Galactic quadrant
A galactic quadrant, or quadrant of the Galaxy, is one of four circular sectors in the division of the Milky Way Galaxy.
Quadrants in the galactic coordinate system
In actual astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the pole of the mapping system. The Sun is used instead of the Galactic Center for practical reasons since all astronomical observations to date have been based on Earth or within the Solar System.Delineation
Quadrants are described using ordinals—for example, "1st galactic quadrant", "second galactic quadrant", or "third quadrant of the Galaxy". Viewing from the north galactic pole with 0 degrees as the ray that runs starting from the Sun and through the galactic center, the quadrants are as follows :- 1st galactic quadrant – 0° ≤ ≤ 90°
- 2nd galactic quadrant – 90° ≤ ≤ 180°
- 3rd galactic quadrant – 180° ≤ ≤ 270°
- 4th galactic quadrant – 270° ≤ ≤ 360°
Constellations grouped by galactic quadrants
| Quad | Constellations |
| NGQ1 | 7 |
| SGQ1 | 11 |
| NGQ2 | 8 |
| SGQ2 | 10 |
| NGQ3 | 9 |
| SGQ3 | 9 |
| NGQ4 | 12 |
| SGQ4 | 22 |
Visibility of each galactic quadrant
Due to the orientation of the Earth to the rest of the galaxy, the 2nd galactic quadrant is primarily only visible from the Northern Hemisphere while the 4th galactic quadrant is mostly only visible from the Southern Hemisphere. Thus, it is usually more practical for amateur stargazers to use the northern or southern celestial hemisphere quadrants. Nonetheless, cooperating or international astronomical organizations are not so bound by the Earth's horizon.Based on a view from Earth, one may look towards major constellations for a rough sense of where the borders of the galactic quadrants are:
- For 0°, look towards the Sagittarius constellation.
- For 90°, look towards the Cygnus constellation.
- For 180°, look towards the Auriga constellation.
- For 270°, look towards the Vela constellation.
Traditional fourfold divisions of the skies (celestial quadrants)
Ancient Mesopotamian formulae spoke of "the four corners of the universe" and of "the heaven's four corners",
and the Biblical Book of Jeremiah echoes this phraseology: "And upon Elam will I bring the four winds from the four quarters of heaven". Astrology too uses quadrant systems to divide up its stars of interest.
The astronomy of the location of constellations uses the Northern and Southern celestial hemisphere divisions into four celestial quadrants each with the entire sky comprising a total of eight regions of equal size and shape - four northern celestial quadrants and four southern celestial quadrants.
Each hemisphere's four quadrants span six of the twenty-four hours of right ascension - hence, the first quadrant runs from 0h00 to 6h00 RA, the second from 6h00 to 12h00 RA, and so on and each quadrant is labelled using NQ or SQ respectively : as an example, "SQ3" would identify Southern Celestial Quadrant 3, the area between 12h00 and 18h00 right ascension in the southern celestial hemisphere.
In fiction
''Star Trek''
"Galactic quadrants" within Star Trek are based around a meridian that runs from the center of the Galaxy through Earth's Solar System, which is not unlike the system used by astronomers. However, rather than have the perpendicular axis run through the Sun, as is done in astronomy, the Star Trek version runs the axis through the galactic center. In that sense, the Star Trek quadrant system is less geocentric as a cartographical system than the standard. Also, rather than use ordinals, Star Trek designates them by the Greek letters Alpha, Beta, Gamma, and Delta.The Canadian Galactic Plane Survey created a radio map of the Galaxy based on Star Treks quadrants, joking that "the CGPS is primarily concerned with Cardassians, while the SGPS focuses on Romulans".