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The galactic coordinate system is a celestial coordinate system centered on the Sun, used to indicate directions into the celestial sphere, that is fixed to the galactic plane and to the line from the Sun to the galactic center. Spherical-coordinate elements:
The designator G has been used (alone) with galactic coordinates to identify an object, probably most often for radio sources. An example is G1.9+0.3.
A current standardization of the system, the IAU galactic coordinate system, was approved by the International Astronomical Union (IAU) in 1958 based upon the data available at the time regarding the locations in the celestial sphere of the galactic plane and the direction toward the galactic center. For the standardized coordinates, the plane and direction chosen had convenient coordinates that were within the range of determinations of the time. The coordinates are naturally unaffected by the precession of the equinoxes, but they are also defined to explicitly ignore new determinations of the plane and direction, and to explicitly ignore subsequent changes to the direction of the galactic center due to the Sun's motion: the Sun's orbit around the Milky Way produces a shift in the actual direction toward the galactic center on the order of five milliarcseconds per year. The galactic coordinates of quasars should be stable over our lifetime, but those of Milky-Way objects still depend upon an epoch due to their peculiar velocities. The directions selected for standardization:
Conversion to/from J2000-based (and future) equatorial coordinates are built on the standard methods to convert between such newer coordinates and B1950.
Rectangular galactic coordinates based on the same galactic plane and line are also used.
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