Galactic Coordinate System and distance

If the Galactic Coordinate System is made up of two values, angles of longitude and latitude, how is distance measured? If you have those two values, you get a line from the sun in a certain direction. There might be multiple objects on that line, how can you specify one of them in the coordinate?

• I prefer the X,Y,Z galactic coordinate system: In that system the sun is at 0,0,0. Positive X is in the direction toward the galactic center (The sun is approximately 70 light years above the galactic plane, depending on what source you read since the plane is not well defined. (ie. Humphreys and Larsen ApJ May 31,1995) This is obviously insignificant compared to the distance to the galactic center. Y is parallel to the plane, perpendicular to the X axis and positive in the direction of galactic rotation (which is counterclockwise if you consider the sun "above" the plane and you are looking " – Jack R. Woods Dec 19 '19 at 1:27
• down" on the plane from the sun). The Z axis then is obviously perpendicular to X and Y and positive Z is in the "Galactic North" direction. (ie. using the approximate figure, the galactic plane would be at Z = -70 LY) Once you have galactic coordinates in this system (there are a couple of good websites that will give you these (year 2000 coordinates)), it's child's play to make a 3D map of local stars. – Jack R. Woods Dec 19 '19 at 1:27

You need three coordinates to represent a point in 3-dimensional space. Galactic coordinates are a 2-dimensional coordinate system on the surface of the abstract celestial sphere. Typically it is easier to measure position on the celestial sphere than it is to measure astronomical distances, so usually the systems are given in those terms. For 3D you need to supplement the Galactic coordinate system with distance as a separate coordinate, this is in fact the natural choice for a 3D spherical coordinate system.

Remember that the galactic coordinate system has its origin in the same place as equatorial coordinates (right ascension, declination), so the distance measurement is the same: you don't have to translate it relative to the Galactic Centre. To measure it, you use the usual techniques used to determine the cosmic distance ladder: parallaxes, standard candles (Cepheid variables, etc.), redshifts...

• Good answer. You'd think the galactic coordinate system's origin would be the center of the galaxy, but, since we don't accurately know how far the center of the galaxy, our own coordinates would be inaccurate. Thus, we center galactic coordinates at our solar system's barycenter (SSB) – user21 Dec 4 '19 at 16:29
• I can accept some concessions made for inaccuracy, and centers of reference are largely arbitrary, anyway - but it offends me that somebody had the audacity to call a reference system "galactic coordinates" when it doesn't even contain all the information necessary to find where something is in the galaxy. Not your fault, though, presumably. – Erhannis Dec 26 '20 at 22:43