| The "Galactic Equator" isn't the equator of the Milky Way Galaxy but it isn't an "arbitrary line", either. |
You may be surprised to learn that the Sun never "crosses the Galactic Equator" because the Sun is always on that "Equator", by definition. In contrast, the Sun is currently almost 100 light years away from the Galactic Plane, and getting further away all the time. The Sun won't return to the Plane for about 30 million years.
Introduction
It's been said that first-hand knowledge is not only more worth acquiring than second-hand knowledge, but is usually much easier and more delightful to acquire.1
This was our own experience regarding the difference between the "Galactic Plane" and the "Galactic Equator". The information we'd found in several online sources indicated that they're identical. We knew this to be wrong, so we searched further, and found the article2 from 1960 in which the International Astronomical Union (IAU) defined the reference plane that has since become known as the "Galactic Equator". We'll soon see that this is an unfortunate misnomer.
Although the IAU's article was apparently not written for the general public, it's quite readable, with a friendly, engaging tone. We encourage everyone to examine it themselves, and hope our treatment of it here will help readers over any difficult spots.
Background
Galactic Equator
Why was the so-called "Galactic Equator" defined by the IAU?
The reference plane that's come to be known as the "Galactic Equator" was defined as part of a Sun-centered reference system for determining directions to astronomical objects. To understand that system, we have to learn a little about the external form and internal structure of our home galaxy, which is called the Milky Way.
External form of the Milky Way Galaxy
Our Galaxy is disk-shaped, of the form known as a "barred spiral". From "above", it should look very much like this:
(This image is from Richard Powell's website, An Atlas of the Universe. See here for copyright information.)
From the side, the Milky Way Galaxy should look like this:
(This image, too, is from Richard Powell's website).
In the sections that follow, our explanation of the "Galactic Equator" will use a simplified cross-section of our Galaxy:
Simplified cross-section of the Milky Way Galaxy.
Internal Structure
The state of knowledge about the internal structure of the Milky Way Galaxy, ca. 1955
By the 1950s, astronomers had analyzed radio-frequency emissions from natural sources in much of the Galaxy. They'd found something interesting about the central region:
The outstanding feature of the whole of the observations is the high degree of flatness of the layer of neutral hydrogen in the region of the Galaxy within about 7 kpc3 of the centre. Within this area nearly all the points of maximum density of hydrogen … lie within 20 pc of the mean plane of the layer [which is called elsewhere] the "H1 Principal Plane".4
The authors noted that outside of the central region, the hydrogen layer was irregular.
Below, we locate the H1 Principal plane on our cross-section of our Galaxy.
The H1 Principal Plane in Our Galaxy.
Astronomers recognized that the Sun might be slightly higher in the Galaxy than the level of the H1 Plane.
Cross-section of our Galaxy, with position of Sun added.
By the 1950s, astronomers knew, to a high degree of accuracy, the direction from the Sun to the true rotational center of our Galaxy. The IAU referred to that center as Sagittarius A.
Below, we add the location of Sgr A to our cross-section.
Cross-section of our Galaxy, with position of Sgr A added.
Reference Systems
The reference system proposed to the IAU, and the one that was adopted
The preceding summarizes the most-relevant parts of what astronomers knew in 1958, when a sub-committee of the IAU recommended that a new Sun-centered "galactic coordinate system" be adopted for specifying directions from the Sun to other objects in space. Now, we'll see how that knowledge influenced the IAU criteria for a galactic coordinate system
Criteria
Criteria for the recommended system
The type of system recommended by the IAU needed both a reference plane through the Sun, and a reference direction from the Sun to some other celestial object. Please note that "reference plane" and "reference direction" are not the standard terms for those features. We use them here because the standard terms are a source of confusion, as will become apparent.
A "straightforward" reference system satisfying the criteria
Regarding the choice of a reference plane, we quote the article we've been following:
The high degree of flatness of the disk of interstellar gas in the inner region must be of fundamental physical significance and the H1 principal plane appears to be the best reference plane for dynamical studies of the whole galaxy. It thus seems reasonable to choose for the equatorial plane for the galactic coordinate system the plane through the Sun parallel to the H1 principal plane.5
Here we see a source of confusion. In the sort of coordinate system desired by the IAU, "equatorial plane" is the correct name for what we've been calling the "reference plane". Since its full name is the "equatorial plane of the galactic reference system", it could easily (and evidently did) come to be called "the galactic equator".
Regarding the reference direction, the simple, obvious choice was the direction from the Sun to Sgr A. So, in its straightforward version, the coordinate system proposed by the IAU would have had the reference plane and direction shown below, on our cross-section.
Reference plane and direction (red arrow) for the straightforward version of the galactic coordinate system.
The Adopted System
The system actually adopted by the IAU
The straightforward version poses mathematical annoyances that the IAU had of course foreseen. The first was that the reference direction doesn't lie within the reference plane. the second is that conversions from other reference systems to this one involve messy numbers. (Remember that ca. 1960, pocket scientific calculators were still the stuff of science fiction.)
Fortunately, a satisfactory solution to all these somewhat-conflicting requirements was easily found. The IAU identified a reference direction that passed almost exactly through Sgr A, lying within a reference plane that was almost parallel to H1, and whose conversions to other systems used nicely rounded numbers.
The key features of the coordinate system identified by the IAU are illustrated below.
Key features of the coordinate system identified by the IAU. The reference plane is almost parallel to H1, and contains a reference direction that passes almost exactly through Sgr A. These modifications made the system mathematically convenient. NOTE that this version of the "reference plane" is what has come to be known as the "Galactic Equator".
Specifically, the pole of the reference plane was RA 12h49m, DEC +27o24' (equinox 1950), and the reference direction was $\Theta$ = 123o (1950). (See original article for sign conventions, etc.6)
The IAU established these values as definitions of the new galactic coordinate system, adding
The above quantities are to be regarded as exact so that the new galactic coordinates may be computed to any desired accuracy in terms of right ascension and declination for the equinox 1950.0.7
Comparison with the Galactic Plane
While the "Galactic Equator" is a mathematically convenient reference based upon a the H1 plane, the Galactic Plane is (roughly speaking) a plane of symmetry of the whole Galaxy's distribution of mass. (See here.) The Galactic Plane and the H1 plane (and therefore the "Galactic Equator") are approximately parallel just because the Galaxy is such a thin disk.
As mentioned earlier, the Sun never crosses the "Galactic Equator", but will cross the Galactic Plane in about 30 million years. Of course that statement needs some explanation.
The Sun never crosses the Galactic Equator because the Sun is always on the Galactic Equator: that Equator passes through the center of the Sun, by definition. We sometimes hear that the Sun aligns with the Galactic Equator near the Winter Solstice of each year, but that's only because we tend to look at things from our from our provincial geocentric perspective. The ecliptic plane (i.e., the plane that contains the Earth's orbit) passes through the Center of the Sun, just like the "Galactic Equator", but the two are not parallel. Therefore, the ecliptic plane and the "Galactic Equator" intersect along a line that passes through the Sun's center. When the Earth crosses that line in its yearly orbit around the Sun, the Sun appears (from our perspective) to "align with the intersection of the ecliptic and the galactic equator". Actually, it's the Earth that's getting in line.
In contrast, the Sun is currently almost 100 light-years away from the Galactic Plane, as explained here.
Misconceptions
Analysis of common misconceptions about the "Galactic Equator"
Given the readability of the article we've been following, and the importance of the so-called "Galactic Equator" to the 2012 scenarios, we're puzzled that so many misconceptions ever arose regarding the "Equator's" physical significance. We're also embarrassed at our own contributions to those misconceptions.
Strangely, debunkers of 2012 as well as its proponents have been seriously mistaken about the "Galactic Equator". Part of the problem may be that popular online references like Wikipedia give incomplete or misleading definitions of it, and don't mention the original IAU article.8
We'll now analyze a few representative mis-characterizations of the "Galactic Equator" that appear in the 2012 literature. The first two are from John Major Jenkins:
…I would like to emphasize that the Galactic equator -the precise edge of our spiraling Galaxy- is the zero point location of the turnabout moment in the cycle of precession.9
and
A slight variation of [the definition of the galactic alignment] replaces "dark rift in the Milky Way" with the more abstract [!] astronomical term "galactic equator". The galactic equator is the precise midline of the Milky Way, a line that we could draw in our mind's eye as we looked at the bright road of the Milky Way in the sky.10
Having familiarized ourselves with the IAU article, we can see that Jenkins' statements are simply baseless.
The next mis-characterization of the "Galactic Equator" is found in Starry Night Education's often-cited debunking of the Galactic alignment:
[The Galactic Equator] was officially defined by the International Astronomical Union in 1959, but it is there by definition, not based on any physical characteristics or markers.11
From the IAU article, we know that this statement is puzzlingly inaccurate: the IAU based the "Galactic Equator" upon important physical characteristics of the Galaxy, which serve as clear markers. Unfortunately, one of the authors of the present article, jim smith, has frequently repeated Starry Night's mis-characterization in his own materials on 2012.
So how does this affect 2012?
Out of curiosity, in what year does the Solstice occur closest to the time when the Sun is aligned with the intersection of the ecliptic and the galactic equator?
First, it is necessary to convert the coordinates of the galactic pole from right ascension (α) and declination (δ) to longitude (λ) and latitude (β) in the ecliptic coordinate system. ε is the obliquity of the ecliptic (1950). From standard spherical geometry, we have:
cosβcosλ = cosδcosα
cosβsinλ = cosδsinαcosε + sinδsinε
Thus tanλ= (cosδsinαcosε + sinδsinε)/cosδcosα
Using the IAU values for the galactic pole coordinates and the obliquity of the ecliptic (1950): α = 12h 49m = 192.25º, δ = +27.4º, ε = 23.446º and entering them into the equations gives
tanλ = ((0.88782 x -0.21218 x 0.91744) + (0.46020 x 0.39789))/(0.88782 x -0.97723)
= (-0.17283 + 0.18311)/(-0.86760)
= -0.01185
Since sinλ is positive and cosλ is negative, λ is in the range 90º to 180º.
Therefore λ = 179.321º.
There is no need to proceed with the calculation of β as it is not required in what follows.
The two points where the galactic equator crosses the ecliptic are 90º in ecliptic longitude from this, namely 89.321º and 269.321º, close to the solstices which are at 90º and 270º. Precession causes the ecliptic longitude of celestial objects to increase with time. We can therefore answer the question as to what date the solstice points cross the galactic equator.
The rate of precession is 5028.8"/century12, ignoring second order terms. This is 0.01397º/year.
The December solstice point was 270º - 269.321º = 0.679º from the galactic equator at the beginning of 1950. The time taken for it to reach 270º is therefore 0.679/0.01397 = 48.6 years.
This gives us a date of 1950 + 48.6 = 1998.6, ie. roughly halfway through the year 1998.
This is in good agreement with the calculation by Meeus, who arrived at a date of May 1998.13
We can also deduce the likely error in this estimate. Repeating the calculation with slightly different input coordinates to see how much the date changes, we find that:
A change of +1m of time in the right ascension of the galactic pole alters the date by -16.4 years
A change of +0.1º in the declination of the galactic pole alters the date by +3.7 years
The IAU estimate of the uncertainty in the location of the galactic pole is 0.5m (0.125º) in RA and 7 arcminutes (0.117º) in Dec. Thus the error in the date of 1998.6 is +/- 12.5 years, ie. the date of the solstice point crossing the galactic equator could fall anytime during the years 1986 to 2011.
Conclusion: A calculation of the date on which the solstice points cross the galactic equator gives 1998, but the probable errors in the position of the galactic pole as defined by the IAU mean that this date could fall anytime between the years 1986 and 2011.
How often does this solstice crossing of the galactic equator occur and what is its physical significance?
We have already seen that precession advances the ecliptic longitude of celestial objects at a rate of 0.01397º/year, so in order to complete a 360º circle, the time taken must be 360/0.01397 or just under 25,800 years.
There are many claims of a unique alignment which occurs every 25,800 or, less accurately, 26,000 years. What does this mean in real physical terms? Well, not much.
Precession is simply the gradual change in the direction in which the Earth's axis points. The line of solstices is an imaginary line which is the projection of the Earth's axis on to the plane of its orbit. Like the axis, this line completes a rotation about the sky every 25,800 years. Twice during each rotation, this line crosses the galactic equator, once with the December solstice lining up roughly with the direction of the center of the galaxy and 12,900 years later with the June solstice facing in this direction.
We need to bear in mind that the direction in which the Earth's axis is pointing is important only in the vicinity of the Earth. It has no importance elsewhere in the solar system or the galaxy. There is no significance in the direction in which the axis or line of solstices happens to be pointing.
Two of the most famous sights in Geneva, Switzerland are the floral clock and the large fountain in the lake. Twice a day, the hour hand of the clock points towards the fountain. Yes, it could be called an alignment, but what does it mean? Nothing, of course. In the same way, the direction in which the Earth's axis or line of solstices is pointing means nothing in relation to distant objects.
Let's put it another way. The Earth is our viewing platform from which we look out at space. It is gradually tilting. How does this affect phenomena arising from the motions of the Sun, stars and galaxy? It requires us to point our telescopes in slightly different directions over time, but it has no effect whatsoever on these phenomena. It's rather like sitting in your backyard and viewing an eclipse. If you tilt your chair slightly backwards, does it alter the eclipse? Of course not.
Claims of a 'once in 26,000 year' physical event arising from any sort of alignment are without foundation. There is no physical relationship between the line of solstices and the galactic plane.
But didn't the Maya ALMOST get it right?
When 2012 proponents are shown that Sun aligned with the Galactic Equator in 1998, and won't align in 2012 as the Maya supposedly predicted, the proponents tend to dismiss the discrepancy as nit-packing. They then reassert the Maya's supposedly godlike powers, this time on the basis that the Maya almost got the year right. (For example, see this review of John Major Jenkins' book, The 2012 Story.) We might expect that having been refuted regarding the Galactic Equator, 2012 proponents will now proclaim that the Maya almost nailed the year of the "H1 Plane" alignment.
However, anyone who argues that the Maya were able to identify either the H1 plane or the "Galactic Equator" by means of godlike powers, must then explain why the Maya weren't able to foresee the need to transmit that information by some more-suitable means than engraving it on one perishable monument in each of two peripheral cities.
The Maya did have alternatives, especially if they could travel through time, as Jenkins claims. Surely they could have foreseen that the Codices would be burned, and that the monuments would be damaged. Why, then, didn't they bake the message into hundreds of clay tablets, then bury them in several different places where they'd be unearthed in the 20th century? King Pakal's tomb in Palenque would have been an ideal spot. The Maya could even have engraved the message in gold sheets, knowing the tomb wouldn't be found until 1952.
