An excerpt from Yesterday's Sky Chapter 5
THE ASTRONOMY OF THE LUNAR NODES
If you are a nerd, welcome to paradise. If you slept through science, but were wide-eyed when English class turned to the Romantic poets, you will probably make a very fine astrologer—but you might want to skip this chapter.
So, what exactly are the nodes of the Moon? Obviously, they are not planets. Since they are the foundation of Evolutionary Astrology and the gateway into karmic analysis, let’s take a little while to understand them rigorously.
Earth orbits the Sun, but since we are sitting on the Earth, it doesn’t look that way. For millennia, our ancestors assumed it was the other way around—that the Sun orbited the Earth. Since astrology is Earth-centered, we stick with that visual illusion in our language and perspective. What we actually see is that the Sun circuits the same band of constellations each year. Astrologers call that band the Zodiac. Astronomers call it the Ecliptic.
Imagine that as the Earth swings around the Sun, it sweeps out a flat circle of glass. That disk is the Plane of the Ecliptic. In our Earth-centered heads, we project this Plane of the Ecliptic out onto an imaginary Celestial Sphere—a kind of vast starry ball with us inside, right in the center. Where the Plane of the Ecliptic touches the Celestial Sphere, we have the “highway” in the sky that the Sun follows in its yearly path. Ditto, pretty much, for all the planets too. That is the Zodiac.
The Moon orbits the Earth. That cycle takes just under 27 days, eight hours. Again, imagine that as the Moon does so, it too sweeps out another flat circle of glass. That is the Plane of the Moon’s orbit. We project it out onto the Celestial Sphere as well, just like with the Ecliptic. (It is helpful to remember that even though in reality the Moon’s orbit is a lot smaller than the Earth’s orbit around the Sun, when they are both projected onto the inside of the imaginary Celestial Sphere, they are the same size.)
Here is the critical point: the Plane of the Moon’s Orbit is inclined by about five degree to the Ecliptic. (5E 8' 40") They do not “lie flat,” in other words. So half the time the Moon is above the Ecliptic, half the time it is below. (And here we shamelessly indulge in the European colonial fantasy that north is “above” and south is “below!”)
If you have got all that, you are ready to understand the lunar nodes:
Where the Moon crosses the Ecliptic heading north is the Moon’s north node. Where it crosses heading south is the Moon’s south node. More precisely, where the plane of the Moon’s orbit rises above the plane of the Ecliptic, we define the north node. Where it sinks below the Ecliptic, we define the south node.
The Moon itself could be anywhere in its journey around the circle. It only actually crosses the Ecliptic twice each month, once going up, once going down.
Those are the nodes.
The Nodal Cycle
Spin a child’s top. Say it is rotating really fast in a clockwise direction. As it gradually runs out of momentum, you see it begin a slow counter-clockwise wobble before it topples over. Similarly as the Moon speeds around the Earth every month, the plane of its orbit is gradually sliding backwards in the opposite direction. In other words, the places where the Moon’s orbit crosses the Ecliptic—the nodes—do not remain in the same degree of the Zodiac for eternity. Instead they move slowly retrograde, taking 6793.39 days to get back to where they started. That works out to 18.5997 years—18 years, seven months, and a few days.
Another way to say it is that it takes either lunar node about a year and a half to move retrograde through each sign of the Zodiac.
Yet another way to say it is that the nodes take just under nineteen days to move through one degree.
Solar and Lunar Eclipses fit into all this nodal theory in a fairly obvious way, if you think about it. A solar eclipse, as everybody knows, is a big deal. People travel long distances to see one. What many do not realize is how close we come to a solar eclipse every month. Every 29 days or so, we have a New Moon. That means that the Sun and the Moon are aligned in the same degree of the Zodiac.
So why don’t we have an eclipse?
The answer is because, even though Sun and Moon are lined up, the Moon is little above or below the Sun—remember: the plane of the Moon’s orbit and the Earth/Sun plane are out of whack by about five degrees.
Where they are not out of whack is the lunar nodes—as we have learned, the nodes are where the Moon’s orbit actually crosses the Ecliptic, or aligns with it. So if we do experience a solar eclipse, that simply means that a New Moon has fallen close to a lunar node. Similarly, we will only have a lunar eclipse if a Full Moon falls near a lunar node.
Little hand-held calculators will figure all this out nowadays. Years ago, our ancestors built a big one to do the same thing. It is called Stonehenge.
Mean Versus True Nodes
There is a lot of fuss in the astrological community about whether to use the mean or the true nodes. People get very passionate about it, even though the difference between mean and true is never very great, not more than a degree and half or so. I have found that if such debates become annoying, a great way to silence them is to ask for a simple astronomical definition of the difference. That usually quiets people down.
The Moon orbits the Earth. That is a simple, practical statement, and basically true. But it is actually more precise to say the Earth and the Moon orbit a common center of gravity. That point is called the barycenter.
Think of the Earth as a big, heavy man dancing with his little tiny wife. Both pairs of hands are joined and they are spinning around merrily. Because he is so much bigger, at first glance he seems to be stable and she seems to be orbiting him. But if you look carefully, you see it is not quite that simple. The center of his body is making little circular gyrations as she pulls him first this way then that.
It is exactly like that with the Earth and the Moon. Their common center of gravity—the barycenter—does not lie at the precise center of the Earth. In fact, it lies about three-quarters of the way from Earth’s center to its surface. It is within the Earth, in other words, but just not at the center. So, like the heavy man, Earth wobbles as the Moon orbits it.
Let’s make the language a little more technical. The small woman pulls the center of the big man’s body toward her as she spins around him. She is much lighter, so the effect on him is slight. But it is real. Like the Moon, her “orbit” around him has a “plane,” but the center of that plane—his hips—is not fixed and stable. It always pulls a bit toward her.
Earlier, we defined the south node of the Moon as the place where the Plane of the Moon’s orbit intersects the Ecliptic heading south—where the Moon would move “below” the Ecliptic. (And remember that we were visualizing the plane as circular piece of glass.) But, just as with our dancers, the “glass” Plane of the Moon’s orbit wobbles. So the exact position of those nodes wobbles too. The true nodes reflect that wobble. The mean nodes do not.
It gets a little more complicated.
The more closely we approach something, the more strongly we feel the pull of its gravity. That is the famous “Inverse Square Law” of physics—halve the distance and the force of gravity quadruples. The Moon’s orbit around the earth is not perfectly circular. Like most astronomical orbits, it follows an ellipse, sometimes closer, sometimes farther from the Earth. This means that the strength of the Moon’s tug on the barycenter varies over the Moon’s monthly orbit. The effect is that the barycenter is basically made even more jittery.
Furthermore, the Sun’s massive gravity is also part of the equation. As we learned earlier, the Moon’s orbital plane around the Earth is not “flat” relative to the plane of the Earth’s orbit around the Sun—it is tilted by about five degrees. So sometimes the tug of the Sun’s gravity is “down.” Other times it is “up.” The effect is to add a further jiggle to the Moon’s orbit.
One can take all of these factors into account, and see that the actual moment-to-moment position of the lunar nodes is quite twitchy. Essentially that is how the true node is calculated—although in reality most astrological computer do not take all of these effects fully into account. Even the “true” node is not really true. The bottom line is the calculated true node’s motion is eccentric. It can even sometimes go briefly Direct.
The mean node assumes the simpler view: that the Moon orbits the center of the Earth. The effect is to smooth everything out a bit.
As I mentioned earlier, the difference between the position of the mean and the true node is always slight, and thus generally inconsequential for our purposes.
I would never be dogmatic about it, but I work with the mean node myself. My reasons are practical more than theoretical. Here is what I have learned and experienced. Occasionally, someone sits with me who has the mean and true nodes separated, as is typical, by a distance of only a degree or so. But for this person that difference is enough to put the two nodes in different signs. Maybe the mean lunar south node is in 29EII 37' and the true one is in 0Ea 43', for example. I view such people as precious—or at least as excellent lab rats! When I was trying to sort all this out I have told such people both of the possible karmic stories and asked them which one felt right. I also compared the two versions with my subjective impressions of them as individuals and with the shapes of their outward lives. Far more valuable than any technical astronomical argument, these human truths are astrology’s Supreme Court.
For me, more often than not, it is the mean lunar node that has carried the day.
See what works best for you.
First, do not apply this test to differences between true and mean nodal house positions. House cusps are inherently a little more blurry than sign cusps—they “flow” a bit more, like seasons changing, while sign cusps tend to operate with more distinct “clicks,” like television channels. A great bugaboo in astrology is the accuracy of someone’s birth time. Even a small error there can throw house cusps off enough to make them dubious for our purposes here. So, if the true node makes better personal sense to you than the mean one in terms of its house position—well, you are probably right about which house the node occupies, but my first guess would be that your birth time is off by a minute or two.
Second, in exploring the distinctions between mean and true nodes, avoid the temptation to look at only one or two charts. Half of the truly dumb ideas in astrology have arisen that way. “My best friend has Jupiter in her first house and she loves cats. Therefore Jupiter in the first house means you love cats.”
We need to cast our nets more widely to see reliable patterns, and it is in those patterns that we mine the alchemical gold.
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