A desktop planetarium, two thousand years before planetariums: nested metal rings, one per circle of the sky (equator, ecliptic, tropics, meridians), with the Earth in the middle, or the Sun after Copernicus. The name comes from Latin armilla, bracelet.
Invented twice, independently: in Greece (attributed to Eratosthenes, 3rd century BC) and in China, where Zhang Heng hooked one to a water wheel so it would turn by itself. It measured star positions AND taught how the sky works, and by the Renaissance it had become shorthand for wisdom itself, showing up in scholars’ portraits. It is also why there is a golden sphere on Portugal’s flag: it was the personal emblem of King Manuel I, the instrument that stood for the whole discoverable world.