= Armillary sphere
{wiki}
An <armillary sphere> is an astronomical model made from rings representing reference circles on the <celestial sphere>. Rotating the rings makes relations among the sky's reference planes visible in three dimensions. A central globe can represent the <Earth>, while inclined rings represent the apparent solar path and other celestial circles.
Royal Museums Greenwich records a 1602 instrument with eight brass rings, a horizon ring, movable solar and lunar ring systems and a small terrestrial globe. Its nested parts make the reference geometry tangible rather than merely drawing intersecting circles on paper.<Armillary sphere AST0633 by Royal Museums Greenwich>[Instrument description, primary sphere and movable systems; object details AST0633, date 1602]{ref}
An <armillary sphere> is useful for explaining why an angular <coordinate> depends on a chosen reference plane. It represents directions and geometric relations; its ring sizes should not be interpreted as a scale model of the distances to the <Sun>, <Moon> or stars.
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