Another coordinate system is sometimes preferred when astronomers are studying the structure of the Galaxy. This system uses the plane of the Galaxy as the reference plane and the centre of the Galaxy as the arbitrary reference point. The coordinates are both expressed in degrees. They are the GALACTIC LONGITUDE (1) and the GALATIC LATTITUDE (b). As the position of the centre of the Galaxy became better established, ‘old’ galactic coordinates (I1, b1) and, much later, ‘new’ galactic coordinates (1II , bII ) came into use. However, only the new galactic coordinates are used now and the superscripts are usually omitted, giving (1, b). The galactic north pole is at right ascension 12hr 49min and declination +27°.4 (1950.0). Galactic longitude increases from 0° (towards the galactic centre) to 360° in the same direction as increasing right ascension. The galactic latitude is +90° at the galactic north-pole, zero on the galactic equator and -90° at the galactic south-pole.

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