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Encyclopedia > Galactic coordinate system
The anisotropy of the star density in the night sky makes the galactic coordinate system very useful for coordinating surveys, both those which require high densities of stars (at low galactic latitudes) and those which require a low density of stars (at high galactic latitudes)
The anisotropy of the star density in the night sky makes the galactic coordinate system very useful for coordinating surveys, both those which require high densities of stars (at low galactic latitudes) and those which require a low density of stars (at high galactic latitudes)

Many galaxies, including the Milky Way in which our Sun and Earth are located, are disk-shaped: the majority of their visible mass (excluding possible dark matter) lies very close to a plane. Also the Milky Way galaxy rotates and so has an axial center as the Earth does. The galactic coordinate system makes use of these properties to define a system of coordinates that is fixed to the rotation of the Sun around the Milky Way's axis of rotation. In this way galactic coordinates abstracts from this rotation to mark the position of stars within this rotation. As objects participate in this rotation their galactic coordinates will remain constant. However as these objects move about within this rotation their galactic coordinates will change. Image File history File linksMetadata Download high-resolution version (1500x772, 453 KB) Original caption: The Infrared Milky Way This panoramic view encompasses the entire sky as seen by Two Micron All-Sky Survey. ... Image File history File linksMetadata Download high-resolution version (1500x772, 453 KB) Original caption: The Infrared Milky Way This panoramic view encompasses the entire sky as seen by Two Micron All-Sky Survey. ... Look up anisotropy in Wiktionary, the free dictionary. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 56,000 light years in diameter and approximately 60 million light years distant. ... The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as the Galaxy), is a barred spiral galaxy of the Local Group. ... The Sun is the star of our solar system. ... Earth (IPA: , often referred to as the Earth, Terra, the World or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. ... It has been suggested that disc (galaxy) be merged into this article or section. ... Unsolved problems in physics: What causes anything to have mass? Mass is a property of a physical object that quantifies the amount of matter and energy it is equivalent to. ... In astrophysics, dark matter is matter that does not emit or reflect enough electromagnetic radiation (such as light, X-rays and so on) to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter. ... Two intersecting planes in three-dimensional space In mathematics, a plane is a fundamental two-dimensional object. ...


As with any spherical coordinate system, galactic coordinates define an equator, poles, meridians perpendicular to the equator and specifies the origin for longitude. By convention galactic latitude and galactic longitude are usually denoted by b and l, respectively. The directions perpendicular to the plane (either b=+90° or b=−90°) point to the galactic poles. Objects that have a galactic latitude — b — (close to) zero (near the galactic equator) lie in the plane of the disk of our Milky Way. A point plotted using the spherical coordinate system In Mathematics, the spherical coordinate system is a coordinate system for representing geometric figures in three dimensions using three coordinates, (ρ, φ, θ), where ρ represents the radial distance of a point from a fixed origin, φ represents the zenith angle from the positive z-axis and...


Within the Milky Way the position of objects (as specified by galactic coordinates) will remain fairly stable, except that the orbits of objects nearer the centre of the galaxy than the Sun will tend to increase their longitude. Orbits of objects further rimwards than the Sun will tend to have their longitude decrease. Objects above the plane of the Galaxy will tend to accelerate downwards, and objects below the plane will tend to accelerate upwards, etc, in accordance with the rules of orbital mechanics. Distant objects outside the Milky Way, and therefore not participating in the rotation of our galaxy, will change position dramatically, yet consistently, with respect to galactic coordinates. For example, consider a galaxy that lies on the galactic equator opposite the Milky Way’s axis of rotation today (say b of 0° and l of 180°). In approximately 110 million years from today the co-ordinate system would rotate so that that galaxy was at a point opposite its current position (b of 0° and l of 0°). As measured in this rotating frame, everything outside the Milky Way revolves around the Milky Way in a period of 220 million years.

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Conceptual definition

The galactic coordinates define a spherical coordinate system with the Sun at the center and a plane parallel to the general orientation of the Milky Way galaxy's central plane as the galactic equator. The longitudinal origin is toward the center of the galaxy. Galactic coordinates are not affected by precession so no precessional epoch need be designated. However, the galactic equator is quite oblique to both the earth's equator and the Earth's ecliptic. Also the Earth's orbit and distance from the Sun make Galactic coordinates more suitable for marking distant objects far beyond our solar system, than for objects within our solar system. For even the nearest stars the Earth's changing position relative to the Sun has little effect on the perceived position of objects outside our Solar System. However, for anything nearer, the galactic coordinates would be of no use.


The latitudinal plane or galactic equator is defined as a plane parallel to the central plane of the Milky Way Galaxy, but bisecting the Sun. Visually one can apprehend the general area of this plane by the band of Milky Way dust visible with the naked eye. Our solar system lies 112.7±1.8 light years (34.56±0.56 pc) [1] above the central plane of the Milky Way. However, since the Sun is estimated to be 26,000 light years to 35,000 light years from the galactic center, it is relatively speaking extremely close to the central plane. Based on these approximations the arc angle is 0.232±.02 degrees. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia Kuklos; or simply the Galaxy) is a barred spiral galaxy in the Local Group, and has special significance to humanity as the location of the solar system, which is located near the Orion... A light-year or lightyear, symbol ly, is the distance light travels in vacuum in one Julian year. ... Stellar parallax motion The parsec (symbol pc) is a unit of length used in astronomy. ...


The galactic equator lies at an angle of approximately 123° from the Earth's equatorial plane. Galactic latitudes are measured in degrees and increase from 0° at the Galactic Equator to 90° at the North Galactic Pole near the star Arcturus and decrease to - 90° at the South Galactic Pole in the constellation Sculptor. Arcturus (α Boo / α Boötis / Alpha Boötis) is the brightest star in the constellation Boötes, and the third brightest star in the night sky, with a visual magnitude of −0. ... This article or section does not cite its references or sources. ... Categories: Astronomy stubs | Modern constellations | Constellations | Sculptor constellation ...


Galactic longitude is measured in degrees as well from 0° to 360° originating in the direction of the axial center of the Milky Way in the constellation Sagittarius. Traveling around the galactic equator we find Cygnus at 90°, Auriga opposite the Galaxy's center at 180°, and Vela at 270°. For the astrological sign, see Sagittarius (astrology). ...


Since galactic coordinates are spherical coordinates they do not define a specific point in the galaxy but rather a ray extending from the center of the galactic equator. However, by including the distance of an object from the center of the galactic equator (the center of the Sun) the coordinates define a unique point in the galaxy. This is analogous to the need to specify altitude above or below sea-level to know an exact point-position in the Earth's coordinate system.


In terms of equatorial coordinates

In 1959, the IAU defined a standard of conversion between the Equatorial coordinate system and galactic coordinate system. Accordingly, the Milky Way's north galactic pole is exactly RA 12h51m26.282s, Dec 27°07′42.01″. 1959 (MCMLIX) was a common year starting on Thursday of the Gregorian calendar. ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ... Equatorial Coordinates Right ascension (abbrev. ... In astronomy, declination (dec) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. ...


The "zero of longitude" point on the galactic coordinates was calibrated to 17h45m37.224s, −28°56′10.23″ (J2000), and its J2000 position angle is 122.932°. Since the plane of the galactic equator lies above the plane through the center of the galaxy the galactic center is offset from the longitudinal origin and is located at 17h45m40.04s, −29°00′28.1″ (J2000). The Galactic Center is the rotational center of the Milky Way galaxy. ...


External references

See also


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The coordinates on a space transform naturally (by pullback) under the group of automorphisms of the space, and the set of all coordinates is a commutative ring called the coordinate ring of the space.
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A coordinate system is a mathematical language that is used to describe geometrical objects analytically; that is, if the coordinates of a set of points are known, their relationships and the properties of figures determined by them can be obtained by numerical calculations instead of by other descriptions.
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