A galactic quadrant, or quadrant of the Galaxy, is one of four circular sectors in the division of the Milky Way Galaxy.
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 (by humans) to date have been based on Earth or within the Solar System.
Quadrants are described using ordinals—for example, "1st galactic quadrant", [1] "second galactic quadrant", [2] or "third quadrant of the Galaxy". [3] 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 (where l is galactic longitude):
| Quad | Constellations (Zodiacal constellations in bold) |
|---|---|
| NGQ1 | 7 (Lyra, Hercules, Serpens, Corona Borealis, Ophiuchus, Cygnus, Boötes) |
| SGQ1 | 11 (Sagittarius, Capricornus, Aquarius, Equuleus, Delphinus, Sagitta, Aquila, Vulpecula, Microscopium, Scutum, Piscis Austrinus) |
| NGQ2 | 8 (Cepheus, Draco, Ursa Minor, Canes Venatici, Ursa Major, Lynx, Camelopardalis, Auriga) |
| SGQ2 | 10 (Pisces, Aries, Taurus, Andromeda, Cassiopeia, Cetus, Perseus, Triangulum, Lacerta, Pegasus) |
| NGQ3 | 9 (Gemini, Cancer, Leo, Canis Minor, Sextans, Hydra, Monoceros, Pyxis, Leo Minor) |
| SGQ3 | 9 (Orion, Eridanus, Caelum, Lepus, Pictor, Columba, Canis Major, Puppis, Fornax) |
| NGQ4 | 12 (Virgo, Libra, Scorpius, Coma Berenices, Corvus, Crater, Lupus, Centaurus, Norma, Crux, Antlia, Vela) |
| SGQ4 | 22 (Circinus, Musca, Telescopium, Triangulum Australe, Apus, Chamaeleon, Corona Australis, Pavo, Indus, Grus, Octans, Sculptor, Phoenix, Reticulum, Dorado, Mensa, Tucana, Volans, Carina, Ara, Hydrus, Horologium) |
Due to the orientation of the Earth with respect 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 celestial 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 quadrants are: [5] (Note: by drawing a line through the following, one can also approximate the galactic equator.)
 | This section needs expansion. You can help by adding to it. (August 2010) |
A long tradition of dividing the visible skies into four precedes the modern definitions of four galactic quadrants. Ancient Mesopotamian formulae spoke of "the four corners of the universe" and of "the heaven's four corners", [6] and the Biblical Book of Jeremiah echoes this phraseology: "And upon Elam will I bring the four winds from the four quarters of heaven" (Jeremiah, 49:36). Astrology too uses quadrant systems to divide up its stars of interest. And the astronomy of the location of constellations sees each of the Northern and Southern celestial hemispheres divided into four quadrants.
"Galactic quadrants" within Star Trek are based around a meridian that runs from the center of the Galaxy through Earth's Solar System, [7] 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 (CGPS) created a radio map of the Galaxy based on Star Trek's quadrants, joking that "the CGPS is primarily concerned with Cardassians, while the SGPS (Southern Galactic Plane Survey) focuses on Romulans". [8]
"Galactic quadrants" within Star Wars canon astrography map depicts a top-down view of the galactic disk, with "Quadrant A" (i.e. "north") as the side of the galactic center that Coruscant is located on. As the capital planet of the Republic and later the Empire, Coruscant is used as the reference point for galactic astronomy, set at XYZ coordinates 0-0-0. Standardized galactic time measurements are also based on Coruscant's local solar day and year.
The Imperium of Man's territory in the Milky Way Galaxy in Warhammer 40,000 is divided into five zones, known as "segmentae". [9] Navigation in the Milky Way is also identified with cardinal directions, indicating distance from the Sol System: for example, Ultima Segmentum, the largest segmentum in the Imperium of Man, is located to the galactic east of the Sol System. The 0° "north" in Imperial maps does not correspond to the 0° in the real-world.
Caelum is a faint constellation in the southern sky, introduced in the 1750s by Nicolas Louis de Lacaille and counted among the 88 modern constellations. Its name means "chisel" in Latin, and it was formerly known as Caelum Sculptorium ; it is a rare word, unrelated to the far more common Latin caelum, meaning "sky", "heaven", or "atmosphere". It is the eighth-smallest constellation, and subtends a solid angle of around 0.038 steradians, just less than that of Corona Australis.
Fornax is a constellation in the southern celestial hemisphere, partly ringed by the celestial river Eridanus. Its name is Latin for furnace. It was named by French astronomer Nicolas Louis de Lacaille in 1756. Fornax is one of the 88 modern constellations.
Ophiuchus is a large constellation straddling the celestial equator. Its name comes from the Ancient Greek ὀφιοῦχος (ophioûkhos), meaning "serpent-bearer", and it is commonly represented as a man grasping a snake. The serpent is represented by the constellation Serpens. Ophiuchus was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. An old alternative name for the constellation was Serpentarius.
Pyxis is a small and faint constellation in the southern sky. Abbreviated from Pyxis Nautica, its name is Latin for a mariner's compass. Pyxis was introduced by Nicolas-Louis de Lacaille in the 18th century, and is counted among the 88 modern constellations.
Sagittarius is one of the constellations of the zodiac and is located in the Southern celestial hemisphere. It is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations. Its old astronomical symbol is (♐︎). Its name is Latin for "archer". Sagittarius is commonly represented as a centaur drawing a bow. It lies between Scorpius and Ophiuchus to the west and Capricornus and Microscopium to the east.
Virgo is one of the constellations of the zodiac. Its name is Latin for maiden, and its old astronomical symbol is . Between Leo to the west and Libra to the east, it is the second-largest constellation in the sky and the largest constellation in the zodiac. The ecliptic intersects the celestial equator within this constellation and Pisces. Underlying these technical two definitions, the sun passes directly overhead of the equator, within this constellation, at the September equinox. Virgo can be easily found through its brightest star, Spica.
Galactic astronomy is the study of the Milky Way galaxy and all its contents. This is in contrast to extragalactic astronomy, which is the study of everything outside our galaxy, including all other galaxies.
In astronomy, coordinate systems are used for specifying positions of celestial objects relative to a given reference frame, based on physical reference points available to a situated observer. Coordinate systems in astronomy can specify an object's position in three-dimensional space or plot merely its direction on a celestial sphere, if the object's distance is unknown or trivial.
The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane parallel to an approximation of the galactic plane but offset to its north. It uses the right-handed convention, meaning that coordinates are positive toward the north and toward the east in the fundamental plane.
Coma Berenices is an ancient asterism in the northern sky, which has been defined as one of the 88 modern constellations. It is in the direction of the fourth galactic quadrant, between Leo and Boötes, and it is visible in both hemispheres. Its name means "Berenice's Hair" in Latin and refers to Queen Berenice II of Egypt, who sacrificed her long hair as a votive offering. It was introduced to Western astronomy during the third century BC by Conon of Samos and was further corroborated as a constellation by Gerardus Mercator and Tycho Brahe. It is the only modern constellation named for a historic person.
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.
The Galactic Center is the rotational center and the barycenter of the Milky Way. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact radio source which is almost exactly at the galactic rotational center. The Galactic Center is approximately 8 kiloparsecs (26,000 ly) away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest, visually close to the Butterfly Cluster (M6) or the star Shaula, south to the Pipe Nebula.
The Orion Arm, also known as the Orion–Cygnus Arm, is a minor spiral arm within the Milky Way Galaxy spanning 3,500 light-years in width and extending roughly 10,000 light-years in length. This galactic structure encompasses the Solar System, including Earth. It is sometimes referred to by alternate names such as the Local Arm or Orion Bridge, and it was previously identified as the Local Spur or the Orion Spur. It should not be confused with the outer terminus of the Norma Arm, known as the Cygnus Arm.
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
The Carina–Sagittarius Arm is generally thought to be a minor spiral arm of the Milky Way galaxy. Each spiral arm is a long, diffuse curving streamer of stars that radiates from the Galactic Center. These gigantic structures are often composed of billions of stars and thousands of gas clouds. The Carina–Sagittarius Arm is one of the most pronounced arms in our galaxy as many HII regions, young stars and giant molecular clouds are concentrated in it.
The Perseus Arm is one of two major spiral arms of the Milky Way galaxy. The second major arm is called the Scutum–Centaurus Arm. The Perseus Arm begins from the distal end of the long Milky Way central bar. Previously thought to be 13,000 light-years away, it is now thought to lie 6,400 light years from the Solar System.
In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler. However, its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve, and its effect on the colors of stars had been observed by a number of individuals who did not connect it with the general presence of galactic dust. For stars lying near the plane of the Milky Way which are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies is roughly 1.8 magnitudes per kiloparsec.
The Milky Way is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος, meaning "milky circle". From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.
| Location | Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow (→) may be read as "within" or "part of". |  | |||||
|---|---|---|---|---|---|---|---|
| Structure |
| ||||||
| Satellite galaxies | |||||||
| Related | |||||||
