Supergalactic Plane
The grand old man of galaxy research, Gérard de Vaucouleurs,
noticed that when you plot all nearby galaxies in a 3D plot, they
lie more or less on a plane (with a certain thickness, of course).
He based a new coordinate system, the supergalactic coordinate
system, on this plane. The Earth is at the origin of this coordinate
system and, hence, the supergalactic plane passes through the Earth.
Objects with supergalactic latitude (SGB) at zero degrees lie on the
supergalactic plane. Another variant of the coordinate system is
supergalactic cartesian coordinate system, with positions of objects
given with cartesian coordinates SGX, SGY and SGZ. The supergalactic
plane is the same as the SGX-SGY -plane. Hence objects that lie on the
supergalactic plane have SGZ value of zero.