A lunar standstill or lunistice is when the Moon reaches its furthest north or furthest south point during the course of a month (specifically a draconic month of about 27.2 days). The declination (a celestial coordinate measured as the angle from the celestial equator, analogous to latitude) at lunar standstill varies in a cycle 18.6 years long between 18.134° (north or south) and 28.725° (north or south), due to lunar precession. These extremes are called the minor and major lunar standstills.
The last minor lunar standstill was in October 2015, and the next one will be in May 2034. The last major lunar standstill was in June 2006, and the next one will be in January 2025.
Presently the northern lunistice occurs when the Moon is seen in the direction of Taurus, northern Orion, Gemini, or sometimes the southernmost part of Auriga (as at the time of a major lunistice). The southern lunistice occurs when the Moon is in Sagittarius or Ophiuchus. Due to precession of the Earth's axis, the northernmost and southernmost locations of the Moon in the sky move westward, and in about 13,000 years the northern lunistice will occur in Sagittarius and Ophiuchus and the southern lunistice in the area of Gemini.
During a minor lunar standstill, tidal forces are slightly increased in some places, leading to increased amplitude of tides and tidal flooding.
At a major lunar standstill, the Moon's range of declination, and consequently its range of azimuth at moonrise and moonset, reaches a maximum. As a result, viewed from the middle latitudes, the Moon's altitude at upper culmination (the daily moment when the object appears to contact the observer's meridian) changes in two weeks from its maximum possible value to its minimum possible value above the horizon, due north or due south (depending on the observer's hemisphere). Similarly, its azimuth at moonrise changes from northeast to southeast and at moonset from northwest to southwest. In a year of a major lunar standstill, solar eclipses occur in March at ascending node and in September at descending node, whereas lunar eclipses at descending node occur in March, lunar eclipses at ascending node occur in September. In a year of a minor lunar standstill the situation is reversed.
The times of lunar standstills appear to have had special significance for the Bronze Age societies who built the megalithic monuments in Britain and Ireland. It also has significance for some neopagan religions. Evidence also exists that alignments to the moonrise or moonset on the days of lunar standstills can be found in ancient sites of other ancient cultures, such as at Chaco Canyon in New Mexico, Chimney Rock in Colorado and Hopewell Sites in Ohio.
Major lunar standstill
A major lunar standstill occurs when the Moon's declination reaches a maximum monthly limit, stopping at 28.725° north or south. An eclipse season near the March equinox has solar and lunar eclipses at an odd-numbered saros, while another eclipse season near the September equinox has solar and lunar eclipses at an even-numbered saros. Between 1951 and 2050, these dates are 29 March 1969, 8 November 1987, 19 June 2006, 29 January 2025 and 10 September 2043.
Minor lunar standstill
A minor lunar standstill occurs when the Moon's declination reaches a minimum monthly limit, stopping at 18.134° north or south. An eclipse season near the March equinox has solar and lunar eclipses at an even-numbered saros, while another eclipse season near the September equinox has solar and lunar eclipses at an odd-numbered saros. Between 1951 and 2050, these dates are 7 December 1959, 19 July 1978, 27 February 1997, 10 October 2015 and 21 May 2034.
Origin of name
The term lunar standstill was apparently first used by engineer Alexander Thom in his 1971 book Megalithic Lunar Observatories. The term solstice, which derives from the Latin solstitium: sol- (sun) + -stitium (a stoppage), describes the similar extremes in the Sun's varying declination. Neither the Sun nor the Moon stands still, obviously; what stops, momentarily, is the change in declination. The word tropic, as in Tropic of Capricorn, comes from ancient Greek meaning "to turn", referring to how ascending (or descending) motion turns to descending (or ascending) motion at the solstice.