The Six Conditions of the Galactic Coordinate System

1. The physical basis for the Galactic System is the set of 21-cm radio observations of the observed distribution of cold, neutral, atomic hydrogen in our Galaxy.
2. The Fundamental Great Circle (FGC) of the system is the median plane of the hydrogen distribution, called the "Galactic Equator."
3. The "poles" of the system are the North Galactic Pole (NGP, denoted " G " in the accompanying diagram, to the right) and the South Galactic Pole (SGP, denoted " G' " in the figure).
4. The secondary great circles are "circles of galactic longitude."
5. The origins associated with each coordinate are:
   • The latitudinal coordinate is zeroed at the Galactic Equator, increasing positively towards the NGP, and decreasing negatively towards the SGP.
   • The Galactic Longitude increases positively eastwards (i.e., counter-clockwise, which is opposite to the direction of galactic rotation) from the center of the Galaxy (denoted by " C " in the diagram) to the intersection of the object's circle of longitude with the Galactic Equator (point 'M' in the diagram).
6. The systemic coordinates are:
   • the Galactic Latitude (b), expressed in arc-units: - 90° ≤ b ≤ + 90°;
   • the Galactic Longitude (l), expressed in arc-units: 0° ≤ l ≤  360°