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I know a galactic co-ordinate system exists already, but it's dependent on the position of Earth. What if Sagittarius A* itself were used as the centre of a compass? What objects would best define its North (opposite direction of Earth), East, and West points, as well as up and down? I know "Elnath" is normally used to define the anticentre, AKA South (I think?).

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  • $\begingroup$ The problem with this map is that the directions are relative to the Earth, not to the centre of the galaxy. We know nothing about North (because it is behind the galaxy's centre) We can't see anything in that direction at all. We know little about West and East, they are too far away and also are hidden behind thick bands of dust. In fact pretty much all the bright star are inside the little circle marking the location of Earth. Elnath, for example is a pretty ordinary star, there are millions like it in the same direction (but most are far far away) $\endgroup$
    – James K
    Apr 23, 2023 at 7:34
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    $\begingroup$ If we could zoom around the Galaxy, and had a good map of it, a coordinate system centred on Sag A* would make sense. But we're currently stuck in the Solar System, so it's more practical to use the SSB (Solar System Barycentre) as the origin. See en.wikipedia.org/wiki/… $\endgroup$
    – PM 2Ring
    Apr 23, 2023 at 9:18
  • $\begingroup$ There are millions of stars in those directions but they're too far away and hidden behind dust and gas. So there is no definite answer to this question. I think it should be closed $\endgroup$
    – James K
    Apr 23, 2023 at 14:52

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From Earth, we cannot see any objects directly behind the Milky Way's center (at the center of the Zone of Avoidance), so we don't have any known objects available to define North in the new compass system you've drawn. Also, as mentioned in the comments, this system is still dependent on the location of the Earth at any given time.

If we want to remove the dependence on Earth, we could arbitrarily choose any two orthogonal directions as North and Up (all the other directions can then be determined). When choosing objects to define North, South, East, West, Up, and Down, it would be best to use distant extragalactic objects that don't change position in our observable sky much as we move around the galaxy.

It might be worth noting that a useful coordinate system with the galactic center at the origin already exists. It is defined as a cylindrical coordinate system with three coordinates. Here we'll call them $(R, \theta, z)$, where $R$ is the distance from the galactic center, $\theta$ is the angle of separation between the Sun and the object as viewed from the Galactic center, and $z$ is the height of the object above or below the galactic disk (where "above" is towards the North Galactic Pole). The Sun is then roughly at cylindrical galactic coordinates $(8.1\ \mathrm{kpc}, 0^\mathrm{o}, 17.4\pm1.9\ \mathrm{pc})^*$. This coordinate system is very useful for analyzing the kinematics and dynamics of our galaxy.

In reality, defining the coordinate system off of the position of the Sun/Earth makes the math easier, since all of the observations we've made are from within the Sun/Earth system.


*See This Wikipedia article and How far is the Earth/Sun above/below the galactic plane, and is it heading toward/away from it?
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