Galactic coordinate system

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. The northern galactic pole is located in the constellation of Coma Berenices while the southern pole lies in Sculptor.[1]

Contents

Notation

See also: List of celestial bodies by galactic coordinate system

The symbols ℓ 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.[2]

Definition

The International Astronomical Union (IAU) defined the galactic coordinate system in reference to the Equatorial coordinate system in 1958[3] 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.[4] 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°.[5] 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″.[6]

The galactic equator runs through the following constellations:[7]

Galactic rotation

See also: Galactic rotation curve

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,[11][12] at a distance of 8 kpc and a velocity of 220 km/s,[13] 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.

See also

References

  1. ^ Kaler, James B.. "Sculptor". Stars. University of Illinois. http://stars.astro.illinois.edu/sow/scl-t.html. Retrieved 2012-01-02. 
  2. ^ Peter Duffett-Smith (1988). Practical Astronomy with Your Calculator (3rd ed.). Cambridge University Press. p. 32; Figure 8. ISBN 0521356997. http://books.google.com/?id=DwJfCtzaVvYC&pg=PA43&dq=%22galactic+coordinate%22. 
  3. ^ replacing an older system introduced in the 1930s."User Manual: The Galactic Coordinate System". Where is M13?. Think Astronomy. 2007. http://www.thinkastronomy.com/M13/Manual/common/galactic_coords.html. Retrieved 2008-02-07. 
  4. ^ James Binney, Michael Merrifield (1998). Galactic Astronomy. Princeton University Press. pp. 30–31. ISBN 0691025657. http://books.google.com/?id=arYYRoYjKacC&pg=PA675&dq=rotation+%22proper+motion%22+galaxy+OR+galactic. 
  5. ^ 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. http://www.journals.uchicago.edu/doi/full/10.1086/424960. Retrieved 2008-02-07. 
  6. ^ Data and scientific papers about Sagittarius A*
  7. ^ SEDS Milky Way Constellations
  8. ^ Peter Schneider (2006). Extragalactic Astronomy and Cosmology. Springer. p. 4, Figure 1.4. ISBN 3540331743. http://books.google.com/?id=uP1Hz-6sHaMC&pg=PA100&dq=rotation+Milky+way. 
  9. ^ Theo Koupelis, Karl F Kuhn (2007). In Quest of the Universe. Jones & Bartlett Publishers. p. 492; Figure 16-13. ISBN 0763743879. http://books.google.com/?id=6rTttN4ZdyoC&pg=PA491&dq=Milky+Way+%22rotation+curve%22. 
  10. ^ 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. http://books.google.com/?id=36K1PfetZegC&pg=PA20&dq=Milky+Way+%22rotation+curve%22. 
  11. ^ David Pratt (2003). "Earth's Meteoric Veil". http://davidpratt.info/dust2.htm#d2. Retrieved January 13, 2011. 
  12. ^ Bruce McClure (2007). "Two Stars Flag the Sun's Path through the Milky Way". http://www.idialstars.com/tspm.htm. Retrieved January 13, 2011. 
  13. ^ 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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