The major lunar standstill: the extreme Moon every 18.6 years

The Sun has its solstices, the moments of the year when it rises and sets at the most extreme points of the horizon. The Moon has its extremes too, but they follow a much slower and less familiar rhythm, completed in almost nineteen years. This phenomenon, called a lunar standstill, was rediscovered by twentieth-century archaeoastronomy and linked, not without debate, to some of the most famous prehistoric monuments.

A Moon that swings

Like the Sun, the Moon rises and sets at points on the horizon that change during its cycle. The Sun reaches its extremes at the solstices, once a year; the Moon reaches them every month, climbing high in the sky and then sinking low over about two weeks. The amplitude of this monthly swing, however, is not constant: it varies with the 18.6-year cycle of the lunar nodes.

When the ascending node coincides with the point of the spring equinox, the tilt of the lunar orbit adds to that of the Earth's axis, and the Moon's declination can reach about 28.6 degrees north and south of the celestial equator. This is the major standstill. Nine years and a few months later, when the two tilts subtract, the range shrinks to about 18.3 degrees: this is the minor standstill.

The standstill of 2024-2025

The major standstill is not a precise instant but a period of a year or two in which the Moon reaches extremes close to the maximum. The last one occurred between 2024 and 2025, and the next will fall around 2043. In those months the Moon, at our latitudes, could climb very high in the sky and, two weeks later, remain very low over the southern horizon, rising and setting at points unusually far to the north or to the south.

Stones and horizons

Several prehistoric monuments have been linked to the lunar standstill. In the Outer Hebrides, at the megalithic complex of Callanish on the Isle of Lewis, the Moon at major standstill seems to skim the southern horizon along a line of hills, a phenomenon observed and discussed by several scholars. At Stonehenge some researchers have proposed that the four Station Stones form a rectangle aligned with the lunar extremes, and between 2024 and 2025 observations were carried out precisely to test this.

In the United States, at Chimney Rock in Colorado, the Moon at major standstill rises between two rock pinnacles visible from a building constructed by the Ancestral Puebloans. According to some scholars, the construction dates, derived from tree rings, coincide with major standstills of the eleventh century, but the interpretation remains a matter of debate.

A memory as long as a generation

In the absence of written documents it is hard to establish what meaning prehistoric communities gave to the standstill, and archaeoastronomers urge caution: with many stones and many celestial bodies it is easy to find chance alignments. The fact remains that so long a cycle, almost a generation, could be grasped only through a memory shared and handed down over time, perhaps through stories and ceremonies.

The lunar standstill reminds us that the lunar sky is not exhausted by the rhythm of the phases. Alongside the month, eclipses and tides there is a slower breath, which brings the Moon to the borders of its path once every eighteen and a half years and which, according to some hypotheses, people five thousand years ago had already learned to recognise.