (gold), on its 25,800-year circle (dotted). Vondrák et al. 2011.

2. The tilt: the sunrise has moved about one degree

The obliquity, the tilt of the Earth's axis to its orbit, has fallen from 23.973° in 2500 BC to 23.436° today (Vondrák et al. 2011, through the ERFA library; Laskar 1986 gives 23.974° for 2500 BC). That is a change of 0.537°. Taking 2700 BC or 2300 BC instead gives 23.992° or 23.954°.

Where the Sun appears on the skyline also depends on how high the skyline is. The horizon was computed from the Environment Agency's 1 m LiDAR terrain model (5 m beyond 2 km), from the centre of the circle (51.1789° N, 1.8262° W) with the eye 1.5 m above the ground, allowing for the curvature of the Earth and terrestrial refraction. Stones and trees are ignored. To the north-east the skyline is a ridge about 2.7 km away, 0.57° high at the 2500 BC sunrise and 0.60° at today's. To the south-west it lies about 4.3 km away, at 0.46–0.49°.

With Bennett's (1982) formula for refraction and a solar semi-diameter of 0.267°:

• Midsummer sunrise, first gleam: 49.33° in 2500 BC, 50.42° today, a shift of 1.09°. With the half disc showing, 49.79° to 50.86°; with the whole disc risen, 50.25° to 51.30°.
• Midwinter sunset, last gleam: 230.02° in 2500 BC, 231.08° today, a shift of 1.06°.

The Sun is 0.53° across, so the sunrise has moved about two widths of the Sun. Moving the date of construction 200 years either way changes these azimuths by less than 0.05°. Refraction near the horizon varies with the weather by a few tenths of a degree; that moves both dates alike and leaves the shift unchanged.

The north-east skyline from the centre of Stonehenge, from Environment Agency LiDAR, with the first gleam of the midsummer Sun in 2500 BC and 2026. Vertical and horizontal scales equal.

3. The ground: Rujm el-Hiri

The third movement is the ground itself. Rujm el-Hiri is a large stone ring in the Golan Heights, about 150 m across, with a central cairn. Aveni & Mizrachi (1998) proposed that its layout held solstice and other sky alignments. Khabarova, Birkenfeld & Eppelbaum (2024) argued from geomagnetic evidence that the whole region has rotated, and that this undermines the observatory interpretation. Their figure (section 3):

"Assuming the average rotation velocity of 10 mm per year and the age of the studied prehistoric site as 4000 years, we will receive an arc rotation of about 40 m. The corresponding direct movement can be estimated as 30 m to its today's position."

The paper gives a distance. It gives no angle through which the site has turned.

The Sea of Galilee and Rujm el-Hiri. Lake outline: Natural Earth.

The rings of Rujm el-Hiri (schematic) and the paper's 30 m shift, drawn to the same scale.

A shift is not a turn

The Sun is so far away that its light arrives in parallel lines. Sliding a monument sideways, by 30 m or 30 km, leaves every bearing within it unchanged: a line that pointed at the solstice sunrise still does. To spoil a solar alignment, the ground has to turn, and by something like the width of the Sun, about 0.5°.

No rotation angle is published for Rujm el-Hiri. As an order of magnitude only: fast block rotations measured from palaeomagnetism run at 1–20° per million years, which over 4,500 years gives 0.005–0.09°. That is well short of a Sun-width. This bound is ours, not the paper's.

Diagram: a monument slid sideways still points at sunrise; turned, it does not. Movement and angles exaggerated.

Stonehenge on the Eurasian plate

Stonehenge sits on the Eurasian plate, which rotates slowly about a pole in North America. Evaluating the plate's rotation at the monument:

• ITRF2014 plate motion model (Altamimi et al. 2017; Eurasia ω = −0.085, −0.531, 0.770 mas a year): 16.5 mm a year north and 16.5 mm a year east, so 23.4 mm a year on a bearing of 045°. Over the 4,525 years since 2500 BC that is 106 m: 75 m north and 75 m east.
• NNR-MORVEL56 (Argus, Gordon & DeMets 2011; pole 48.85° N, 106.50° W, 0.223 ± 0.009° per million years): 21.7 mm a year on 043°, 98 m since 2500 BC.

These speeds are measured against the average motion of all the plates (the no-net-rotation frame), which is a convention. Relative to Africa or North America the figures differ; the rotation of the ground under Stonehenge, which is what matters for an alignment, does not depend on the frame chosen.

The Eurasian plate (shaded) and its boundaries, PB2002 (Bird 2003), with Stonehenge's motion. Inset: the monument's 106 m since 2500 BC, to scale.

Has Stonehenge turned?

The component of the plate's rotation about the local vertical at Stonehenge is 2.5 arcseconds anticlockwise since 2500 BC (ITRF2014; 1.8 arcseconds with NNR-MORVEL56), or 0.0007°. Moving 75 m north also raises the latitude by 0.00067°, which moves the solstice sunrise by 2.5 arcseconds, also anticlockwise. The monument and the sunrise have turned the same way by the same amount, so the alignment between them has changed by well under an arcsecond.

A larger term is polar motion: the slow drift of the spin axis relative to the whole crust. At today's secular rate of about 3.5–4 milliarcseconds a year, much of it from rebound after the last ice age and not necessarily constant over 4,500 years, it amounts to roughly 16–18 arcseconds of pole shift, of order 0.005°.

Taking everything together, the ground under Stonehenge has turned by less than a hundredth of a degree against the sky. The easing of the tilt has moved the solstice sunrise about a hundred times further. Stonehenge has moved about 100 m, but its solstice line has not turned.

Change in the solstice sunrise since 2500 BC: precession 0°, tilt about 1°, drift under 0.01°.

Corrections to the 2025 post

The post of 8 January 2025, "The tectonic movement of Stonehenge", reached the right conclusion: continental drift has not changed Stonehenge's solstice alignment by any measurable amount. Four of its figures need correcting:

• "approximately 25 millimeters per year": about 22–23 mm a year (21.7–23.4 mm, depending on the model). "About 2 cm a year" stands.
• "125 meters northward" in 5,000 years: about 100 m north-east since 2500 BC, of which about 75 m is northward. Over a full 5,000 years the total would be about 115 m.
• "rotating clockwise": the Eurasian plate rotates anticlockwise about its pole in North America; at Stonehenge, about 2.5 arcseconds anticlockwise since 2500 BC.
• "one-thousandth of a degree of latitude": 0.00067°.

Notes on the film

Every number in the film was computed for it from open data, with the scripts in the film's folder: the horizon from Environment Agency LiDAR, the Sun's positions with the ERFA implementation of Vondrák et al. (2011), the plate motion from the published rotation poles, and the path of the celestial pole. Published horizon profiles were used only as a cross-check. The plan of Stonehenge is from T. Daw's 3D model; the rings of Rujm el-Hiri are a schematic; maps use Natural Earth and the PB2002 plate boundaries. The film quotes only the 2024 paper's own figures for Rujm el-Hiri. Music: original.

Sources

• Altamimi, Z., Métivier, L., Rebischung, P., Rouby, H. & Collilieux, X. 2017. ITRF2014 plate motion model. Geophysical Journal International 209, 1906–1912. doi:10.1093/gji/ggx136
• Argus, D. F., Gordon, R. G. & DeMets, C. 2011. Geologically current motion of 56 plates relative to the no-net-rotation reference frame. Geochemistry, Geophysics, Geosystems 12, Q11001. doi:10.1029/2011GC003751
• Aveni, A. & Mizrachi, Y. 1998. The geometry and astronomy of Rujm el-Hiri, a megalithic site in the southern Levant. Journal of Field Archaeology 25, 475–496. doi:10.1179/009346998792005261
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• Khabarova, O., Birkenfeld, M. & Eppelbaum, L. V. 2024. Discussion points of the remote sensing study and integrated analysis of the archaeological landscape of Rujm el-Hiri. Remote Sensing 16, 4239. doi:10.3390/rs16224239
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