Illustration of the method Shapley used to determine the position of the center of the Milky Way galaxy.  He determined that the distribution of globular clusters was centered around a point in the disk of the galaxy about 10 kpc away from the Sun in the direction of the constellation Sagittarius.  These massive star clusters would be orbiting around the center of mass of the galaxy, therefore the center of the distribution of these clusters revealed the true center of the Milky Way was not the Sun as previous star counts had indicated.

This figure illustrates the geometric relationship between the 3-D globular cluster distribution and the 2-D plot we use in our lab to determine (like Shapley) the distance and direction to the galactic center.  Once the center was determined a new "celestial" coordinate system (imaginary system of lines on the celestial sphere indicating directions on the sphere) could be defined.  This coordinate system is called galactic coordinates and these map out positons on the celestial sphere relative to the band of light we call the Milky Way .   The galactic equator (or 0^o galactic latitude) is defined as the great circle which traces out the center of the  Milky Way on the celestial sphere and 0^o galactic longitude is defined to be in the direction of the galacitic center.