English: Spherical triangles for deriving relationship between equatorial and galactic coordinate systems. Yellow plane is celestial equator, P and P' are celestial poles. Green plane is galactic plane, G and G' are galactic poles. B is the direction towards galactic center, which is now used as a starting point for measuring galactic longitude. Prior to 1959, starting point was intersection of galactic and equatorial plane (point C).
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