# Galactic coordinate system

Galactic coordinate system
Artist's depiction of the Milky Way galaxy, showing the galactic longitude relative to the sun.

The galactic coordinate system (GCS) is a celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy. The "equator" is aligned to the galactic plane. Similar to geographic coordinates, positions in the galactic coordinate system have latitudes and longitudes.

## Notation

The symbols and b are used to represent the galactic longitude and latitude, respectively. The galactic longitude is measured in the plane of the galaxy using an axis pointing from the Sun to the galactic center. The galactic latitude is measured from the plane of the galaxy to the object using the Sun as vertex.[1]

## Definition

The galactic coordinates use the Sun as vertex. Galactic longitude is measured with baseline the direction to the center of the galaxy from the Sun in the galactic plane, while the galactic latitude b is measured between the object and the galactic plane with origin at the Sun

The International Astronomical Union (IAU) defined the galactic coordinate system in reference to the Equatorial coordinate system in 1958[2] The north galactic pole is defined to be at right ascension 12h 49m, declination +27.4° (B1950) in the constellation Coma Berenices, and the zero of longitude 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, the poles themselves at ±90° and the galactic equator being zero.[3] The south galactic pole is in the constellation Sculptor.

The equivalent system referred to as J2000 has the north galactic pole at 12h 51m 26.282s +27° 07′ 42.01″ (J2000) (192.859508, 27.128336 in decimal degrees), the zero of longitude at the position angle of 122.932°.[4] The point in the sky at which the galactic latitude and longitude are both zero is 17h 45m 37.224s −28° 56′ 10.23″ (J2000) (266.405100, -28.936175 in decimal degrees). This is offset slightly from the radio source Sagittarius A*, which is the best physical marker of the true galactic center. Sagittarius A* is located at 17h 45m 40.04s −29° 00′ 28.1″ (J2000), or galactic longitude 359° 56′ 39.5″, galactic latitude −0° 2′ 46.3″.[5]

The galactic equator runs through the following constellations:[6]

## Galactic rotation

Galaxy rotation curve for the Milky Way. Vertical axis is speed of rotation about the galactic center. Horizontal axis is distance from the galactic center. The sun is marked with a yellow ball. The observed curve of speed of rotation is blue. The predicted curve based upon stellar mass and gas in the Milky Way is red. Scatter in observations roughly indicated by gray bars. The difference is due to dark matter or perhaps a modification of the law of gravity.[7][8][9]
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. For this image the Mollweide projection has been applied, typical in maps using galactic coordinates.

The galactic coordinates approximate a coordinate system centered on the Sun's location. While its planets orbit counterclockwise, the Sun itself orbits the galactic center in a nearly circular path called the solar circle in a clockwise direction as viewed from the galactic north pole,[10][11] at a distance of 8 kpc and a velocity of 220 km/s,[12] which gives an approximate galactic rate of rotation (here at the location of our solar system) of 200 million years/cycle. At other locations the galaxy rotates at a different rate, depending primarily upon the distance from the galactic center. The predicted rate of rotation based upon known mass disagrees with the observed rate, as shown in the galaxy rotation curve and this difference is attributed to dark matter, although other explanations are continually sought, such as changes in the law of gravitation. The differing rates of rotation contribute to the proper motions of the stars.

## References

1. ^ Peter Duffett-Smith (1988). Practical Astronomy with Your Calculator (3rd ed.). Cambridge University Press. p. 32; Figure 8. ISBN 0521356997.
2. ^ replacing an older system introduced in the 1930s."User Manual: The Galactic Coordinate System". Where is M13?. Think Astronomy. 2007. Retrieved 2008-02-07.
3. ^ James Binney, Michael Merrifield (1998). Galactic Astronomy. Princeton University Press. pp. 30–31. ISBN 0691025657.
4. ^ Reid, M.J.; Brunthaler, A. (2004 2004). "The Proper Motion of Sagittarius A*". The Astrophysical Journal (The American Astronomical Society) 616 (2): 883. arXiv:astro-ph/0408107. Bibcode 2004ApJ...616..872R. doi:10.1086/424960. Retrieved 2008-02-07.
5. ^ Data and scientific papers about Sagittarius A*
6. ^ SEDS Milky Way Constellations
7. ^ Peter Schneider (2006). Extragalactic Astronomy and Cosmology. Springer. p. 4, Figure 1.4. ISBN 3540331743.
8. ^ Theo Koupelis, Karl F Kuhn (2007). In Quest of the Universe. Jones & Bartlett Publishers. p. 492; Figure 16-13. ISBN 0763743879.
9. ^ Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An Introduction to Galaxies and Cosmology. Cambridge University Press. p. 21; Figure 1.13. ISBN 0521546230.
10. ^ David Pratt (2003). "Earth's Meteoric Veil". Retrieved January 13, 2011.
11. ^ Bruce McClure (2007). "Two Stars Flag the Sun's Path through the Milky Way". Retrieved January 13, 2011.
12. ^ F. Combes, Keiichi Wada (2008). "Mapping the Milky Way and the Local Group". Mapping the Galaxy and Nearby Galaxies. Springer. p. 19. ISBN 0387727671.

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