Galactic coordinate system

The Galactic coordinate system is defined by the International Astronomical Union (IAU) in 1958, replacing an earlier system introduced in 1932. The reference plane is the mean plane of the Milky Way's disk, and the zero point of galactic longitude is the direction of the galactic center, as determined from radio observations of Sagittarius A*. The north galactic pole is located in the constellation Coma Berenices, at right ascension 12h 51m 26.28s and declination +27° 07′ 42.01″ (J2000). The system is a right-handed, spherical coordinate system, with galactic latitude measured from the galactic plane (positive toward the north) and galactic longitude measured counterclockwise (as seen from the north) from the galactic center direction.

Transformations between equatorial and Galactic coordinates are performed using a rotation matrix that accounts for the orientation of the galactic plane relative to the celestial equator. The transformation parameters are well-established for the J2000 epoch, with the galactic north pole at (RA, Dec) = (192.8595°, 27.1284°) and the galactic center at (l = 0°, b = 0°) corresponding to equatorial coordinates (RA, Dec) = (17h 45m 37.2s, −28° 56′ 10″). These transformations are essential for mapping surveys, such as the Sloan Digital Sky Survey and Gaia, which often present data in Galactic coordinates to facilitate studies of the Milky Way's structure.

Galactic coordinates are widely used in studies of the Milky Way's structure, including mapping spiral arms, the central bar, and the distribution of interstellar dust and gas. Radio and infrared surveys, such as the HI4PI survey of neutral hydrogen and the Spitzer GLIMPSE survey, use Galactic coordinates to organize data across the plane. The system also underpins the definition of the Galactic plane's inclination and the Sun's position relative to it, which is about 25 parsecs above the plane.¹ In extragalactic astronomy, Galactic coordinates help correct for foreground extinction by the Milky Way's disk.

The Galactic coordinate system has a subtle but important quirk: the zero point of longitude is not exactly the dynamical center of the Milky Way, but rather the radio source Sagittarius A*, which is believed to coincide with the supermassive black hole at the center. The offset between the two is negligible for most purposes, but it is a defined convention. Additionally, the system is sometimes used in a heliocentric (Sun-centered) sense, but it is not an inertial frame; the Sun orbits the galactic center, so the system rotates slowly over time. For precise astrometry, the system is tied to the International Celestial Reference Frame (ICRF), and the transformation parameters are updated with precession models. A lesser-known fact is that the galactic plane is not perfectly flat; it warps in the outer regions, so the coordinate system's reference plane is an idealized mean, not a physical surface.

Galactic longitude

Angular coordinate measured along the galactic plane, from 0° to 360°, with 0° pointing toward the galactic center.

Galactic latitude

Angular distance north or south of the galactic plane, ranging from -90° to +90°.

Sagittarius A*

The compact radio source at the center of the Milky Way, coinciding with the supermassive black hole.