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The galactic coordinate system is a
celestial coordinate system Astronomical coordinate systems are organized arrangements for specifying positions of satellites, planets, stars, galaxies, and other celestial objects relative to physical reference points available to a situated observer (e.g. the true horizo ...
in
spherical coordinates In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the ''radial distance'' of that point from a fixed origin, its ''polar angle'' meas ...
, with the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
as its center, the primary direction aligned with the approximate center of the Milky Way
Galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
, 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.


Spherical coordinates


Galactic longitude

Longitude (symbol ) measures the
angular distance Angular distance \theta (also known as angular separation, apparent distance, or apparent separation) is the angle between the two sightlines, or between two point objects as viewed from an observer. Angular distance appears in mathematics (in p ...
of an object eastward along the galactic equator from the Galactic Center. Analogous to terrestrial longitude, galactic longitude is usually measured in degrees (°).


Galactic latitude

Latitude (symbol ) measures the
angle In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle. Angles formed by two rays lie in the plane that contains the rays. Angles a ...
of an object northward of the galactic equator (or midplane) as viewed from Earth. Analogous to terrestrial
latitude In geography, latitude is a coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pole ...
, galactic latitude is usually measured in degrees (°).


Definition

The first galactic coordinate system was used by William Herschel in 1785. A number of different coordinate systems, each differing by a few degrees, were used until 1932, when
Lund Observatory Lund Observatory is the official English name for the astronomy department at Lund University. Between 1867-2001 "Lund Observatory" was also the name of the Observatory building, which is now referred to as the "Lund Old Observatory". As of Janu ...
assembled a set of conversion tables that defined a standard galactic coordinate system based on a galactic north pole at RA , dec +28° (in the
B1900.0 In astronomy, an epoch or reference epoch is a moment in time used as a reference point for some time-varying astronomical quantity. It is useful for the celestial coordinates or orbital elements of a celestial body, as they are subject to pert ...
epoch convention) and a 0° longitude at the point where the galactic plane and equatorial plane intersected. In 1958, the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...
(IAU) defined the galactic coordinate system in reference to radio observations of galactic neutral hydrogen through the hydrogen line, changing the definition of the Galactic longitude by 32° and the latitude by 1.5°. In the
equatorial coordinate system The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fu ...
, for equinox and equator of 1950.0, the north galactic pole is defined at right ascension ,
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
+27.4°, in the constellation
Coma Berenices 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 hemisphe ...
, with a probable error of ±0.1°. Longitude 0° 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, with a plane passing through the Sun and parallel to the galactic equator being 0°, whilst the poles are ±90°. Based on this definition, the galactic poles and equator can be found from spherical trigonometry and can be precessed to other
epochs In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...
; see the table. The IAU recommended that during the transition period from the old, pre-1958 system to the new, the old longitude and latitude should be designated and while the new should be designated and . This convention is occasionally seen. Radio source
Sagittarius A* Sagittarius A* ( ), abbreviated Sgr A* ( ), is the supermassive black hole at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, vi ...
, which is the best physical marker of the true Galactic Center, is located at , (J2000). Rounded to the same number of digits as the table, , −29.01° (J2000), there is an offset of about 0.07° from the defined coordinate center, well within the 1958 error estimate of ±0.1°. Due to the Sun's position, which currently lies north of the midplane, and the heliocentric definition adopted by the IAU, the galactic coordinates of Sgr A* are latitude south, longitude . Since as defined the galactic coordinate system does not rotate with time, Sgr A* is actually decreasing in longitude at the rate of galactic rotation at the sun, , approximately 5.7
milliarcsecond A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...
s per year (see Oort constants).


Conversion between equatorial and galactic coordinates

An object's location expressed in the
equatorial coordinate system The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fu ...
can be transformed into the galactic coordinate system. In these equations, is right ascension, is
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
. NGP refers to the coordinate values of the north galactic pole and NCP to those of the north celestial pole. : \begin \sin(b) &= \sin(\delta_\text)\sin(\delta)+\cos(\delta_\text)\cos(\delta)\cos(\alpha-\alpha_\text) \\ \cos(b) \sin(l_\text-l) &= \cos(\delta)\sin(\alpha-\alpha_\text) \\ \cos(b) \cos(l_\text-l) &= \cos(\delta_\text) \sin(\delta) - \sin(\delta_\text) \cos(\delta) \cos(\alpha - \alpha_\text) \end The reverse (galactic to equatorial) can also be accomplished with the following conversion formulas. : \begin \sin (\delta) &= \sin (\delta_\text) \sin(b) + \cos (\delta_\text) \cos(b) \cos(l_\text - l) \\ \cos (\delta) \sin(\alpha - \alpha_\text) &= \cos(b) \sin(l_\text - l) \\ \cos (\delta) \cos(\alpha - \alpha_\text) &= \cos (\delta_\text) \sin(b) - \sin(\delta_\text) \cos(b) \cos(l_\text - l) \end


Rectangular coordinates

In some applications use is made of rectangular coordinates based on galactic longitude and latitude and distance. In some work regarding the distant past or future the galactic coordinate system is taken as rotating so that the -axis always goes to the centre of the galaxy.For example There are two major
rectangular In Euclidean plane geometry, a rectangle is a quadrilateral with four right angles. It can also be defined as: an equiangular quadrilateral, since equiangular means that all of its angles are equal (360°/4 = 90°); or a parallelogram containin ...
variations of galactic coordinates, commonly used for computing space velocities of galactic objects. In these systems the -axes are designated , but the definitions vary by author. In one system, the axis is directed toward the Galactic Center ( = 0°), and it is a
right-handed In human biology, handedness is an individual's preferential use of one hand, known as the dominant hand, due to it being stronger, faster or more dextrous. The other hand, comparatively often the weaker, less dextrous or simply less subject ...
system (positive towards the east and towards the north galactic pole); in the other, the axis is directed toward the galactic anticenter ( = 180°), and it is a left-handed system (positive towards the east and towards the north galactic pole).


In the constellations

The galactic equator runs through the following constellations: * Sagittarius * Serpens *
Scutum The ''scutum'' (; plural ''scuta'') was a type of shield used among Italic peoples in antiquity, most notably by the army of ancient Rome starting about the fourth century BC. The Romans adopted it when they switched from the military formatio ...
* Aquila *
Sagitta Sagitta is a dim but distinctive constellation in the northern sky. Its name is Latin for 'arrow', not to be confused with the significantly larger constellation Sagittarius 'the archer'. It was included among the 48 constellations listed by t ...
* Vulpecula * Cygnus * Cepheus * Cassiopeia *
Camelopardalis Camelopardalis is a large but faint constellation of the northern sky representing a giraffe. The constellation was introduced in 1612 or 1613 by Petrus Plancius. Some older astronomy books give Camelopardalus or Camelopardus as alternative fo ...
*
Perseus In Greek mythology, Perseus ( /ˈpɜːrsiəs, -sjuːs/; Greek: Περσεύς, translit. Perseús) is the legendary founder of Mycenae and of the Perseid dynasty. He was, alongside Cadmus and Bellerophon, the greatest Greek hero and slayer of ...
*
Auriga AURIGA (''Antenna Ultracriogenica Risonante per l'Indagine Gravitazionale Astronomica'') is an ultracryogenic resonant bar gravitational wave detector in Italy. It is at the Laboratori Nazionali di Legnaro of the Istituto Nazionale di Fisica Nuclea ...
*
Taurus Taurus is Latin for 'bull' and may refer to: * Taurus (astrology), the astrological sign * Taurus (constellation), one of the constellations of the zodiac * Taurus (mythology), one of two Greek mythological characters named Taurus * ''Bos tauru ...
* Gemini * Orion * Monoceros *
Canis Major Canis Major is a constellation in the southern celestial hemisphere. In the second century, it was included in Ptolemy's 48 constellations, and is counted among the 88 modern constellations. Its name is Latin for "greater dog" in contrast to ...
*
Puppis Puppis is a constellation in the southern sky. Puppis, the Latin translation of "poop deck", was originally part of an over-large constellation Argo Navis (the ship of Jason and the Argonauts), which centuries after its initial description, was ...
* Vela *
Carina Carina may refer to: Places Australia * Carina, Queensland, a suburb in Brisbane * Carina Heights, Queensland, a suburb in Brisbane * Carina, Victoria, a locality in Mildura Serbia * Carina, Osečina, a village in the Kolubara District ...
*
Crux Crux () is a constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross. It lies on the southern end of the Milky Way's visible band. The name ''Crux'' is Latin for c ...
*
Centaurus Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. I ...
*
Circinus Circinus is a small, faint constellation in the southern sky, first defined in 1756 by the French astronomer Nicolas-Louis de Lacaille. Its name is Latin for compass, referring to the drafting tool used for drawing circles (it should not be co ...
* Norma * Ara * Scorpius * Ophiuchus


See also

* * * * *


References


External links


Universal coordinate converter

Galactic Coordinate System - Wolfram Demonstration


The
Internet Encyclopedia of Science David Darling (born 29 July 1953 in Glossop, Derbyshire) is an English astronomer, freelance science writer, and musician. Darling has published numerous popular science works, including ''Life Everywhere: The Maverick Science of Astrobiolo ...
* Fiona Vincent
Positional Astronomy: Galactic coordinates
University of St Andrews (Aien aristeuein) , motto_lang = grc , mottoeng = Ever to ExcelorEver to be the Best , established = , type = Public research university Ancient university , endowment ...

An Atlas of the Universe
{{Portal bar, Stars, Spaceflight, Outer space, Solar System Astronomical coordinate systems Milky Way Orientation (geometry)