In its simplest form, consisting of a ring fixed in the plane of the equator, the armilla is one of the most ancient of astronomical instruments. Slightly developed, it was crossed by another ring fixed in the plane of the meridian. The first was an equinoctial, the second a solstitial armilla. Shadows were used as indices of the sun's positions, in combinations with angular divisions. When several rings or circles were combined representing the great circles of the heavens, the instrument became an armillary sphere.
The spherical astrolabe from medieval Islamic astronomy.Eratosthenes most probably used a solstitial armilla for measuring the obliquity of the ecliptic. Hipparchus probably used an armillary sphere of four rings. Ptolemy describes his instrument in the Syntaxis (book v. chap. i). It consisted of a graduated circle inside which another could slide, carrying to small tubes diametrically opposite, the instrument being kept vertical by a plumb-line.
Armillary spheres were developed by the Greeks and were used as teaching tools already in the 3rd century B.C.. In larger and more precise forms they were also used as observational instruments.