Lunar standstill

Meaning

Every month the Moon travels from its farthest north to its farthest south in declination and back, as the Sun does in a year. But how far it goes changes slowly over 18.6 years. At one end of that cycle, the major lunar standstill, the Moon reaches about 28°43′ north and south each month. At the other, the minor lunar standstill, it reaches only about 18°10′. For a year or two around each end, the monthly limits hardly change from month to month, which is why they are called standstills. Lunistice, from the Latin for the Moon and a standing still, is a rarer name, formed like solstice.

Why it happens. The Moon's orbit is tilted to the ecliptic by about 5.145° on average [2] [7]. The line where the two cross, the line of the Moon's Nodes, turns slowly backwards through the zodiac once in about 18.6 years. The Moon's greatest declination depends on which way the tilt points:

• When the ascending Node is near 0° Aries, the Moon is at its highest north latitude near 0° Cancer and its lowest south latitude near 0° Capricorn. The two tilts add: the Moon reaches about ε + i = 23.44° + 5.15° ≈ 28.6°. This is the major standstill.
• When the ascending Node is near 0° Libra, the tilts work against each other: the Moon reaches only about ε − i ≈ 18.3°. This is the minor standstill.

In between, a good approximation for the year's reach is ε + 5.145° × cos (Node's longitude). The actual extremes are a little wider than these round figures, about 28.72° at a major and 18.13° at a minor standstill, because the tilt of the Moon's orbit itself varies, between about 5.00° and 5.30°, in a cycle of about 173 days. It is greatest when the Sun lies along the line of the Nodes [7]. At a standstill the Nodes lie near the equinox points, so the most extreme monthly limits (widest at a major standstill, narrowest at a minor one) come around the equinoxes, in March and September.

Origin and history

The term is usually credited to the engineer Alexander Thom, who used it in Megalithic Lunar Observatories (1971), his study of alignments at prehistoric stone sites in Britain and Brittany [1]. Thom argued that some monuments were laid out on the risings and settings of the Moon at its extremes; many archaeologists have questioned how many of the alignments he proposed were intended.

Sites often discussed:

• Callanish (Calanais), Isle of Lewis. At the major standstill the Moon near its southern extreme stays very low, skimming along the range of hills to the south known locally as the Sleeping Beauty [8]. The 2024–25 event was studied there in detail [3]; whether the builders intended any lunar alignment is debated.
• Stonehenge. The long side of the rectangle formed by the four Station Stones points toward the southernmost moonrise at a major standstill. English Heritage, with Oxford, Leicester and Bournemouth universities and the Royal Astronomical Society, observed the 2024–25 standstill there; it notes that whether the alignment was deliberate has long been debated [4].
• Chimney Rock, Colorado. At the major standstill the Moon, seen from the Ancestral Puebloan great house on the ridge, rises between two natural rock pillars, Chimney Rock and Companion Rock. The Forest Service describes it as one of several sky alignments the Ancestral Puebloans built into the design of their community [5].

How it is used

Dates. The last major standstill centred on the start of 2025, when the Moon's mean Node passed 0° Aries (late January 2025, by the Swiss Ephemeris). By the same computation the Moon's greatest geocentric declinations were about 28°43′ N on 7 March 2025 (with 28°42′ N on 24 September 2024 almost as far) and about 28°44′ S on 22 March 2025. Reijs gives 24 September 2024 and 22 March 2025 as the dates of the northern and southern limits seen from Callanish [3]. Public observing events ran through 2024 and 2025 [6]. The previous major standstill was in 2006 and, by the same computation, the next falls around 2043. The last minor standstill was in autumn 2015, when the Node passed 0° Libra and the monthly limits fell to about 18°08′; the next is around 2034.

Seen from a place. These figures are geocentric, from the Earth's centre. Seen from the surface the Moon is shifted by parallax, which lowers its apparent limit; at Callanish the apparent limits of 2024–25 were about 28.62° N and 28.70° S [3].

Out of bounds. The Moon can go out of bounds only when its reach for the year exceeds the obliquity, that is when the ascending Node is within 90° of 0° Aries (between 0° Capricorn and 0° Cancer, passing through Aries). That covers about half of every 18.6-year cycle, around each major standstill. Near a minor standstill the Moon cannot go out of bounds at all.

On this site. The Declination panel shows the Moon's reach for the chart's year, worked out as the chart's obliquity plus 5.145° × the cosine of the Node's longitude, draws it as dashed lines on the declination strip, and says whether the birth Moon was within 1° of that limit. Its note gives the cycle's range as about 18°08′ to 28°43′, the Moon's actual extremes; the simple formula itself gives a slightly narrower range, about 18°17′ to 28°35′ with today's obliquity.

Examples

• Early 2025, Node near 0° Aries, obliquity 23.436°: reach ≈ 23.436° + 5.145° × cos 1.5° ≈ 28.58°, or 28°35′. The actual extreme, 28°43′–28°44′, is slightly larger, as explained above.
• September 2026, mean Node near 27°51′ Aquarius (327.85°): reach ≈ 23.436° + 5.145° × 0.847 ≈ 27.79°, or 27°47′. The Moon can still go out of bounds, by up to about 4.4°.
• October 2015, Node near 0° Libra: reach ≈ 23.44° − 5.145° ≈ 18.29°. The actual monthly limits fell to about 18°08′.