Lunistices

Though the sky is filled with immeasurable objects – galaxies, stars, asteroids, Uranus – two of them dominate our perspectives: the Sun and the Moon.

Most celestial bodies feature tangible but often imperceptible cycles, thanks to Earth’s rotation, but our star and our satellite grace us with significant patterns.

Our definitions of time and seasons both come from the Sun. A year is how long it takes us to go once around the star. Our seasons arise due to our planet’s axial tilt, which is approximately 23 degrees in relation to the ecliptic, the plane of orbit around the Sun. When a hemisphere is tilted toward the Sun, we experience spring and summer; while tilted away, we live in autumn and winter.

As we go around the Sun, think about us flying around in a straight line, but what we consider the top (i.e. the North Pole) is angled 23 degrees.

Our orbit creates extremely stable patterns on Earth. Though we go around the Sun, on our planet, the star appears to change its position in the sky as the year progresses.

Thanks to seasons, we know we get more sunlight during the summer and less during the winter. However, the longer sunlight also comes with a Sun higher in the sky and vice versa. The Sun also rises and sets at different places during the year. On the Northern Hemisphere’s summer solstice, the sun rises and sets farther north than any other day; during the winter solstice, it rises and sets farther south than any other day. It’s no coincidence that the Earth’s axial tilt – ~ 23 degrees – matches the farthest latitudinal march north and south the Sun makes during the year. We call these two latitudes the Tropics of Cancer and Capricorn. At the Tropic of Cancer, on the summer solstice, the Sun will stand directly above an observer at noon.

These characteristics happen like clockwork, year after year. You could stand outside your house and mark the point where the Sun rises on any date you want, come back a year later, and watch it hit the same spot again.

With such rigorous periodicity and life-giving qualities, it’s no surprise that ancient humans noted the patterns of the Sun and erected incredible monuments to it.

Likewise, the Moon displays steadfast patterns.

Over 29.5 days, Luna revolves through a cycle of phases, from new to quarter to full to quarter to new. This period led to the adoption of the “moonth,” also known as a month.

Though the Moon does not affect life on Earth remotely as much as the Sun (it does exert tidal pulls and helps stabilize our tilt, which leads to a stable climate), its frequent changes likely impressed themselves greatly upon an ancient person. The Sun is always a blinding disc, but the Moon shakes up its shapes daily.

But the Moon’s periodic shapeshifting is not the end of its patterns.

You might recall from our investigation into the Anatomy of a Lunar Eclipse that the Moon’s orbit is tilted in relation to the ecliptic. This slight offset prohibits eclipses from happening every month. You might also wonder if this tilt could cause a similar situation to the Sun, in which the body rises and sets at different places depending on the date.

Yes!

Though we might never notice the location of the Moon’s rising and setting in modern times, our satellite’s positioning on the horizon does shift. The Sun’s declination (the astronomical equivalent of latitude) appears to change more than 46 degrees between the solstices. That’s a noticeable amount. Think about all the places you’ve seen the Moon on the horizon. Does it oscillate within a 10-degree band, the arc it would cover if its orbital tilt accounted for its shifting phenomenon?

No!

As it turns out, the Moon does rise and set predictably, but its pattern is far more complex and wide-ranging than the Sun’s.

Unlike the Sun, which switches its apparent northward and southward motions twice a year, the Moon achieves this distinction monthly. That is, the Moon’s northernmost rising and setting and its southernmost rising and setting during one cycle happen about two weeks apart, coinciding approximately with its phases. This fluctuation seems intuitive. However, unlike the Sun, the northernmost and southernmost points are not always the same.

These extremities vary in a complex cycle that lasts 18.6 years!

This process introduces a few neat pieces of vocabulary. The word “solstice” comes from the Latin roots for “Sun” and “standing still.” At the solstice, the Sun seems to momentarily pause, as it stops its movement and reverses course. The Moon achieves a similar apparent cessation in its monthly fluctuations between north and south risings and settings. In the middle of the sequence, it moves rapidly across the horizon from day to day, but at the extremes, it moves slowly. This pause is known as a lunistice or a lunar standstill.

As we learned, this monthly movement is not the end of the Moon’s pattern, though. Over 18.6 years, the Moon’s declination cycles through two sets of extrema. These spots, the absolute northernmost and southernmost points on the horizon from which the Moon rises and sets and the minimum northernmost and southermost, are called the major lunar standstill and the minor lunar standstill. During the major standstill, the gap between the Moon’s rising and setting extremes is a whopping 57 degrees! During a minor standstill, the difference is just 37 degrees.

If this explanation seems confusing or hard to visualize, it’s not you. This process is relatively complex, especially because we are not attuned to thinking about the Moon’s positioning.

Here’s an attempt at a simplified recap of the Moon’s movement:

If you watched the spot on the horizon where the Moon rises for a month, you would watch it swing from a northern extreme to a southern extreme every two weeks. The north-south gap is surprisingly large. However, at the edges, you would notice the rising location’s change begin to slow down or even “pause” for a few days. We call this spot the lunistice or lunar standstill. That’s the basic pattern. However, if you watched the Moon repeat this cycle for a few months, you would start to notice that the lunistice extremes do not remain constant. Unlike the Sun, which swings like a pendulum between two points throughout the year, the Moon’s extremes change over time. Over an 18.6-year period, the standstill points move between two meta-extremes. At the major lunar standstill, the gap between the northernmost and southernmost rises is the largest. In 9.3 years, the Moon reaches the minor standstill point, where the northernmost and southernmost rises are closest together during the cycle. In another 9.3 years, the Moon once again hits a major standstill. So, you can think of the Moon as if it’s a pendulum that does not swing between two constant extremes, but a pendulum whose extremes are constantly moving, albeit on a slow scale.

What causes this 18.6-year cycle?

In short, the Sun’s gravitational pull. Earth has by far the biggest influence on the Moon, but the Sun is so massive that it exerts a noticeable pull. Because the Moon’s orbit is slightly tilted, the Sun causes a “wobble” in the orbit. The Moon continues to spin and move around Earth, but its orbital plane essentially spins around Earth, instead of remaining in a normal orbit. Scientists call this precession. Think of the Moon’s orbit around the Earth as if it were a ring. Instead of the Moon going around this ring like it’s a fixed rollercoaster, the tracks themselves spin around the Earth. This reality can be hard to visualize, but the orbit’s “track” returns to the same position after 18.6 years.

Once again, it’s no surprise that the maximum gap in declination for the Moon works out to a nice equation. With the Earth’s axial tilt – 23 degrees – we got the tropics. For the Moon’s gap, the addition of the Earth’s axial tilt and the Moon’s extra five degrees add up to a maximum declination of about 28.5 degrees (or 57 degrees difference between the northernmost and southernmost). This gap occurs at a major standstill. At a minor standstill, we take Earth’s axial tilt and subtract the Moon’s orbital tilt to get the 37-degree gap!

At lunistices, we get a little time to appreciate the moon rising in nearly the same spot. Unfortunately, if you don’t purposefully look for it, you might miss the phenomenon.

However, at a major or minor standstill, things are a bit different. The Moon tends to stick around the extremes for longer at these points, up to a year in either direction! So, when the Moon is near a major standstill and rising farther north than it ever does, we can witness the phenomenon for many months. The spot won’t be exactly the same but will be close enough to the naked eye to be nearly imperceptible.

As we write this article, we are in the midst of a major lunar standstill. The Moon is rising farther north and south right now than it ever does. It swings between these extremes every month. The result is a Moon that is all over the sky right now. If you spend a month or two keeping tabs on Luna, you’ll see her seemingly all over the place.

Because of the Moon’s periodic terms, lunistices tend to happen near equinoxes and eclipses, too! The upcoming lunar eclipse and nearby spring equinox are not a coincidence with this major standstill.

The complicated lunar cycle can get lost in the shuffle of modern life. Keeping tabs on a cycle that lasts 18.6 years and involves orbital precessions can be rough even if you know about it. The following video might help you visualize it a bit better:

The Sun was not the only body to which ancient humans built monuments and calculators.

As we’ll see in our next investigation, ancient civilizations developed some staggering calendars to the Moon.