The complex stone monument at Calanais is world-famous for marking an extraordinary natural phenomenon that happens every 18.6 years, at the time of the moon’s Major Standstill, when its rising and setting positions are at their southernmost point and furthest apart.

At this time, when viewed from the northern end of the Avenue at Calanais, the full moon’s path from rising to setting means that it appears to skim across the horizon (which is shaped like a woman lying on her back – the ‘Cailleach Na Mointeach’, or ‘Old Woman of the Moors’), then disappear from view briefly before shining into the centre of the circle at the southern end of the Calanais monument as it sets.

It is believed that the design of the monument at Calanais was deliberately changed, probably around 2500 BC, to re-orientate the monument so that it was aligned on this remarkable lunar event. Before that, it had existed as a free-standing stone circle with a tall central stone, built around 2900 BC, whose orientation is believed to have been concerned principally with marking winter solstice (i.e. an event connected with the sun, not the moon).

There is evidence for a small chamber tomb being constructed inside the stone circle around 2500 BC, and it may well be that that the northern Avenue and perhaps also the East, West and South rows were added around this time. It was the construction of the Avenue that directed people’s gaze and movement towards the position of the Major Lunar Standstill.

The last time the moon’s Major Standstill was marked was 2006, and it is due to happen again in 2025.

Please note the following dates (all British Summer Time):

12 June                               Moonrise 00:01   Moonset 04:00           Moon’s Declination    -29° 18´

9-10 July                             Moonrise 21:30   Moonset 02:53           Moon’s Declination    -29° 17´

5-6 August                          Moonrise 20:39    Moonset 00:36          Moon’s Declination    -29° 21´

1 Sep                                  Moonrise 18:30    Moonset 22:21          Moon’s Declination    -29° 31´

The phenomenon will also occur at different times in 2026, and to a lesser extent in 2027, and 2028, as the orbital plane of the moon gets higher each month.

View of The Major Lunar Standstill Season is Here! (equinoxpub.com)

It is a film made 19 years ago! I did not find a film of this year! I asked, but there was no answer.