Supergalactic coordinate system


In astronomy, supergalactic coordinates are coordinates in a spherical coordinate system which was designed to have its equator aligned with the supergalactic plane, a major structure in the local universe formed by the preferential distribution of nearby galaxy clusters towards a plane. The supergalactic plane was recognized by Gérard de Vaucouleurs in 1953 from the Shapley-Ames Catalog, although a flattened distribution of nebulae had been noted by William Herschel over 200 years earlier. Vera Rubin had also identified the supergalactic plane in the 1950s, but her data remained unpublished.
By convention, supergalactic latitude is usually abbreviated SGB, and supergalactic longitude as SGL, by analogy to and conventionally used for galactic coordinates. The zero point for supergalactic longitude is defined by the intersection of this plane with the galactic plane.

Definition

So the transformation from a triple of Cartesian supergalactic coordinates to a triple of galactic coordinates is
The left column in this matrix is the image of the origin of the supergalactic system in the galactic system, the right column in this matrix is the image of the north pole of the supergalactic coordinates in the galactic system, and the middle column is the cross product.