[NMSU, N. Vogt]

When astronomers study the structure of the Galaxy, they often find it more convenient to define the positions of stars in the galactic coordinate system rather than the equatorial coordinate system. The reference plane of the galactic coordinate system is the disk of our Galaxy and the intersection of this plane with the celestial sphere is known as the galactic equator, which is inclined by about 63° to the celestial equator. Galactic latitude, b, is analogous to declination, but measures distance north or south of the galactic equator, attaining +90° at the north galactic pole (NGP) and -90° at the south galactic pole (SGP). The galactic latitude of the star X is arc YX and is north.

Galactic longitude, l, is analogous to right ascension and is measured along the galactic equator in the same direction as right ascension. The zero-point of galactic longitude is in the direction of the Galactic Centre (GC), in the constellation of Sagittarius; it is defined precisely by taking the galactic longitude of the north celestial pole to be exactly 123°. The galactic longitude of the star X is given by the angle between GC and Y.