The Moon's 18.6-Year Cycle, and Why Northern Autumn Crescents Hug the Horizon Until the Early 2030s

The Short Answer

The Moon's orbit is tilted about 5 degrees to the Sun's path, the ecliptic, and the direction of that tilt turns round once every 18.6 years. When the tilt adds to the tilt of the Earth's axis, at a major standstill, the Moon swings every month to 28.7 degrees north and south of the celestial equator. That happened in 2006 and again in 2024 and 2025, and will next in 2043. When the two tilts work against each other, at a minor standstill, the Moon only reaches 18.6 degrees: in 2015 and next in 2034.

The same wobble changes how high the new crescent stands after sunset, and that matters for anyone looking for the hilal. At 45° N, a Moon 24 hours past conjunction:

• in February to April stands on average 10.7° above the horizon at sunset in 2024 to 2031, against 7.5° in 2035 to 2040;
• in August to October stands on average only 0.5° above it in 2024 to 2031, against 6.7° in 2035 to 2040.

So for most of this decade the northern autumn crescents are the hardest of the cycle, and the spring ones the easiest. The pattern turns round in the late 2030s.

What the 18.6-Year Cycle Is

The Moon's orbit crosses the ecliptic at two points, the nodes. They are not fixed: the Sun's pull makes them slide slowly backwards round the sky, one full circuit in 18.6 years. As they move, the Moon's orbit tilts sometimes the same way as the Earth's axis and sometimes the opposite way. The Moon's monthly swing north and south of the equator therefore grows and shrinks:

• major standstill: the two tilts add up, and the Moon reaches about 28.7 degrees (2006, 2024 to 2025, 2043);
• minor standstill: they work against each other, and it reaches only about 18.6 degrees (2015, 2034).

In a major standstill year the full moon of winter rides even higher than usual in the northern sky and the full moon of summer even lower, and moonrise and moonset swing far round the horizon. The Moon's changing path across the sky describes the monthly side of this.

Why It Moves the Crescent

The young crescent sits a few degrees from the Sun along the Moon's own path. Whether it stands high or low at sunset depends mostly on how steeply that path meets the horizon, which changes with the seasons. The 18.6-year cycle adds a second effect on top: the Moon can be up to 5.1 degrees above or below the ecliptic where it stands that evening. When the cycle puts the Moon above the ecliptic at the place in the sky where the evening crescent is in spring, the spring crescent is lifted; six months later, on the opposite side, the autumn crescent is pushed down.

That is why the lower panel of the figure follows the cycle but is not centred on the standstill itself: what matters is where the nodes are relative to the Sun at the time of the crescent, not the Moon's largest declination. Over 2024 to 2031 the nodes sit where they lift spring crescents and lower autumn ones in the north. By the late 2030s they have moved half way round and the effect is reversed.

In the autumns of 2024 to 2031, 13 of the 25 crescents in this sample at 45° N were already below the horizon when the Sun set. In 2035 to 2040 none of the 19 were.

What It Means for the Hilal

• In the north, this decade: the months whose crescent is sought in late summer and autumn are harder than usual; those of late winter and spring easier. Over these years Ramadan and the two Eids fall in winter and late autumn (Ramadan begins on 8 February 2027 and on 5 January 2030), so this affects other months more than them.
• In the south: the cycle helps southern spring crescents (August to October) in the same years: at 45° S they average 10.9 degrees, against 8.6 in 2035 to 2040.
• Near the equator: the crescent stands higher in every season, but the cycle still moves it: in 2024 to 2031 it averages 8.7 degrees in February to April and 9.0 in August to October, against 10.6 and 11.8 in 2035 to 2040.

Remember that 24 hours after conjunction is a fixed reference, not the age of the Moon on any real evening. The crescent of a given month can be younger or older, and the visibility map shows the real evening for any place.

Frequently Asked Questions

Is the major standstill over?

The Moon's monthly swing was largest in 2024 and 2025 and is now slowly shrinking, to a minimum in 2034. The effect on the crescent lags behind it by a few years and is strongest around 2027 to 2032.

Did the major standstill affect the Ramadan crescent?

Not much. Ramadan has fallen in late winter and spring in these years, when the crescent at mid-northern latitudes is already high, and the cycle lifted it a little more. The big effect is on the late summer and autumn months.

Why do the yearly dots in the figure scatter so much?

Each dot is the average of only three new moons, and each of those can differ from the average by several degrees because the Moon's speed and distance change through its orbit. The lines, three-year running means, show the cycle more clearly.

Where can I see this for my own location?

Use the visibility map and the Sky Dome for any evening; this post compares years at one latitude to isolate the cycle.

See Also

• Why the new crescent leans: its tilt and height by latitude and season
• Why the Moon's path across the sky changes every month
• The four kinds of lunar month
• Why eclipses come in pairs
• The day the Moon does not rise: why moonrise skips a day every month

References

• Meeus, J. (1998). Astronomical Algorithms (2nd ed.). Willmann-Bell. The lunar node and the Moon's declination.
• The numbers and the figure in this post are reproduced by docs/research/moon-cycles.mjs and computed by packages/astronomy/src/moonCycles.ts and crescentSeason.ts in the Hilal Vision source.