Definition
See also: Galactic plane and Galactic centerIn 1958, the International Astronomical Union (IAU) defined the galactic coordinate system in reference to the 21-cm radio emissions of galactic neutral hydrogen, replacing a system first defined in 1932. In the equatorial coordinate system, for equinox and equator of 1950.0, the north galactic pole is defined at right ascension 12h 49m, declination +27°.4, in the constellation Coma Berenices, 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, the galactic equator being 0°, the poles ±90°. Based on this definition, the galactic poles and equator can be found from spherical trigonometry and can be precessed to other epochs; see the table.
| right ascension | declination | constellation | |
| north pole (+90° latitude) | 12h 49m
(12h 51m.4) |
+27°.4
(+27°.13) |
Coma Berenices |
| south pole (-90° latitude) | 0h 49m
(0h 51m.4) |
-27°.4
(-27°.13) |
Sculptor |
| galactic center (0° longitude) | 17h 42m.4
(17h 45m.6) |
-28°.92
(-28°.94) |
Sagittarius |
| anti-center (180° longitude) | 5h 42m.4
(5h 45m.6) |
+28°.92
(+28°.94) |
Auriga |
Radio source Sagittarius A*, which is the best physical marker of the true galactic center, is located at 17h 45m 40s.0409, -29° 00' 28".118 (J2000). Rounded to the same number of digits as the table, 17h 45m.7, -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.
Read more about this topic: Galactic Coordinate System
Famous quotes containing the word definition:
“Scientific method is the way to truth, but it affords, even in
principle, no unique definition of truth. Any so-called pragmatic
definition of truth is doomed to failure equally.”
—Willard Van Orman Quine (b. 1908)
“I’m beginning to think that the proper definition of “Man” is “an animal that writes letters.””
—Lewis Carroll [Charles Lutwidge Dodgson] (1832–1898)
“According to our social pyramid, all men who feel displaced racially, culturally, and/or because of economic hardships will turn on those whom they feel they can order and humiliate, usually women, children, and animals—just as they have been ordered and humiliated by those privileged few who are in power. However, this definition does not explain why there are privileged men who behave this way toward women.”
—Ana Castillo (b. 1953)