Many galaxies, including the Milky Way in which our Sun and Earth are located, are disk-shaped: the majority of their visible mass (excluding possible dark matter) lies very close to a plane. It is sometimes convenient to use this galactic plane as the basis of a galactic coordinate system, where the directions perpendicular to the plane point to the galactic poles, creating a spherical coordinate system. In actual usage, these terms are nearly always used to refer specifically to the plane and poles and coordinate system of the Milky Way rather than any other galaxy. By convention galactic latitude and galactic longitude are usually denoted by b and l, respectively.

Defining the galactic plane is slightly imprecise and arbitrary since the stars are not perfectly coplanar. In 1959, the IAU defined the postion of the Milky Way's north galactic pole to be exactly RA=12h49m, Dec=27°24′ in the then-used B1950 epoch; in the currently-used J2000 epoch, after precession is taken into account, its position is RA 12h51m26.282s, Dec 27°07′42.01″. This position is in Coma Berenices, near the bright star Arcturus; likewise, the south galactic pole lies in the constellation Sculptor.

The "zero of longitude" of galactic coordinates was also defined in 1959 to be at position angle 123° from the north galactic pole. Thus the zero longitude point on the galactic equator was at 17h42m26.603s, −28°55′00.445″ (B1950) or 17h45m37.224s, −28°56′10.23″ (J2000), and its J2000 position angle is 122.932°. The galactic center is located at position angle 31.72° (B1950) or 31.40° (J2000) east of north.

Recent observations using the Hipparcos satellite have permitted a more accurate determination of the galactic pole than data available in 1959; nevertheless, the coordinate system defined in 1959 remains in use. Current data indicates that the true position of the galactic pole is l_g=0.°004±0.°039; b_g=89.°427±0.°035, and that the Sun is located approximately 34.56±0.56 pc (112.7±1.8 ly) above the galactic plane [1]. Since the Sun is about 30,000 ly from the galactic center, it is relatively speaking extremely close to the galactic plane.

External references

• http://arxiv.org/pdf/astro-ph/0408107 (appendix provides numbers used above)
• http://star-www.st-and.ac.uk/~fv/webnotes/chapter8.htm