A jointed ring placed at the upper end of a → planispheric astrolabe astrolabe. 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 (online museo galileo, VirtualMuseum).
From L. thronus, from Gk. thronos "elevated seat, chair, throne," from PIE root *dher- "to hold firmly, support;" cf. L. firmus "firm, steadfast, strong, stable," Skt. dharma- "statute, law;" Pers. dâr-, dâštan "to have, to possess," → property.
Korsi "throne, chair, seat," from Ar. kursī.