Quadrant (astronomy)

A galactic quadrant, or quadrant of the Galaxy, is one of four circular sectors in the division of the Milky Way Galaxy.

In actual astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system, which places the Sun as the pole of the mapping system. The Sun is used instead of the Galactic Center for practical reasons since all astronomical observations (by humans) to date have been based on Earth or within the solar system.

Quadrants are described using ordinals—for example, "1st galactic quadrant" "second galactic quadrant," or "third quadrant of the Galaxy." Viewing from the north galactic pole with 0 degrees (°) as the ray that runs starting from the Sun and through the galactic center, the quadrants are as follow:

Due to the orientation of the Earth with respect to the rest of the Galaxy, the 2nd galactic quadrant is primarily only visible from the northern hemisphere while the 4th galactic quadrant is mostly only visible from the southern hemisphere. Thus, it is usually more practical for amateur stargazers to use the celestial quadrants. Nonetheless, cooperating or international astronomical organizations are not so bound by the Earth's horizon.

Based on a view from Earth, one may look towards major constellations for a rough sense of where the borders of the quadrants are: (Note: by drawing a line through the following, one can also approximate the galactic equator.)

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