The Stonehenge Avenue Alignment and the significance of Sidbury Hill

8 miles northeast of Stonehenge is an Iron Age hillfort called Sidbury Hill that stands on the Summer Solstice Sunrise line.

Stonehenge to Sidbury Hill along the Avenue Alignment on Sir Norman Lockyer's Azimuth of 49° 34' 18"

Sidbury Hill is a prominent feature on the east of Salisbury Plain, rising to 223m above Ordnance Datum (OD) at its highest point.

The hillfort encloses about 17 acres of land around the summit and there are commanding views of the surrounding landscape, stretching north to Tan Hill and Milk Hill on the Marlborough Downs, and south beyond Salisbury (whose cathedral spire is visible on clear days). Looking southwest, back along the solstice line, the far horizon reaches to Melbury Hill, south of Shaftesbury in Dorset, almost 30 miles distant.

Intersection of the Avenue Alignment with Sidbury Hill, just south of the hillfort at a point 219m OD

Due to the tree growth on Larkhill immediately northeast of the Cursus, the far horizon is now obscured when viewed from Stonehenge. Old photos show a landscape almost entirely without trees before the 20th century, leading to the intriguing possibility that Sidbury and Stonehenge have been intervisible in the past.

Certainly the elevation profile suggests that the summit of Sidbury ought to be visible from the higher ground only a few hundred metres southwest of the monument at 109m OD on Normanton Gorse ridge.

Elevation Profile from Normanton Gorse ridge at 109m OD (extreme left of plot) to Sidbury Hill at 219m OD

Where the solstice line crosses Larkhill ridge there is a clear view across the Avon valley towards Sidbury Hill, so in 2006 I decided to see exactly where sunrise appeared at the Summer Solstice.

Blue line indicates the line from Stonehenge to Sidbury Hill. Arrow marks the position where the photos (below) were taken

Summer Solstice Sunrise out of Sidbury Hill's southern flank, taken from Larkhill on the Avenue alignment June 23 2006

The Earth's axis presently has a tilt of about 23.5°, and this tilt varies on a cycle lasting around 41,000 years. At the time that Stonehenge was built the axial tilt was 24°. This change in what's known as the "obliquity of the ecliptic" has the effect of shifting the positions of sunrise and set at the solstices.

In simple terms, 4,500 years ago the Sun would have appeared to the left of the position shown above. Imagine a gap of one Sun diameter between our modern solstice sunrise and that of 2,500BC.

Summer Solstice Sunrise position in 2,500BC - just over 1° further North than the position in our era

The calculated position of the Summer Solstice Sunrise in 2,500BC agrees reasonably well with the point where the Avenue's alignment intersects Sidbury Hill.

2006 was also the year of the Major Lunar Standstill - the point in the Moon's 18.61 year long cycle when the range of its possible rising and setting azimuths is at its greatest. At the extreme of the range the Moon can rise far further north that the Sun can, even at Summer Solstice.

I was struck by the thought that perhaps Sidbury Hill was significant for both Sun and Moon, so I returned to the same point in Larkhill on the Avenue alignment and waited for Moonrise at its northernmost position.

Maximum northerly Moonrise from Larkhill on the Avenue alignment, August 2006

The montage of the lunstice and solstice pictures is revealing. Sidbury Hill spans the gap between the extreme positions of the Sun and Moon when viewed from the Larkhill ridgeline.

Montage of northernmost Moonrise and Summer Solstice Sunrise with Sidbury Hill between

The Moon is similarly affected by the change in obliquity of the ecliptic so 4,500 years ago it, too, would appear 1° further north at its most northerly rising (to the left in the images above) than in our era.

It's important to realise that the montage view above doesn't represent the view that would be seen if the same photo was taken from Stonehenge or the Normanton Gorse ridgeline - the angle that Sidbury Hill would subtend along the horizon from Stonehenge would be much less because Stonehenge is an extra 1.78 miles distant. Summer Solstice Sunrise position relative to Sidbury Hill remains the same but the Moon's rising position shifts away.

Indeed, if you draw a line from Stonehenge in the direction of the northernmost extreme Moonrise then you miss Sidbury Hill's northerly edge by a good margin. That line intersects with a completely different archaeological feature - the Class II henge just north of Weather Hill Firs on Everleigh Down.

Stonehenge to Weather Hill Firs Henge along the maximum northerly Moonrise azimuth line

Recent discoveries at the east end of Larkhill ridge in connection with a housing development have found some very interesting archaeology.

Not only is there a 4th millennium BC, early Neolithic, causewayed enclosure (perhaps the companion to Robin Hood's Ball which lies 2.7 miles west near the other end of the same ridgeline) but there is also a mysterious "hengiform" enclosure surrounded by a ring of large postholes and a series of large pits that appear to lead away from one of the entrances of the newly found causewayed enclosure.

Perhaps significantly, this line of pits seems to point towards the northernmost Moonrise position (information from a discussion after a presentation of these findings at the Wiltshire Archaeology Conference, April 2017).

The line from Stonehenge to Weather Hill Firs Henge passes very close to this new causewayed enclosure, as the following image shows. Note that the outline of the new enclosure south of the chalk track is my speculative inference based on the size of Robin Hood's Ball, evidence for the new enclosure has only been excavated on the northeast side.

Larkhill East Causewayed Enclosure and the northernmost Moonrise azimuth line from Stonehenge (in red)

It should come as no surprise that this spot on Larkhill ridge is a significant location in this part of Salisbury Plain - it overlooks the Avon Valley to the northeast and east and the Stonehenge bowl to the southwest. The hills that frame the eastern horizon from here run from imposing Beacon Hill at the southern end to prominent Sidbury Hill at the northern.

We can only guess at what remains might have existed below ground along the rest of the Larkhill ridge before the military occupied it in the early 20th Century, initially in tents and latterly with the garrison infrastructure.

The discoveries arising from the Army Rebasing Programme (providing new homes for 4000 troops returning from Germany) have been astonishing and have the potential to shed new light on our understanding of this phenomenal landscape.

It serves - once again - to highlight just how arbitrarily drawn were the borders of the Stonehenge World Heritage Site - the northern edge of it lies along the Packway, the road running east west in the image above.