What is the Minor Standstill of the Moon?

Abstract

2014–2015 was the period of the minor standstill of the Moon. Archaeoastronomy has shown that many prehistoric monuments in northwest Europe have horizon alignments on the Sun’s solstices and the Moon’s standstills (North 1996; Ruggles 1999). These are horizon range properties of the risings and settings of the Sun and the Moon. In northern latitudes, at summer solstice, the Sun rises in the northeast and sets in the northwest, while at winter solstice, the Sun rises in the southeast and sets in the southwest (Figure 1). For a given latitude and horizon elevation, the horizon positions for these solar events are fixed.1 The unaided human eye cannot detect any change in its horizon rise or set position during a solstice for three days before or after the day of the solstice (Ruggles 1999, 25) and, for the monument builders of prehistoric Europe, each solstice is therefore a seven-day event. After the week of the summer solstice, the Sun begins to rise and set further south on the horizon. Pendulum-like, these horizon positions are slow to change close to the solstices and fastest in mid-swing during the equinoxes. At winter solstice, the Sun once again rises in an apparently fixed horizon position for a week, but now in the southeast. A monument that has a horizon alignment on a solstice Sun (summer or winter, rise or set) is therefore a selection of one seven-day event of sunrises or sunsets out of a total 353 (365–14+2) events in any one year. There are important differences when a monument is aligned on the Moon. Unlike the Sun, the limits to the Moon’s horizon range fluctuate. When the Moon’s range is at its greatest extent, what Thom (1971) called the major standstill of the Moon, at the latitude of Stonehenge, the Moon’s horizon range is 10° of azimuth further north or south on the horizon than the Sun ever reaches at each of its four solstice positions. Nine or so years later, the Moon’s horizon range has shrunk to about 10° within the extreme horizon

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