darkdave3000 said:
Are we (Earth & Sun) considered part of the galactic plane? Or are we slightly above/below it? I'm asking because I want to make sense of your picture. Does the picture show the galactic plane being a 1 dimensional line (because it is perpendicular to us)?
Also is the intersection occurring right on the galactic core?
I think there's something that should have been made clear earlier. There's a difference between the supergalactic plane - the structure, and supergalactic plane - the equatorial plane of the coordinate system.
The former is a roughly (with emphasis on roughly) pancake-shaped arrangement of nearby galaxies (of which Milky Way is a part), with some spatial extent and shape that is not all that precisely known, and not necessarily all that planar. The latter is a plane bisecting the celestial sphere of any observer, that was chosen to coincide roughly with the plane of the structure (as it was thought to be some years back), and which is precisely defined.
In the discussion so far, I was focusing on the coordinate system, but I'm starting to think you were thinking of the structure instead.
The relationship between the two is like the relationship between the plane of the Milky Way and the equatorial plane of the galactic coordinate system. Or the plane of Earth's equator and the equatorial plane of the equatorial coordinate system. You can be some place in space w/r to and you can point to some location of the former, but the latter - while parallel to the physical planes - extend to infinity and pass through the observer (bisecting the celestial sphere in two).
So, you can say you are somewhere w/r to the galactic plane (above, below), but the galactic plane in the coordinate system passes through the observer. You can say you are somewhere w/r to the plane of the Earth's equator, but the equatorial plane of the coordinate system passes through the observer. Same with the physical supergalactic plane and the equatorial plane of the supergalactic coordinate system.
It doesn't matter where you are w/r to the physical structure as far as using the relevant coordinate system goes. You can always use those coordinates, even if you're so far away from the structure (e.g. the Milky Way), that you can't see it. At which point it would no longer have much physical significance, and be just a largely arbitrary way of dividing the celestial sphere.
If you want to read about the physical structure, follow the paper linked to at the bottom of the Wiki page on the coordinate system (
here it is for convenience). Note that they're using Cartesian supergalactic coordinates, which have the Z axis pointing from the Earth towards the north pole of the spherical SGP coordinates, and X towards the 0,0 point of the same (Y being at 90 degrees to both).
They have some helpful planar projections of galactic distributions in Fig.1, and it's worth noting the general 'uh, not that much or a plane, really' conclusion.Back to that 2MASS picture - treat the supergalactic plane as if we're being inside another galaxy, only larger. We see the Milky Way as a line on the sky, because we're inside of it. Same with the SGP. Just as you can see the band of the MW angled w/r to the ecliptic at 60-odd degrees, because the Solar System has the planets orbit in a plane at that angle to the galactic plane, you can similarly see the band of the SGP (such as it is) angled w/r to the galactic plane because the plane of galactic rotation is at close to 90 degrees to the SGP.
The intersection is not at the galactic centre, but at the galactic coordinates given by Wikipedia. So over 137 degrees away from the galactic centre.
One can imagine seeing the same picture drawn in galactic coordinates (whose 0,0 point is towards the centre).
It'd be rotated so that the plane of the Milky Way goes along the equator, the SGP is angled at 6-odd degrees to the horizontal (at the equator), and offset to the sides where it'd intersect the equator at roughly 137 and 137+180 degrees galactic longitude.
And indeed, here's such one picture:
And here's the same thing with coordinates drawn:
The SGP is visible as the roughly ring-like structure that crosses the equator at approx 150 and 330 degrees.
Remember that this is the Mollweide projection, similar to map projections of Earth globe of the same shape. It distorts shapes more and more towards the perimeter of the projection. So while SGP is a lopsided ring on the above two maps, it is a line (or more precisely, a great circle) running across the entire sky as viewed by an observer.
