Astonomy: Armillary Sphere - 16th century. From Tycho Brahe. An armillary sphere (also known as a spherical astrolabe, armilla, or armil) is a model of the celestial sphere, invented by Eratosthenes in 255 BC. Its name comes from the Latin armilla (circle, bracelet), since it has a skeleton made of graduated metal circles linking the poles and representing the equator, the ecliptic, meridians and parallels. Usually a ball representing the Earth or, later, the Sun is placed in its center. It is used to demonstrate the motion of the stars around the Earth. ?006 Fortean / TopFoto