‘Once in a blue moon’ is a phrase used to express something that happens very rarely. In fact, the phenomenon is not as uncommon as the idiom implies, appearing roughly every 33 months. Rather less frequent, and much less well known, though, is another moon-related event known as a Major Lunar Standstill, which takes place every 18.6 years. While such subjects might sound more astronomical than archaeological, there is an intriguing link with one of Britain’s most famous ancient monuments. Stonehenge is best-known for its solar alignments, attracting huge crowds to witness the interaction of sun and sarsens during the summer and winter solstices. It has also been suggested, however, that some elements of its design were deliberately sited to align with the moon, specifically during a Major Lunar Standstill – and, with one such event taking place in 2024 and 2025, a team of archaeologists are hoping to shed more (moon)light on the matter.
If you got up to watch the sunrise every morning, you might notice that the point on the eastern horizon where the sun appears gradually shifts over the course of the year. This movement reaches its northern limit at the summer solstice, before heading back in the other direction and reaching its southern limit six months later at the winter solstice, before the cycle begins again. The position of the moonrise changes too, but on a much shorter cycle, moving from northern to southern extreme in around 27 days. (It is also more difficult to chart, as its timing varies dramatically and is invisible to the naked eye when it takes place in daylight.) At the same time, these northern and southern limits are themselves moving, albeit much more gradually, and the point when they are furthest apart, achieved every 18.6 years, is known as a Major Lunar Standstill.
When this occurs, the moon can be seen rising and setting much further north and south than usual, and the effect lasts for around 1-2 years at a time. Might Neolithic communities on Salisbury Plain have recognised that, once in every generation, the moon was strikingly ‘out of place’ in the sky – and might they have chosen to commemorate such observations when constructing Stonehenge?
Of moons and monuments
As mentioned above, Stonehenge is most commonly ᴀssociated with the sun. The main axis of its ‘final’ design (the stage that saw the erection of its sarsen monoliths and trilithons, taking the form we are familiar with today) is oriented on the summer solstice sunrise and the winter solstice sunset. But, like the moon, Stonehenge has pᴀssed through multiple phases, changing its appearance each time – and the monument’s earliest incarnation hints at an initial interest in lunar alignments.
Centuries before enormous uprights rose above Salisbury Plain, Stonehenge’s original design comprised a simple, circular bank and ditch. Just inside this line a ring of 56 pits – today known as the Aubrey Holes, after the antiquarian who first documented them – were dug; it has been suggested that they may have once held timber posts or small stones, possibly even representing an original setting for the bluestones that were brought to the site from west Wales and now stand at the heart of the monument. A key aspect of this early version of Stonehenge appears to have been as a cemetery, though, as cremated human remains were placed in some of the Aubrey Holes.
Analysis of these remains has indicated that they were already old at the point of their burial – suggesting that they may have been carefully curated ancestral remains – and that they had been brought from somewhere far to the west of Salisbury Plain (see CA 237 and CA 344). Given the evident importance of these burials to the builders of Stonehenge, it seems telling that the cremated remains were not placed in all of the Aubrey Holes, and there is a strong concentration in the south-east of the circle. This cluster is aligned with the most southerly rising position of the moon during a Major Lunar Standstill, and the same direction was marked with three timber posts set into the surrounding bank.
When Stonehenge took its ‘final’ form, there are hints that this lunar link continued to be significant, even as the newly installed sarsens placed a new emphasis on the movement of the sun. Just outside the main circle of the monument, four smaller stones were set in a nearly perfect rectangle. Known as the Station Stones, they are made from the same sarsen as their larger neighbours, sourced from West Woods about 30 miles to the north (CA 367), and are thought to belong to the same construction phase. Despite their modest scale, the Station Stones appear to have had an important role to play. The rectangle that they form (today, only two stones remain in their original position: the missing pair are represented by modern markers) brackets the curved arrangements of sarsens at a right angle to the monument’s main solar axis. The short side of the rectangle runs parallel to this line, pointing towards the midsummer sunrise and midwinter sunset, while the long side cuts across this orientation, pointing instead to the southernmost moonrise and the northernmost moonset during a Major Lunar Standstill.
Because the short axis of the Station Stones ties in with the solstice alignment, it is thought that they were purposefully positioned as part of the monument’s architecture. But, given this solar link, was their rectangle intended as another piece of apparatus for sighting the sun, with their lunar long axis merely a coincidental consequence of this orientation, or were they truly deliberately designed for the Major Lunar Standstill? Playing devil’s advocate, one might wonder if such infrequent events would actually have been noticed by Neolithic observers, and if they would have been able to record this information and pᴀss it down the generations in a pre-literate society. Even so, the apparent importance of this orientation to Stonehenge’s earliest use does suggest that it was seen as somehow special – and perhaps the Station Stones represent this significance being later literally ‘set in stone’.
To investigate these questions more closely, Dr Amanda Chadburn, Professor Clive Ruggles, and Dr Fabio Silva are working with English Heritage (in whose care Stonehenge lies) and the Royal Astronomical Society to explore the effects of the present Major Lunar Standstill at Stonehenge. Until the phenomenon ends in mid-2025, they will be regularly visiting the site to observe and document various moonrises and -sets at their northern and southern extremes, when the moon is in alignment with the Station Stones.
For Amanda (Universities of Oxford and Bournemouth), who worked as English Heritage’s lead advisor on Stonehenge and Avebury for more than 20 years, and Clive, Emeritus Professor of Archaeoastronomy at the University of Leicester, the lunar event represented an unmissable opportunity. They had worked together on related research since 2009, but neither could recall anyone documenting the last Major Lunar Standstill at Stonehenge. With almost 20 years until the next chance, they put in a research proposal to English Heritage. Meanwhile, Fabio (Deputy Head of Bournemouth University’s Department of Archaeology, and co-founder of the Journal of Skyscape Archaeology) was exploring the same phenomenon at Chimney Rock, a National Monument in Colorado, USA. Working with Professor Erica Ellingson from the University of Colorado, he had been studying a pair of natural stone pillars through which you can see the moon rise during a Major Lunar Standstill. The site is of long-held significance to local Native American communities, and Fabio wanted to test if similar observations could be made at Stonehenge. When the two teams realised the similarity of their proposed projects, they agreed to work together.
One of the project’s key aims is to ground-truth the theoretical sightlines proposed for the Station Stones, and to document the effect of the moon on this orientation in real time. The researchers also hope to observe how the moon interacts with the monument’s other stones at this time, and how the impact of this effect changes depending on the phase of the moon, and when moonrise or moonset occurs close to dawn or dusk, when the sky is lighter.
So far, the study has not been entirely straightforward – although in theory the researchers have the chance to watch the lunar alignment in action every fortnight, they are reliant on moonrise taking place after dark and in clear conditions – and so far they have been frequently thwarted by bad weather. One such occasion was the night after the summer solstice: this was set to be a full moon, and therefore a particularly good opportunity to appreciate the impact of a lunar extreme, so the team planned to livestream moonrise on English Heritage’s YouTube channel. Unfortunately, again the moon was obscured by clouds, but the livestream (still available online) proved very popular, featuring explanations by the researchers, English Heritage Properties Historian Dr Jennifer Wexler and Senior Historic Property Curator Dr Heather Sebire, and Professor Erica Ellingson, as well as a specially commissioned piece by Orkney musician Erland Cooper.
Happily, the team had better luck the following night. Heather Sebire describes what they saw: ‘It was absolutely spectacular; the moon did exactly what we had hoped. It came up so low in the sky that it looked absolutely huge, and shone across the Station Stones and through the monument. After it rose, the moon cast shadows inside the circle, throwing shadows from the outer circle lintels on to the giant trilithons and shining shafts of light through Stonehenge.’
