Extreme Moon: The Major Lunar Standstills of 2024 – 2025
Every 18.6 years, our Moon reaches the extremes of its orbit around the Earth. We observe this as the Moon rises and sets at its most northerly and southerly positions on the horizon.
Upcoming Events at Griffith Observatory
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Observing Unusual Moonrises and Moonsets
The major lunar standstill results from the rotation of the Moon’s inclined orbit over an 18.6-year cycle. Because the greatest monthly excursion of the Moon changes very little during a standstill, we can observe the phenomenon for the rest of 2024 and much of 2025. The last time this happened was in 2005, when the Observatory was still being renovated and expanded. Unlike an eclipse, a standstill can be observed at any location on Earth.
The full Moon is the most spectacular time to observe the major lunar standstill. Griffith Observatory is planning a series of activities for notable moonrises and moonsets over the next 18 months. The Observatory has moonset lines inlaid on the lower West Terrace. They point to the exact spots (northerly and southerly) on the western horizon where the major lunar standstill Moon will set.
What Will We Observe?
We observe the major lunar standstill by watching where the Moon rises on the eastern horizon and where it sets on the western horizon. In this way, the major lunar standstill is similar to observation of the Sun at the solstices. The solstices represent the most southern (winter) and most northern (summer) sunrise and sunset positions on the horizon. The Observatory has sunset lines inlaid on its lower West Terrace (as shown, at winter solstice).
To the unaided eye, the position of sunset at the solstices does not seem to change for several days. Because the sunset seems to stand still, the time when this happens is called a solstice, which means “Sun still” in Latin.
In one year, the Sun moves gradually from one extreme to another and then back again. These extreme positions for sunrise and sunset stay the same for thousands of years. People throughout history have observed the rising and setting Sun as a reliable way of calibrating the calendar.
Tracking the Moving Moon
The Moon’s motion and timing are much more complicated. The Moon reaches both a northernmost and southernmost position each month, doing in 29 days what the Sun does in a year. From month to month, however, the northern and southern limits of moonrise and moonset change. Over an 18.6 year cycle, the greatest northern and southern positions of the Moon extend beyond those of the Sun. And just as the Sun lingers at its limits at each solstice, once the Moon reaches these extremes, its limits don’t seem to change. But where “Sun still” (solstice) lasts a couple days, the Major Lunar Standstill lasts for two years!
What we observe during this time is the Moon rising on the horizon north or south of the solstice Sun and rising higher in the sky than the summer solstice Sun or lower in the sky than the winter solstice Sun. It does this every month for nearly two years. For an observer, this means the location of the rising Moon quickly travels roughly 70 degrees on the eastern horizon in a month and does the same for the setting Moon on the western horizon.
While this rapid back and forth travel on the horizons occurs, we still observe the Moon going through its familiar phases. That means each day the Moon rises and sets in a slightly different phase and just less than an hour later in time. This can make observing the major lunar standstill challenging. The best standstill viewing is when the Moon is full. Griffith Observatory shall be highlighting some of the most visible occurrences of the major lunar standstill through the end of 2025.
The Moon's Orbit Causes the Standstills
The reason we observe the Moon at extreme positions on the horizon every 18.6 years is the complex motion of the Moon’s orbit. There are two key factors. First, the Moon’s orbit is tilted a bit compared to the Earth’s orbit around the Sun. Second, because of the gravitational pull of the Sun, the Moon’s orbit swivels, which varies the angle of lunar travel relative to Earth.
The image here illustrates the geometry of one point in the 18.6-year cycle, specifically the point of the major lunar standstill. The ecliptic – the path that the Sun appears to take in the sky – is represented as an orbit with a flat plane running through it. We also see the celestial equator – the equator of the Earth projected onto the sky of stars – represented as a plane. The Moon’s orbit is also shown as a plane. The observations we make of the Moon are a product of the relationship between and the movement of these three planes. The lunar orbit is tilted about five degrees to the ecliptic, and the ecliptic is tilted 23.5 degrees to the celestial equator. The direction of the tilt of the Moon’s orbit is not always the same (due to the swiveling). When the tilts of the ecliptic and the Moon’s orbit add together, to make the total apparent tilt about 28.5 degrees, the Moon is at its most extreme inclinations. This is observed on Earth as the major lunar standstill.
