The galactic plane is the plane in which the majority of a flattened galaxy's mass lies. The most commonly discussed example is the plane on which the Earth's galaxy, the Milky Way, lies, which is also called the Milky Way's plane or plane of the Milky Way.
One direction perpendicular to the plane is called below or south, while the opposite direction is called above or north. From Earth, due galactic south is marked by the south galactic pole, which lies in Sculptor, and due galactic north is marked by the north galactic pole, which lies in Coma Berenices, near the bright starArcturus. These are defined to lie roughly in the same directions as the Earth's north and south poles.
It is sometimes convenient to use this galacticplane as the basis of a galactic coordinate system, where the directions perpendicular to the plane point to the galactic poles, creating a spherical coordinate system.
In 1959, the IAU defined the position of the Milky Way'snorthgalactic pole to be exactly RA=12h49m, Dec=27°24′ in the then-used B1950 epoch; in the currently-used J2000.0 epoch, after precession is taken into account, its position is RA 12h51m26.282s, Dec 27°07′42.01″.
This position is in Coma Berenices, near the bright starArcturus; likewise, the southgalactic pole lies in the constellation Sculptor.
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