An Etymological Dictionary of Astronomy and Astrophysics

The body of the → planispheric astrolabe which is a thin circular plate, with a hole in the center. It has a thicker, raised, and graduated edge, called the → limb. The hollow of the mater holds the → tympanum and the rotating → rete. The upper part of the mater carries a jointed ring, called the → throne. By slipping one’s thumb into the ring, one raises the instrument so that its weight and symmetrical design keeps it perpendicular to the ground. On the back of the mater are engraved several circular scales (online museo galileo, VirtualMuseum).

See also: From L. mater, → mother.