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Galactic plane
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<p>The galactic plane is the <a href="/facts/Plane_(geometry)/tAUtRFcn">plane</a> on which the majority of a disk-shaped <a href="/facts/Galaxy/zCyUvEf7">galaxy</a>'s <a href="/facts/Mass/HHdc8XmF">mass</a> lies. The directions <a href="/facts/Perpendicular/u0k8xqx4">perpendicular</a> to the galactic plane point to the galactic poles. In actual usage, the terms <i>galactic plane</i> and <i>galactic poles</i> usually refer specifically to the plane and poles of the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a>, in which Planet <a href="/facts/Earth/HLLmDfj0">Earth</a> is located. </p><p>Some <a href="/facts/Irregular_galaxy/P95rRpYR">galaxies are irregular</a> and do not have any well-defined disk. Even in the case of a <a href="/facts/Barred_spiral_galaxy/cntSCFX4">barred spiral galaxy</a> like the Milky Way, defining the galactic plane is slightly imprecise and arbitrary since the stars are not perfectly <a href="/facts/Coplanar/dSTa1bcb">coplanar</a>. In 1959, the <a href="/facts/International_Astronomical_Union/GMFPImXo">IAU</a> defined the position of the Milky Way's north galactic pole as exactly <a href="/facts/Right_ascension/z4CEsTFu">RA</a> = 12h 49m , <a href="/facts/Declination/5X4S4k9u">Dec</a> = 27° 24′ in the then-used <a href="/facts/B1950/5wkqN4MJ">B1950</a> <a href="/facts/Epoch_(astronomy)/RDdj6SmA">epoch</a>; in the currently-used <a href="/facts/J2000/RDdj6SmA">J2000</a> epoch, after <a href="/facts/Precession/44Sk1PMD">precession</a> is taken into account, its position is RA 12h 51m 26.282s, Dec 27° 07′ 42.01″. This position is in <a href="/facts/Coma_Berenices/k2phuQ8b">Coma Berenices</a>, near the bright <a href="/facts/Star/PFbd6ICx">star</a> <a href="/facts/Arcturus/8W0qslPJ">Arcturus</a>; likewise, the south galactic pole lies in the <a href="/facts/Constellation/nBQqgW8A">constellation</a> <a href="/facts/Sculptor_(constellation)/WPe7YtEv">Sculptor</a>. </p><p>The <i>zero of longitude</i> of <a href="/facts/Galactic_coordinate_system/Un6BambN">galactic coordinates</a> was also defined in 1959 to be at position angle 123° from the north <a href="/facts/Celestial_pole/5Gj9JJbK">celestial pole</a>. Thus the zero longitude point on the galactic equator was at 17h 42m 26.603s, −28° 55′ 00.445″ (B1950) or 17h 45m 37.224s, −28° 56′ 10.23″ (J2000), and its J2000 position angle is 122.932°. The <a href="/facts/Galactic_Center/TI4MwWwM">Galactic Center</a> is located at position angle 31.72° (B1950) or 31.40° (J2000) east of north. </p>