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.