The lunar standstill and the 18.6-year nodal cycle
Alexander Thom, Megalithic Lunar Observatories, 1971 — who put the English phrase into circulation
The physics long predates the phrase. The Moon's orbit is tilted about 5° to the ecliptic, and the line where the two planes cross — the nodes — slides backwards through the whole zodiac every 18.6 years. That regression is why the Moon's reach in declination swells and shrinks: at a major standstill the tilt adds to the obliquity and the Moon swings to about 28.5° north and south; nine years later the tilt subtracts and it manages only about 18.3°. Astronomers had described and named the cycle long before Thom; the engineer's contribution was the English word, coined while arguing that megalithic sites were aligned to the extremes.
What it measures
How far north or south of the equator the Moon can reach in a given season, which is the obliquity of the ecliptic (about 23.4°) plus or minus the Moon's own orbital tilt, depending on where the nodes currently sit. It is the mechanism behind an out-of-bounds Moon: for roughly half the 18.6-year cycle the Moon exceeds the Sun's limit about twice a month, and for the other half it cannot do so at all.
Where practitioners disagree
- Is the standstill a moment or a season?
A single extreme date — or the year-or-more around it during which the Moon repeatedly reaches near-maximum
The Moon reaches a fresh extreme roughly every 27.2 days, so the 'standstill' is a long season of high extremes rather than one event. Popular accounts often give a single date.
This engine: This engine states no standstill dates. It reports the Moon's actual declination for the chart's own instant, which is the fact underneath.
Still open
Whether Thom coined “lunar standstill” or merely popularised it is not settled: every source that credits him hedges the claim, and none quotes the book directly.
Sources
- Alexander Thom, Megalithic Lunar Observatories (1971)
- Jean Meeus, Astronomical Algorithms
- Astronomical Almanac, 2010 edition