‘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?
