The Galactic Coordinate System
The galactic coordinate system is the key to understanding where objects are located within the Galaxy. It was established in 1958 by the International Astronomical Union and is useful for specifying an object's location relative to the Sun and the galactic core of the Milky Way.
The galactic coordinate system is a 2-D spherical coordinate system with us (or the Sun) at its center. It has latitude and longitude lines, similar to Earth's. In fact, a good analogy is to imagine yourself standing at the center of a hollow Earth looking at the latitude and longitude lines on the Earth's surface. The galactic coordinate system is similar except we are looking out at the celestial sphere.
There is a one-to-one mapping between the galactic coordinate system and the more familiar equatorial coordinate system. Relatively simple equations can be used to convert from one to the other.
The galactic equator (i.e., 0º galactic latitude) is coincident with the plane of the Milky Way Galaxy and is shown as the red circle in the image above. Galactic latitude is the angle above or below this plane (e.g. the yellow angle above). Thus, objects with a galactic latitude near 0º will be located within the Milky Way's spiral arms. Objects with a positive galactic latitude will be above the arms in the northern galactic hemisphere; objects with a negative galactic latitude will be below, in the southern galactic hemisphere.
Galactic longitude is measured from 0º to 360º, counter clockwise as seen from the north galactic pole. 0º galactic longitude is arbitrarily defined as the direction pointing to our galactic center. Within the plane of our galaxy (0º galactic latitude), the main points of longitude and the Milky Way constellations which lie in their directions are as follows:
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• 0º is in the direction of Sagittarius
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• 90º is in the direction of Cygnus
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• 180º is in the direction of the galactic anti-center in Auriga
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• 270º is in the direction of Vela
Now consider an object and its galactic coordinates. Any other object lying along the same line of sight will have the same coordinates but only differ in its distance component. An object's distance is not part of its galactic coordinates. However, knowledge of an object's galactic latitude, longitude and distance, does allow us to uniquely locate it within the 3-D space around the Milky Way. Our Galaxy uses this information to plot deep sky objects in its Galaxy View.