The galactic coordinate system
is a celestial coordinate system
which is centered on the Sun
and is aligned with the apparent center of the Milky Way galaxy
. The "equator" is aligned to the galactic plane. Similar to geographic coordinates, positions in the galactic coordinate system have latitudes and longitudes.
The International Astronomical Union
(IAU) defined the galactic coordinate system in reference to the Equatorial coordinate system
in 1958. The north galactic pole is defined to be at right ascension
), and the zero of longitude is the great semicircle that originates from this point along the line in position angle
123° with respect to the equatorial pole. The galactic longitude increases in the same direction as right ascension. Galactic latitude is positive towards the north galactic pole, the poles themselves at ±90° and the galactic equator being zero.
The equivalent system referred to as J2000 has the north galactic pole at (J2000), the zero of longitude at the position angle of 122.932°. The point in the sky at which the galactic latitude and longitude are both zero is (J2000). This is offset slightly from the radio source Sagittarius A*, which is the best physical marker of the true galactic center. Sagittarius A* is located at (J2000), or galactic longitude , galactic latitude .
The symbols l and b are used to represent the galactic longitude and latitude, respectively.