Galactic coordinate system
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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.
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.
Within the Milky Way the position of objects (as specified by galactic coordinates) will remain relatively stable. However distant objects outside the Milky Way and therefore not participating in the rotation of our galaxy will change position dramatically 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 that same galaxy would revolve around the Milky Way to a point opposite its current position (b of 0° and l of 0°). This makes galactic coordinates Milky Way centric coordinates. Everything outside the Milky Way revolves around the Milky Way in a period of 220 million years.
Conceptual definition
Conceptually, 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 effected 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. This is due to the Earth's distance from the Sun and its orbit around the Sun. 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 coplanar with 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) [link] above the central plane of the Milky Way. However, since the Sun is about 30,000 light years from the galactic center, it is relatively speaking extremely close to the central plane.
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.
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 galaxies center at 180° and Vela at 270°.
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″.
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).
External references
- http://heasarc.gsfc.nasa.gov/cgi-bin/Tools/convcoord/convcoord.pl (Universal coordinate converter)
- http://violet.pha.jhu.edu/cgi-bin/eqtogal_tool (Equatorial/Galactic conversion tool)
- http://star-www.st-and.ac.uk/~fv/webnotes/chapter8.htm
- [Where is M13? User Manual]
See also
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