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
(such as the Virgo cluster
, the Great Attractor
and the Pisces-Perseus supercluster
) towards a (two-dimensional) plane
. The supergalactic plane was recognized by Gérard de Vaucouleurs
in 1953 from the Shapley-Ames catalogue
, although a flattened distribution of nebulae
had been noted by William Herschel
over 200 years earlier.
By convention, supergalactic latitude and supergalactic longitude are usually denoted by SGB and SGL, respectively, by analogy to b and l conventionally used for galactic coordinates. The zero point for supergalactic longitude is defined by the intersection of this plane with the galactic plane.
- The north supergalactic pole (SGB=90°) lies at galactic coordinates (l =47.37°, b =+6.32°). In the equatorial coordinate system (epoch J2000), this is approximately (RA=18.9 h, Dec=+15.7°).
- The zero point (SGB=0°, SGL=0°) lies at (l=137.37°, b=0°). In J2000 equatorial coordinates, this is approximately (2.82 h, +59.5°).