Lunar standstill

The Moon’s rising and setting points shift along the horizon over the course of its monthly cycle. Their northern and southern limits also change from month to month. Near a major lunar standstill, these limits approach their furthest positions; near a minor standstill, the monthly range is more restricted. “Standstill” does not mean that the Moon stops moving. Rather, its extreme limits change so slowly around the peak that the pattern persists over an extended interval.

The Moon’s orbit is inclined by about five degrees to the plane of Earth’s orbit around the Sun. The Sun’s gravitational influence causes the orientation of the lunar orbital plane to turn over time. The resulting cycle takes about 18.6 years. When the orbital tilt and the tilt of Earth’s equator relative to the ecliptic combine in the same direction, the Moon reaches its greatest apparent north–south excursions, producing a major standstill. When they partly offset one another, the range is smaller and a minor standstill occurs.

A major standstill can be followed by comparing where the Moon rises or sets on successive dates, especially near its monthly extreme positions. The Moon’s horizon position moves rapidly back and forth through the month, even while the standstill’s overall limits change little. The full Moon can make a striking observation when it rises or sets near those limits, but the exact direction and altitude depend on the observer’s location and the horizon. Unlike an eclipse, a lunar standstill is not confined to a narrow viewing path; the Moon’s extreme horizon positions can be observed from different places on Earth.

The long lunar cycle has been proposed as a possible subject of alignments or imagery at some archaeological sites. Such interpretations require care: a structure aligned with a lunar extreme does not by itself prove that its builders intended to track the 18.6-year cycle. The High Altitude Observatory at the National Center for Atmospheric Research describes proposed interpretations at Mesa Verde and other sites as plausible conjectures and notes that alternative explanations exist.