So, what exactly are the nodes of the Moon? Well, they are shadow, not physical, planets, also known as Rahu, the North Node, and Ketu, the South Node. They are deemed to be critical points in a birth chart, paving the way for karmic analysis and judgment. Lets have a closer look at their significance and determine which is the better calculation for them, Mean Node or True Node?
The Earth orbits the Sun, but since we are sitting on planet 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 consisting of signs and constellations, also known as rashis and nakshatras, 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 perspective, 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. This can be applied to all the planets. 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!”
Now lets come back to the topic at hand, the lunar nodes, Rahu & Ketu. Where the Moon crosses the Ecliptic heading north, that is regarded as the Moon’s north node, Rahu. Where it crosses heading south, this is regarded as the Moon’s south node, Ketu. 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.
So, these 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 see this: it takes either lunar node about a year and a half to move retrograde through each sign of the Zodiac.
Yet another way to see this: is that the nodes take just under nineteen days to move through one degree.
Eclipses
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 realise 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 synch by about five degrees.
Where they are not out of synch 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.
Mean Vs. True Node
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 great, not more than a degree and half or so. Though this may affect the nakshatra placement of the Node, and hence the calculation, especially if you are experiencing a nodal dasha.
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.
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. 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 node calculation reflects that wobble. The mean node calculation does not.
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 software do not take all of these effects fully into account. Even the “true” node is not really true then. 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 normally see the mean node calculations for general transits. However, when examining a birth chart will also take a look at the nakshatra placement of Rahu and Ketu, as this can make a difference in the planets that are impacted. I have seen this practice work well, as there are certainly times when the Mean Node is more applicable and relatable for a person in their birth chart, and other times, it is True Node. This becomes relevant if they are undergoing the dashas of the Nodes.
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Text from Article taken from Chapter 5 of Steven Forrest’s book, Yesterday’s Sky, and is used by permission.
